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If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - - - - -Title: Airplane Photography - - -Author: Herbert Eugene Ives - - - -Release Date: November 12, 2016 [eBook #53508] - -Language: English - -Character set encoding: UTF-8 - - -***START OF THE PROJECT GUTENBERG EBOOK AIRPLANE PHOTOGRAPHY*** - - -E-text prepared by Richard Tonsing and the Online Distributed Proofreading -Team (http://www.pgdp.net) from page images generously made available by -Internet Archive (https://archive.org) - - - -Note: Project Gutenberg also has an HTML version of this file - which includes the 208 original illustrations. - See 53508-h.htm or 53508-h.zip: - (http://www.gutenberg.org/files/53508/53508-h/53508-h.htm) - or - (http://www.gutenberg.org/files/53508/53508-h.zip) - - - Images of the original pages are available through - Internet Archive. See - https://archive.org/details/airplanephotogra00ivesuoft - - -Transcriber's note: - - Text enclosed by underscores is in italics (_italics_). - - Text enclosed by equal signs is in bold face (=bold=). - - A carat character is used to denote superscription. A - single character following the carat is superscripted - (example: 100,000^2). - - An underscore followed by one or more characters enclosed - by curly brackets indicates that the the character(s) within - the curly brackets is/are subcripted (example: A_{1}). - - - - - -AIRPLANE PHOTOGRAPHY - - -[Illustration] - - -AIRPLANE PHOTOGRAPHY - -by - -HERBERT E. IVES - -Major, Aviation Section, Signal Officers Reserve Corps, United States -Army; Lately Officer in Charge of Experimental Department, Photographic -Branch, Air Service - -208 ILLUSTRATIONS - - - - - - - -[Illustration] - -Philadelphia and London -J. B. Lippincott Company - -Copyright, 1920, by J. B. Lippincott Company - -Printed by J. B. Lippincott Company -at the Washington Square Press -Philadelphia, U. S. A. - - - - - TO MY WIFE - - A HELPFUL CRITIC, EVEN THOUGH SHE NEITHER PHOTOGRAPHS NOR FLIES - - - - - PREFACE - - -Airplane photography had its birth, and passed through a period of -feverish development, in the Great War. Probably to many minds it -figures as a purely military activity. Such need not be the case, for -the application of aerial photography to mapping and other peace-time -problems promises soon to quite overshadow its military origin. It has -therefore been the writer's endeavor to treat the subject as far as -possible as a problem of scientific photography, emphasizing those -general principles which will apply no matter what may be the purpose of -making photographs from the air. It is of course inevitable that whoever -at the present time attempts a treatise on this newest kind of -photography must draw much of his material from war-time experience. If, -for this reason, the problems and illustrations of this book are -predominantly military, it may be remembered that the demands of war are -far more severe than those of peace; and hence the presumption is that -an account of how photography has been made successful in the military -plane will serve as an excellent guide to meeting the peace-time -problems of the near future. - -It is assumed that the reader is already fairly conversant with ordinary -photography. Considerable space has indeed been devoted to a discussion -of the fundamentals of photography, and to scientific methods of study, -test, and specification. This has been done because aerial photography -strains to the utmost the capacity of the photographic process, and it -is necessary that the most advanced methods be understood by those who -would secure the best results or contribute to future progress. No -pretence is made that the book is an aerial photographic encyclopædia; -it is not a manual of instructions; nor is its appeal so popular as it -would be were the majority of the illustrations striking aerial -photographs of war subjects. It is hoped that the middle course steered -has produced a volume which will be informative and inspirational to -those who are seriously interested either in the practice of aerial -photography or in its development. - -The writer is deeply in debt to many people, whose assistance of one -sort or another has made this book possible. First of all should be -mentioned those officers of the English, French and Italian armies -through whose courtesy it is that he can speak at first hand of the -photographic practices in these armies at the front. It is due to -Lieutenant Colonel R. A. Millikan that the experimental work of which -the writer has had charge was initiated in the United States Air -Service. To him and to Major C. E. Mendenhall, under whom the work was -organized in the Science and Research Division of the Signal Corps, are -owing the writer's thanks for the opportunities and support given by -them. A similar acknowledgment is made to Lieutenant Colonel J. S. -Sullivan, Chief of the Photographic Branch of the Army Air Service, for -his interest and encouragement in the compilation of this work, and for -the permission accorded to use the air service photographs and drawings -which form the majority of the illustrations. - -The greatest debt of all, however, is to those officers who have formed -the staff of the Experimental Department. To mention them by name: -Captain C. A. Proctor, who was charged with our foreign liaison, and who -acted as deputy chief during the writer's absence overseas; Captain A. -K. Chapman, in charge of the work on optical parts, and later chief of -our Rochester Branch; Captain E. F. Kingsbury, who had immediate charge -of camera development; Lieutenant J. B. Brinsmade and Mr. R. P. -Wentworth, who handled the experimental work on camera mountings and -installation; Lieutenant A. H. Nietz, in charge of the Langley Field -Laboratory of the Experimental Department; Mr. R. B. Wilsey and -Lieutenant J. M. Hammond, who, with Lieutenant Nietz, carried on the -experimental work on sensitized materials. A large part of what is new -and what is ascribed in the following chapters to “The American Air -Service” is the work of this group of men. Were individual references -made, in place of this general and inclusive one, their names would -thickly sprinkle these pages. It has been a rare privilege to have -associates so able, enthusiastic, and loyal. - - THE AUTHOR - - NOVEMBER, 1919 - - - - - CONTENTS - - - I. INTRODUCTORY - - CHAPTER PAGE - - 1. GENERAL SURVEY 15 - - 2. THE AIRPLANE CONSIDERED AS A CAMERA PLATFORM 20 - - - II. THE AIRPLANE CAMERA - - 3. THE CAMERA—GENERAL CONSIDERATIONS 39 - - 4. LENSES FOR AERIAL PHOTOGRAPHY 44 - - 5. THE SHUTTER 68 - - 6. PLATE-HOLDERS AND MAGAZINES 87 - - 7. HAND-HELD CAMERAS FOR AERIAL WORK 95 - - 8. NON-AUTOMATIC AERIAL PLATE CAMERAS 102 - - 9. SEMI-AUTOMATIC AERIAL PLATE CAMERAS 116 - - 10. AUTOMATIC AERIAL PLATE CAMERAS 124 - - 11. AERIAL FILM CAMERAS 130 - - 12. MOTIVE POWER FOR AERIAL CAMERAS 145 - - 13. CAMERA AUXILIARIES 163 - - - III. THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS - - 14. THEORY AND EXPERIMENTAL STUDY OF METHODS OF CAMERA SUSPENSION 179 - - 15. PRACTICAL CAMERA MOUNTINGS 193 - - 16. INSTALLATION OF CAMERAS AND MOUNTINGS IN PLANES 208 - - - IV. SENSITIZED MATERIALS AND CHEMICALS - - 17. THE DISTRIBUTION OF LIGHT, SHADE AND COLOR IN THE AERIAL VIEW 221 - - 18. CHARACTERISTICS OF PHOTOGRAPHIC EMULSIONS 227 - - 19. FILTERS 239 - - 20. EXPOSURE OF AERIAL NEGATIVES 247 - - 21. PRINTING MEDIA 252 - - 22. PHOTOGRAPHIC CHEMICALS 257 - - - V. METHODS OF HANDLING PLATES, FILMS AND PAPERS - - 23. THE DEVELOPING AND DRYING OF PLATES AND FILMS 267 - - 24. PRINTING AND ENLARGING 279 - - - VI. PRACTICAL PROBLEMS AND DATA - - 25. SPOTTING 291 - - 26. MAP MAKING 304 - - 27. OBLIQUE AERIAL PHOTOGRAPHY 320 - - 28. STEREOSCOPIC AERIAL PHOTOGRAPHY 329 - - 29. THE INTERPRETATION OF AERIAL PHOTOGRAPHS 351 - - 30. NAVAL AERIAL PHOTOGRAPHY 368 - - - VII. THE FUTURE OF AERIAL PHOTOGRAPHY - - 31. FUTURE DEVELOPMENTS IN APPARATUS AND METHODS 383 - - 32. TECHNICAL AND PICTORIAL USES 388 - - 33. EXPLORATION AND MAPPING 401 - - - - - I - INTRODUCTORY - - - - - AIRPLANE PHOTOGRAPHY - - - - - CHAPTER I - GENERAL SURVEY - - -=Aerial Photography from Balloons and Kites.=—Photography from the air -had been developed and used to a limited extent before the Great War, -but with very few exceptions the work was done from kites, from -balloons, and from dirigibles. Aerial photographs of European cities had -figured to a small extent in the illustration of guidebooks, and some -aerial photographic maps of cities had been made, notably by the Italian -dirigible balloon service. Kites had been employed with success to carry -cameras for photographing such objects as active volcanoes, whose -phenomena could be observed with unique advantage from the air, and -whose location was usually far from balloon or dirigible facilities. - -As a result of this pre-war work we had achieved some knowledge of real -scientific value as to photographic conditions from the air. Notable -among these discoveries was the existence of a veil of haze over the -landscape when seen from high altitudes, and the consequent need for -sensitive emulsions of considerable contrast, and for color-sensitive -plates to be used with color filters. - -The development of aerial photography would probably however have -advanced but little had it depended merely on the balloon or the kite. -As camera carriers their limitations are serious. The kite and the -captive balloon cannot navigate from place to place in such a way as to -permit the rapid or continuous photography of extended areas. The kite -suffers because the camera it supports must be manipulated either from -the ground or else by some elaborate mechanism, both for pointing and -for handling the exposing and plate changing devices. The free balloon -is at the mercy of the winds both as to its direction and its speed of -travel. The dirigible balloon, as it now exists after its development -during the war, is, it is true, not subject to the shortcomings just -mentioned. Indeed, in many ways it is perhaps superior to the airplane -for photographic purposes, since it affords more space for camera and -accessories, and is freer from vibration. It is capable also of much -slower motion, and can travel with less danger over forests and -inaccessible areas where engine failure would force a plane down to -probable disaster. But the smaller types as at present built are not -designed to fly so high as the airplane, and the dirigibles, both large -and small, are far more expensive in space and maintenance than the -plane. For this one reason especially they are not likely to be the most -used camera carriers of the aerial photographer of the future. Inasmuch -as the photographic problems of the plane are more difficult than those -of the dirigible and at the same time broader, the subject matter of -this book applies with equal force to photographic procedure for -dirigibles. - -=Development of Airplane Photography in the Great War.=—The airplane has -totally changed the nature of warfare. It has almost eliminated the -element of surprise, by rendering impossible that secrecy which formerly -protected the accumulation of stores, or the gathering of forces for the -attack, a flanking movement or a “strategic retreat.” To the side having -command of the air the plans and activities of the enemy are an open -book. It is true that more is heard of combats between planes than of -the routine task of collecting information, and the public mind is more -apt to be impressed by the fighting and bombing aspects of aerial -warfare. Nevertheless, the fact remains that the chief use of the -airplane in war is _reconnaissance_. The airplane is “the eye of the -army.” - -In the early days of the war, observation was visual. It was the task of -the observer in the plane to sketch the outlines of trenches, to count -the vehicles in a transport train, to estimate the numbers of marching -men, to record the guns in an artillery emplacement and to form an idea -of their size. But the capacity of the eye for including and studying -all the objects in a large area, particularly when moving at high speed, -was soon found to be quite too small to properly utilize the time and -opportunities available in the air. Moreover, the constant watching of -the sky for the “Hun in the sun” distracted the observer time and time -again from attention to the earth below. Very early in the war, -therefore, men's minds turned to photography. The all-seeing and -recording eye of the camera took the place of the observer in every kind -of work except artillery fire control and similar problems which require -immediate communication between plane and earth. - -The volume of work done by the photographic sections of the military air -service steadily increased until toward the end of the war it became -truly enormous. The aerial negatives made per month in the British -service alone mounted into the scores of thousands, and the prints -distributed in the same period numbered in the neighborhood of a -million. The task of interpreting aerial photographs became a highly -specialized study. An entirely new activity—that of making photographic -mosaic maps and of maintaining them correct from day to day—usurped -first place among topographic problems. By the close of the war scarcely -a single military operation was undertaken without the preliminary of -aerial photographic information. Photography was depended on to discover -the objectives for artillery and bombing, and to record the results of -the subsequent “shoots” and bomb explosions. The exact configurations of -front, second, third line and communicating trenches, the machine gun -and mortar positions, the “pill boxes,” the organized shell holes, the -listening posts, and the barbed wire, were all revealed, studied and -attacked entirely on the evidence of the airplane camera. Toward the end -of the war important troop movements were possible only under the cover -of darkness, while the development of high intensity flashlights -threatened to expose even these to pitiless publicity. - -=Limitations to Airplane Photography Set by War Conditions.=—The ability -of the pilot to take the modern high-powered plane over any chosen point -at any desired altitude in almost any condition of wind or weather gives -to the plane an essential advantage over the photographic kites and -balloons of pre-war days. There are, however, certain disadvantages in -the use of the plane which must be overcome in the design of the -photographic apparatus and in the method of its use. Some few of these -disadvantages are inherent in the plane itself; for instance, the -necessity for high speed in order to remain in the air, and the -vibration due to the constantly running engine. Others are peculiar to -war conditions, and their elimination in planes for peace-time -photography will give great opportunities for the development of aerial -photography as a science. - -Chief among the war-time limitations is that of economy of space and -weight. A war plane must carry a certain equipment of guns, -radio-telegraphic apparatus and other instruments, all of which must be -readily accessible. Many planes have duplicate controls in the rear -cockpit to enable the observer to bring the plane to earth in case of -accident to the pilot. Armament and controls demand space which must be -subtracted from quarters already cramped, so that in most designs of -planes the photographic outfit must be accommodated in locations and -spaces wretchedly inadequate for it. Economy in weight is pushed to the -last extreme, for every ounce saved means increased ceiling and radius -of action, a greater bombing load, more ammunition, or fuel for a longer -flight. Hence comes the constant pressure to limit the weight of -photographic and other apparatus, even though the tasks required of the -camera constantly call for larger rather than smaller equipment. - -To another military necessity is due in great measure the forced -development of aerial photographic apparatus in the direction of -automatic operation. The practice of entrusting the actual taking of the -pictures to observers with no photographic knowledge, whose function was -merely to “press the button” at the proper time, necessitated cameras as -simple in operation as possible. The multiplicity of tasks assigned to -the observer, and in particular the ever vital one of watching for enemy -aircraft, made the development of largely or wholly automatic cameras -the war-time ideal of all aerial photographic services. Whether the -freeing of the observer from other tasks will relegate the necessarily -complex and expensive automatic camera to strictly military use remains -to be seen. - - - - - CHAPTER II - THE AIRPLANE CONSIDERED AS A CAMERA PLATFORM - - -An essential part of the equipment of either the aerial photographer or -the designer of aerial photographic apparatus is a working knowledge of -the principles and construction of the airplane, and considerable actual -experience in the air. Conditions and requirements in the flying plane -are far different from those of the shop bench or photographic studio. -As a preliminary to undertaking any work on airplane instruments a good -text-book on the principles of flight should be studied. Such general -ideas as are necessary for understanding the purely photographic -problems are, however, outlined in the next paragraphs. - -[Illustration: - - FIG. 1.—The elements of the plane.] -=Construction of the Airplane.=—The modern airplane (Fig. 1) consists of -one or more _planes_, much longer across than in the direction of flight -(_aspect ratio_). These are inclined slightly upward toward the -direction of travel, and their rapid motion through the air, due to the -pull of the _propeller_ driven by the _motor_, causes them to rise from -the earth, carrying the _fuselage_ or body of the airplane. In the -fuselage are carried the pilot, observer, and any other load. Wheels -below the fuselage forming the _under-carriage_ or _landing gear_ serve -to support the body when running along the ground in taking off or -landing. The pilot, sitting in one of the _cockpits_, has in front of -him the _controls_, by means of which the motion of the plane is guided -(Figs. 2 and 3). These consist of the engine controls—the _contacts_ for -the ignition, the _throttle_, the oil and gasoline supply, air pressure, -etc., and the steering controls—the _rudder bar_, the _stick_ and the -_stabilizer control_. The rudder bar, operated by the feet, controls -both the _rudder_ of the plane, which turns the plane to right or left -in the air, and the _tail skid_, for steering on the ground. The _stick_ -is a vertical column in front of the pilot which, when pushed forward or -back, depresses or raises the _elevator_ and makes the machine dive or -climb. Thrown to either side it operates the _ailerons_ or wing tips, -which cause the plane to roll about its fore and aft axis. The -stabilizer control is usually a wheel at the side of the cockpit, whose -turning varies the angle of incidence of the small stabilizing plane in -front of the elevator, to correct the balance of the plane under -different conditions of loading. - -[Illustration: - - FIG. 2.—Forward cockpit of DeHaviland 4, showing instrument board.] - -[Illustration: - - FIG. 3.—Rear cockpit of DeHaviland 4, showing rear “stick” and rudder - bar.] - -The _fuselage_ consists usually of a light hollow framework of spruce or -ash, divided into a series of bays or compartments by upright members, -connecting the _longerons_, which are the four corner members, running -fore and aft, of the plane. Diagonally across the sides and faces of -these bays are stretched taut piano wires, and the whole structure is -covered with canvas or linen fabric. Cross-wires and fabric are omitted -from the top of one or more bays to permit their being used as cockpits -for pilot and observer. In later designs of planes the wire and fabric -construction has been superseded by ply-wood veneer, thereby -strengthening the fuselage so that many of the diagonal bracing wires on -the inside are dispensed with. This greatly increases the accessibility -of the spaces in which cameras and other apparatus must be carried. - -[Illustration: - - FIG. 4.—Biplane in flight.] - -The fuselage differs greatly in cross-section shape and in roominess -according to the type of engine. In the majority of English and American -planes, with their vertical cylinder or V type engines, the fuselage is -narrow and rectangular in cross-section. In many French planes, radial -or rotary engines are used and the fuselage is correspondingly almost -circular, and so is much more spacious than the English and American -planes of similar power. The shape and size of the plane body has an -important bearing on the question of camera installation. - -[Illustration: - - FIG. 5.—A single-seater.] - -=Types of Planes.=—The most common type of plane is the _biplane_ (Fig. -4), with its two planes, connected by struts and wires, set not directly -over each other, but _staggered_, usually with the upper plane leading. -_Monoplanes_ were in favor in the early days of aviation, and -_triplanes_ have been used to some extent. According to the position of -the propeller planes are classified as _tractors_ or _pushers_, tractors -being at present the more common form. Planes are further classified as -_single-seaters_ (Fig. 5), _two-seaters_, and _three-seaters_. These -motor and passenger methods of classification are now proving inadequate -with the rapid development of planes carrying two, three, and even more -motors, divided between pusher and tractor operation, and carrying -increasingly large numbers of passengers. Aside from structure, planes -may be further classified according to their uses, as _scout_, _combat_, -_reconnaissance_, _bombing_, etc. Planes equipped with floats or -pontoons for alighting on the water are called _seaplanes_ (Fig. 182), -and those in which the fuselage is boat-shaped, to permit of floating -directly on the water, are _flying boats_ (Fig. 183). - -=The Plane in the Air.=—The first flight of the photographic observer or -of the instrument expert who is to work upon airplane instruments is -very profitably made as a “joy ride,” to familiarize him with conditions -in the air. His experience will be somewhat as follows: - -The plane is brought out of the hangar, carefully gone over by the -mechanics, and the engine “warmed up.” The pilot minutely inspects all -parts of the “ship,” then climbs up into the front cockpit. He wears -helmet and goggles, and if the weather is cold or if he expects to fly -high, a heavy wool-lined coat or suit, with thick gloves and moccasins, -or an electrically heated suit. The passenger, likewise attired, climbs -into the rear cockpit and straps himself into the seat. He finds himself -sitting rather low down, with the sides of the cockpit nearly on a level -with his eyes. To either side of his knees and feet are taut wires -leading from the controls to the elevator, stabilizer, tail skid and -rudder. If the machine is dual control, the stick is between his knees, -the rudder bar before his feet. None of these must he let his body -touch, so in the ordinary two-seater his quarters are badly cramped. - -At the word “contact” the mechanics swing the propeller, and, sometimes -only after several trials, the motor starts, with a roar and a rush of -wind in the passenger's face. After a moment's slow running it is -speeded up, the intermittent roar becomes a continuous note, the plane -shakes and strains, while the mechanics hold down the tail to prevent a -premature take-off. When the engine is properly warmed up it is -throttled to a low speed, the chocks under the wheels are removed, the -mechanics hold one end of the lower wing so that the plane swings around -toward the field. It then “taxis” out to a favorable position facing -into the wind with a clear stretch of field before it. After a careful -look around to see that no other planes are landing, taking off, or in -the air near by, the pilot opens out the engine, the roar increases its -pitch, the plane moves slowly along the ground, gathers speed and rises -smoothly into the air. Near the ground the air is apt to be “bumpy,” the -plane may drop or rise abruptly, or tilt to either side. The pilot -instantly corrects these deviations, and the plane continues to climb -until steadier air is reached. - -At first the passenger's chief impressions are apt to be the deafening -noise of the motor, the heavy vibration, the terrific wind in his face. -If he raises his hand above the edge of the cockpit he realizes the -magnitude of wind resistance at the speed of the plane, and hence the -importance of the _stream-line section_ of all struts and projecting -parts. - -When he reaches the desired altitude the pilot levels off the plane and -ceases to climb. Now his task is to maintain the plane on an even keel -by means of the controls, correcting as soon as he notes it, any -tendency to “pitch,” to “roll” or to “yaw” off the course. The resultant -path is one which approximates to level straight flying to a degree -conditioned by the steadiness of the air and the skill of the pilot. If -he is not skilful or quick in his reactions he may keep the plane on its -level course only by alternately climbing and gliding, by flying with -first one wing down and then the other, by pointing to the right and -then to the left. The skilled photographic pilot will hold a plane level -in both directions to within a few degrees, but he will do this easily -only if the plane is properly balanced. If the load on the plane is such -as to move the center of gravity too far forward with respect to the -_center of lift_ the plane will be _nose-heavy_, if the load is too far -back it will be _tail-heavy_. Either of these conditions can be -corrected, at some cost in efficiency, by changing the inclination of -the stabilizer. When the plane reaches high altitudes in rare air, where -it can go no further, it is said to have reached its _ceiling_. It here -travels level only by pointing its wings upward in the climbing -position, so that the fuselage is no longer parallel to the direction of -flight. An understanding of these peculiarities of the plane in flight -is of prime importance in photographic map making, where the camera -should be accurately vertical at all times. - -The direction and velocity of the wind must be carefully considered by -the pilot in making any predetermined course or objective. The progress -of the plane due to the pull of the propeller is primarily with -reference to the air. If this is in motion the plane's _ground speed_ -and direction will be altered accordingly. In flying with or against the -wind the ground speed is the sum or difference, respectively, of the -plane's _air speed_ (determined by an air speed indicator) and the speed -of the wind. If the predetermined course lies more or less across the -wind the plane must be pointed into the wind, in which case its travel, -with respect to the earth, is not in the line of its fore and aft axis. -The effect of “crabbing,” as it is called, on photographic calculations -is discussed later (Figs. 136 and 138). - -When the plane has reached the end of its straight course and starts to -turn, its level position is for the moment entirely given up in the -operation of _banking_ (Fig. 6). Just as the tracks on the curve of a -railroad are raised on the outer side to oppose the tendency of the -train to slip outward, so the plane must be tilted, by means of the -ailerons, toward the inside of the turn. A point to be clearly kept in -mind about a bank is that if correctly made a plumb line inside the -fuselage will continue to hang vertical _with respect to the floor of -the plane, and not with respect to the earth_, for the force acting on -it is the combination of gravity and the acceleration outward due to the -turn. Only some form of gyroscopically controlled pointer, keeping its -direction in space, will indicate the inclination of the plane with -respect to the true vertical. If the banking is insufficient the plane -will _side slip_ outward or _skid_; if too great, it will side slip -inward. - -[Illustration: - - FIG. 6.—Banking.] - -As part of the “joy ride” the pilot may do a few “stunts,” such as a -“stall,” a “loop,” a “tail spin,” or an “Immelman.” From the -photographic standpoint these are of interest in so far as they bear on -the question of holding the camera in place in the plane. The thing to -be noted here is that (particularly in the loop), if these maneuvers are -properly performed, there is little tendency toward relative motion -between plane and apparatus. In a perfect loop it would, for instance, -be unnecessary, due to the centrifugal force outward, for the observer -to strap himself in. It is, however, =un=wise to place implicit -confidence in the perfection of the pilot's aerial gymnastics. No -apparatus should be left entirely free, although, for the reason given, -comparatively light fastenings are usually sufficient. - -When nearing the landing field the pilot will throttle down the engine -and commence to glide. If he is at a considerable altitude he may come -down a large part of the distance in a rapid spiral. As the earth is -approached the air pressure increases rapidly, and the passenger, if -correctly instructed, will open his mouth and swallow frequently to -equalize the air pressure on his ear drums. Just before the ground is -reached the plane is leveled off, it loses speed, and, if the landing is -perfect, touches and runs along the ground without bouncing or bumping. -Frequently, however, the impact of the tail is sufficiently hard to -cause it to bump badly, with a consequent considerable danger to -apparatus of any weight or delicacy. This is especially apt to occur in -hastily chosen and poorly leveled fields such as must often be utilized -in war or in cross-country flying. - -=Appearance of the Earth from the Plane.=—The view from the ordinary -two-seater is greatly restricted by the engine in front and by the -planes to either side and below (Figs. 7, 8, and 9). By craning his neck -over the side, or by looking down through an opening in the floor, the -passenger has an opportunity to learn the general appearance of the -subject he is later to devote his attention to photographing. Perhaps -the most striking impression he receives will be that of the _flatness_ -of the earth, both in the sense of absence of relief and in the sense of -absence of extremes of light and shade. The absence of relief is due to -the fact that at ordinary flying heights the elevations of natural -objects are too small for the natural separation of the eyes to give any -stereoscopic effect. The absence of extremes of light and shade is in -part due to the fact that the natural surfaces of earth, grass and -forest present no great range of brightness; in part to the small -relative areas of the parts in shadow; in considerable part to the layer -of atmospheric haze which lies as an illuminated veil between the -observer and the earth at altitudes of 2000 meters and over (Figs. 10 -and 11). Due to the combination of these factors the earth below -presents the appearance of a delicate pastel. - -As the gaze is directed away from the territory directly below, the -thickness of atmosphere to be pierced rapidly increases, until toward -the horizon (which lies level with the observer here as on the ground) -all detail is apt to be obliterated to such an extent that only on very -clear days can the horizon itself be definitely found or be -distinguished from low lying haze or clouds (Fig. 4). - -[Illustration: - - FIG. 7.—The view ahead.] - -[Illustration: - - FIG. 8.—The view astern.] - -=Airplane Instruments.=—Mounted on boards in front of the pilot and -observer are various instruments to indicate the performance of engine -and plane (Fig. 2). Those of interest to the photographic observer are -the _compass_, the _altimeter_, the _air speed indicator_, the -_inclinometers_. - -[Illustration: - - FIG. 9.—The view between the wings.] - -[Illustration: - - FIG. 10.—Appearance of the earth from a low altitude—3000 feet or - less.] - -The _compass_ is usually a special airplane compass, with its “card” -immersed in a damping liquid. Like most of the direction indicating -instruments on a plane its indications are only of significance when the -plane is pursuing a steady course. On turns or rapid changes of -direction of any sort perturbations prevent accurate reading. - -The _altimeter_ is of the common aneroid barometer type. On American -instruments it is usually graduated to read in 100-foot steps. While -somewhat sluggish, it is quite satisfactory for all ordinary -determinations of altitude in photographic work. Were primary map making -to be undertaken, where the scale was determinable only from the -altitude and focal length of the lens, the ordinary altimeter is hardly -accurate enough. - -[Illustration: - - FIG. 11.—Appearance of the earth from a high altitude—10,000 feet or - more.] - -The _air speed indicator_ consists of a combination of Venturi and Pitot -tubes, producing a difference of pressure when in motion through the air -which is measured on a scale calibrated in air speed. This instrument is -important for determining, in combination with wind speed, the _ground -speed_ of the plane, on the basis of which is calculated the interval -between exposures to secure overlapping photographs. Its accuracy is -well above that necessary for the purpose. - -_Inclinometers_ for showing the lateral and fore and aft angle of the -plane with the horizontal, are occasionally used, and have also been -incorporated in cameras. The important point to remember about these -instruments is that they are controlled not alone by gravity but as well -by the acceleration of the plane in any direction. They consequently -indicate correctly only when the plane is flying straight. On a bank the -lateral indicator continues to indicate “vertical” if the bank is -properly calculated for the turn. - - - - - II - THE AIRPLANE CAMERA - - - - - CHAPTER III - THE CAMERA—GENERAL CONSIDERATIONS - - -=Chief Uses of an Airplane Camera.=—The kinds of camera suitable for -airplane use and the manner in which they must differ from cameras for -use on the ground are determined by consideration of the nature of the -work they must do. Four kinds of pictures constitute the ordinary -demands upon the aerial photographer. These are single objectives or -_pin points_, _mosaic maps_ of strips of territory or large areas, -_oblique views_, and _stereoscopic views_. Each of these presents its -own peculiar problems influencing camera design. - -_Pinpoints_ consist of such objects as gun emplacements, railway -stations, ammunition dumps, and other objects of which photographs of -considerable magnification are desired for study. Here the instrumental -requirements are _sufficient focal length of lens_ to secure an image of -adequate size; _means for pointing_ the camera accurately; enough -_shutter speed_ to counterbalance the speed of the plane; sufficiently -_wide lens aperture_ to give adequate exposure with the shutter speed -required; _means of supporting_ the camera to protect it from the -vibration of the plane. - -_Mosaic maps_ are built up from a large number of photographs of -adjacent areas. In addition to the above requirements, mosaic maps -demand _lenses free from distortion_ and covering as large a plate as -possible, in order to keep to a minimum the number of pictures needed to -cover a given area; means for keeping the camera _accurately vertical_, -and means for _changing the plates or films_ and resetting the shutter -rapidly enough to avoid gaps between successive pictures. At low -altitudes and high ground speeds the interval between exposures becomes -a matter of only a few seconds. - -_Oblique views_ are made at angles of from 12 to 35 degrees from the -horizontal, usually from comparatively low altitudes. They have been -found to be particularly suitable for the use of men who have no -training in photographic interpretation, being more like the pictures -with which the men are familiar. Distributed among the infantry before -an attack, they have proved indispensable aids to the proper knowledge -of the ground to be covered. The additional requirement here is for -_high shutter speed_ to eliminate the effect of the relatively very -rapid movement of the foreground. - -_Stereoscopic views_ are among the most useful of all airplane pictures. -They are made from successive exposures, the separation of the points of -view being obtained not by two lenses at the distance of the eyes apart, -but by the motion of the plane. For this purpose the views should -overlap by at least 60 per cent; this, therefore, requires a very _short -interval between exposures_. For stereo-oblique views this may mean that -they are taken at intervals as short as one or two seconds. - -=Chief Differences between Ground and Air Cameras.=—Certain definite -differences are thus seen to stand out between airplane cameras and the -ordinary kind. It is essential that the apparatus for use in the air -shall have high lens and shutter speed, means for rapid changing of -plates, and anti-vibration suspension. Without these features a camera -is of little use for aerial work. These requirements lead inevitably to -greater complexity of design. One simplification over ground cameras, -however, is brought about by the fact that all exposures are made on -objects beyond the practical infinity point of the lens; consequently, -all cameras are fixed focus. This fixed focus feature is a positive -advantage in construction, since it permits of the simple rigid box -form, desirable and necessary to withstand the strains due to the weight -of the lens and the stresses from the plane. But with the abandonment of -all provision for focussing in the air must go special care that the -material used in constructing the camera body is as little subject as -possible to expansion and contraction with temperature, since there is -often a drop of 30 to 40 degrees Centigrade from ground to upper air. -The effect of change of temperature on focus will be treated in the -discussion of lenses. - -In addition to these differences, we must keep in mind certain -requirements which are conditioned by the nature and place of aerial -navigation. Thus all mechanical devices which will fail to function at -the low temperatures and pressures met at high altitudes are entirely -unsuitable. Experience has shown, too, that we must avoid all mechanism -depending primarily on _springs_ and on the action of _gravity_. -Vibration, and the motion of the plane in all three dimensions, conspire -to render mechanical motions unreliable when actuated by these agencies. -All plate changing, shutter setting, and exposing operations should be -as nearly as possible positively controlled motions. Because of the cold -of the upper air all knobs, levers and catches must be made extra large -and easy to handle with heavy gloves. Circular knurled heads to such -parts as shutter setting movements are to be avoided in favor of -bat-wing keys or levers. Grooves for the reception of magazines must be -as large and smooth as possible, and guides to facilitate the magazines' -introduction should be provided (Fig. 50). No releases or adjustments -which depend upon hearing or upon a delicate sense of touch are feasible -in airplane apparatus. Wherever possible, large _visible_ indicators of -the stage of the cycle of operations should be provided. Loose parts are -to be shunned, as they are invariably lost in service. Complete -operating instructions should be placed on the apparatus wherever -possible, to minimize the confusion due to changing and uninstructed -personnel. - -=The Elements of the Airplane Camera.=—Disregarding its means of -suspension, the airplane camera proper consists essentially of _lens_, -_camera body_, _shutter_, and _plate or film holding and changing box_. - -In certain of the aerial cameras developed early in the war all of these -elements were built together in a common enclosure. Later it was -generally recognized that a unit system of interchangeable parts is -preferable. In the case of the lens there arose various requirements for -focal length, from 25 to 120 centimeters, according to the work to be -done. Rather than use an entirely different camera for each different -kind of work, it is better to have lenses of various focal lengths, -mounted in tubes or cones, all built to attach to the same camera body. -In the case of the shutter it is desirable to be able to repair or -calibrate periodically. By making the shutter a removable unit, the -provision of a few spares does away with the need for putting the whole -camera out of commission. Similar considerations hold with reference to -other parts. - -A further material advantage that comes from making airplane cameras in -sections is the greater ease with which they are inserted in the plane, -usually through the openings between diagonal cross-wires. It is in fact -only by virtue of this possibility of breaking up into small elements -that some of the larger cameras could be inserted in the common types of -reconnaissance plane. Illustrations of the building up of cameras from -separate removable elements are given in the detailed discussion of the -individual types. - -=Types of Airplane Cameras.=—During the course of the war airplane -cameras have been classified on various bases, in different services. In -the French service, where the deMaria type of camera was standardized -early in the war, the usual classification was based on focal length; -thus the standard cameras were spoken of as the 26, the 50 and the 120 -(centimeter). A further distinction was then made according to the size -of plate, this being originally 13 × 18 centimeters for the 26 -centimeter, and 18 × 24 centimeters for the larger cameras. In the -English service the 4 × 5 inch plate was used almost exclusively, and -their various types of cameras were known by serial letters—C, E, L, -etc. Both these modes of classification became inadequate with the -ultimate agreement to standardize on the 18 × 24 centimeter size for all -plates, and to carry lenses of all focal lengths in interchangeable -elements. - -For purposes of description and discussion, it is most convenient to -classify cameras according to their method of operation and the -sensitive material employed. On this basis we may distinguish among -cameras using _plates_ three kinds—_non-automatic cameras_, -_semi-automatic cameras_, and _automatic cameras_. We may similarly -discuss _film cameras_, but having treated the plate cameras -comprehensively, it will be found that the discussion of all types of -film camera can be handled most conveniently by studying the differences -in construction and operation introduced by the characteristics of film -as compared to plates. - - - - - CHAPTER IV - LENSES FOR AERIAL PHOTOGRAPHY - - -=General Considerations.=—The design and selection of lenses for aerial -photography present on the whole no problems not already encountered in -photography of the more familiar sort. Indeed, the lens problem in the -airplane camera is in some particulars more simple than in the ground -camera. For instance, there is no demand for depth of focus—all objects -photographed are well beyond the usually assumed “infinity focus” of -2000 times the lens diameter. Such strictly scientific problems of -design as pertain to aerial photographic lenses are ones of degree -rather than of kind. Larger aperture, greater covering power, smaller -distortion, more exquisite definition—these always will be in demand, -and each progressive improvement will be reflected in advances in the -art of aerial photography. But many lens designs perfected before the -war were admirably suited, without any change at all, for aerial -cameras. - -Of the utmost seriousness, however, with the Allies, was the problem of -securing lenses of the desired types in sufficient numbers. The -manufacture of the many varieties of optical glass essential to modern -photographic lenses was almost exclusively a German industry, which had -to be learned and inaugurated in Allied countries since 1914. In -consequence of this entirely practical problem of quantity production -without the glasses for which lens formulæ were at hand, some new lens -designs were produced. Whether any of these possess merits which will -lead them to be preferred over pre-war designs, when the latter can -again be manufactured, remains to be seen. - -While the glass problem was still unsolved, aerial cameras had to be -equipped with whatever lenses could be secured by requisition from -pre-war importation and manufacture, and later, with lenses designed to -utilize those glasses whose manufacture had been mastered in the allied -countries. It is important that the historical aspect of this matter be -well understood by the student of aerial photographic methods, for the -use of these odd-lot lenses reacted on the whole design of aerial -cameras and on the methods of aerial photography, particularly in -England and the United States. Almost without exception the available -lenses were of short focus, considered from the aerial photographic -standpoint; that is, they lay between eight and twelve inches. This set -a limit to the size of the airplane camera, quite irrespective of the -demands made by the nature of the photographic problem. Lenses of these -focal lengths produced images which, for the usual heights of flying, -were generally considered too small, and which were, therefore, almost -always subsequently enlarged. Such was the English practice, which was -followed in the training of aerial photographers in America, where -exactly similar conditions held at the start with respect to available -lenses. French glass and lens manufacturers did succeed in supplying -lenses of longer focus (50 centimeters), in numbers sufficient for their -own service, although never with any certainty for their allies. The -French, therefore, almost from the start, built their cameras with -lenses of long focus, and made contact prints from their negatives. - -Practices adopted under pressure of an emergency to meet temporary -practical limitations often come to dominate the whole situation. This -is particularly true of aerial photography in the British and American -services. The small apparatus built around the stop-gap short focus -lenses fixed the plane designer's idea of an airplane camera, and the -space it should occupy. This was directly reflected in the designs of -the English planes, and the American planes copied after them. Meanwhile -the American photographic service in France associated itself with the -French service, adopting its methods and apparatus, and using French -planes whose designs were not being followed in American construction. -The task of harmonizing the photographic practice as taught in America, -following English lines, with French practice as followed in the theater -of war, and of adapting planes built on English designs so that they -could carry French apparatus, was a formidable one, not likely to be -soon forgotten by any who had a part in it. - -=Photographic Lens Characteristics.=—Whole volumes have been written on -the photographic lens, and on the optical science utilized and indeed -brought into being by its problems. Such works should be consulted by -those who intend to make a serious study of the design of lenses for -aerial use. No more can be attempted, no more indeed is relevant here, -than an outline review of the chief characteristics and errors of -photographic lenses, considering them with special reference to aerial -needs. - -The modern photographic lens is, broadly speaking, a development of the -simple convex or converging lens. Its function is the same: to form a -real image of objects placed before it. But the difference in -performance between the simple lens and the modern photographic -objective is enormous. The simple lens forms a clear image only close to -its axis, for light of a single color, and as long as its aperture is -kept quite small as compared to the distance at which the image is -formed. The photographic lens, on the other hand, is called upon to -produce a clear image with light of a wide range of spectral -composition, sharply defined over a flat surface of large area, and it -must do this with an aperture that is large in comparison with the focal -length, whereby the amount of light falling on the image surface shall -be a maximum. This ideal is approximated to a really extraordinary -degree by the scientific combination and arrangement of lens elements -made from special kinds of glass in the best photographic lenses of the -anastigmat type. The result is of necessity a set of compromises, -whereby the outstanding errors are reduced to a size judged permissible -in view of the work the lens is to do. These errors or _aberrations_ are -briefly reviewed below, in order that the reader may readily grasp the -terms in which the performance and tolerances in aerial lenses are -described. - -[Illustration: - - FIG. 12.—Diagrammatic representation of spherical aberration.] - -_Spherical Aberration and Coma._—Suppose we focus on a screen, by means -of a simple convex lens the image of a distant point of light. Suppose -for simplicity that this image is located on the axis of the lens and -that light of only one color is used, such as yellow. It will be found -that the smallest image that can be obtained is not a point, but a small -disc. This is due to the fact that the rays of light passing through the -outer portions of the lens are bent more than those passing through the -lens in the region near the center. This effect is shown in Fig. 12 by -the usual mode of representing it graphically. Here the figures 1, 2, 3, -4, represent distances from the axis of the lens, and the letters A_{1}, -A_{2}, A_{3}, A_{4}, the points of convergence of the rays from 1, 2, 3, -4, etc. These distances projected upward on to the produced lens points -form a curve which shows at a glance the extent and direction of the -error due to each part of the lens. This information is of value where -the lens is fitted with an adjustable diafram. With some types of -correction sharper definition may be obtained by reducing the aperture. -With others, however, diaframing impairs definition, by destroying the -balance between under and over correction which averages to make a good -image. In aerial lenses it is not customary to use diaframs, as all the -light possible is desired. Consequently the reduction of spherical -aberration must be accomplished by proper choice of lens elements and -their arrangement. - -Off the axis of the lens the image of a point source takes on an -irregular shape, due to oblique spherical aberration or _coma_. - -_Chromatic Aberration._—Because of the inherent properties of the glass -of which it is made, a simple collective lens does not behave in the -same way with respect to light of different colors. If one attempts, -with such a lens, to focus upon a screen the image of a distant white -light, it will be found that the blue rays will not focus at the same -point as the red rays, but will come together nearer the lens. Modern -photographic lenses are compounded of two or more kinds of glass in such -a way as to largely eliminate this defect, the presence of which is -detrimental to good definition. Such lenses are called achromatic, and -the property of a lens by virtue of which this defect is eliminated is -called its _chromatic correction_. - -Chromatic correction is never perfect, but two colors of the spectrum -can be brought to a focus in the same plane, and to a certain extent the -departure of other colors from this plane can be controlled. Off the -axis of the lens outstanding chromatic aberration results in a -difference in the size of images of different colors, known as _lateral -chromatism_. - -Like spherical aberration, chromatic aberration is a contributing factor -to the size of the image of a point source, which determines the -defining power of a lens. It is, however, an error whose effect is to -some extent dependent on the kind of sensitive plate used. Two lenses -may give images of the same size (in so far as it is governed by -chromatic aberration), if a plate of narrow spectral sensitiveness is -used, while giving images of different size on panchromatic plates of -more extended color sensibility. The choice of the region of the -spectrum for which chromatic correction is to be made is thus governed -by the color of the photographically effective light. While in ordinary -photography the blue of the spectrum is most important, in aerial work -where color filters are habitually used with isochromatic plates the -green is most important, and color correction centered about this region -constitutes a real difference of design peculiar to aerial lenses. -Similarly the general use of deep orange or red filters with red -sensitive plates, for heavy mist penetration, would call for a shift of -correction to that part of the spectrum. - -_Astigmatism and Covering Power._—Suppose the lens forms at some point -off its axis an image of a cross. Suppose one of the elements of the -cross to be on a radius from the center of the field, the other element -parallel to a tangent. The rays forming the images of these two elements -of the cross are subject to somewhat different treatment in their -passage through the lens. The curvature of the lens surfaces is on the -whole greater with respect to the rays from the radial element than to -those from the tangential element. They are therefore refracted more -strongly and come to a focus nearer the lens. The arms of the cross are -consequently not all in focus at once. This error, termed _astigmatism_, -is rather well shown in Fig. 15, where the images of the outlying -concentric circles are sharp in the radial, but blurred in the -tangential direction. - -Astigmatism can be largely compensated for, and its character -controlled. The most usual correction brings the two images in focus -together both at the axis, and on a circle at some distance out. This -second locus of coincidence may or may not be in the same plane as the -first, depending on which disposition produces the best average -correction. The mean between the two foci determines the focal plane of -the lens, which is in general somewhat curved. The _covering power_ of a -lens is given by the size of the field which is sufficiently flat and -free from astigmatism for the purpose for which the lens is used. This -is largely determined by the astigmatism, but the other aberrations are -also important. - -_Illumination._—The amount of light concentrated by the lens on each -elementary area of the image determines its brightness or illumination. -The ideal image would, of course, be equally bright over its whole area -of good definition, and for lenses of narrow angle this is approximately -true. But when it is desired to cover a wide angle the question of -illumination becomes serious. The relationship between angle from the -axis and illumination is that illumination is proportional to the fourth -power of the cosine of the angle. This relationship is shown in the -following table: - - Angle Image brightness - 0° 100 per cent. - 10° 94.1 per cent. - 20° 78.0 per cent. - 30° 56.2 per cent. - 40° 34.4 per cent. - 50° 17.1 per cent. - -If the field of view is 60°, which corresponds to an 18 × 24 centimeter -plate with a lens of 25 centimeter focus, the brightness is only 56 per -cent., and the necessary exposure at the edge approximately 1.8 times -that at the center. This effect is shown in Fig. 15. It is very -noticeable if the exposure is so short as to place the outlying areas in -the under-exposure period. - -[Illustration: - - FIG. 13.—Barrel and pin-cushion distortion.] - -_Distortion._—Sometimes a lens is relatively free from all the -aberrations, mentioned above, so that it gives sharp, clear images on -the plate, yet these images may not be exactly similar to the objects -themselves as regards their geometrical proportions; in other words, the -image will show distortion. Lens distortion assumes two typical forms, -illustrated in Fig. 13, which shows the result of photographing a square -net-work with lenses suffering in the one case from “barrel” distortion -and in the other from “pin-cushion” distortion. In the first the corners -are drawn in relative to the sides; in the latter case the sides are -drawn in with respect to the corners. Either sort is a serious matter in -precision photography, such as aerial photographic mapping aspires to -become. It must be reduced to a minimum and its amount must be -accurately known if negatives are to be measured for the precise -location of photographed objects. In general symmetrical lenses give -less distortion than the unsymmetrical (Fig. 14). - -[Illustration: - - FIG. 14.—Arrangement of elements in two lenses suitable for aerial - work: _a_, Zeiss Tessar; two simple and one cemented components - (unsymmetrical); _b_, Hawkeye Aerial; two positive elements of heavy - barium crown, two negative of barium flint, uncemented - (symmetrical).] - -=Lens Testing and Tolerances for Aerial Work.=—Simple and rapid -comparative tests of lenses may be made by photographing a _test chart_, -consisting of a large flat surface on which are drawn various -combinations of geometrical figures—lines, squares, circles, -etc.—calculated to show up any failures of defining power. For testing -aerial lenses the chart should be as large as possible, so that it may -be photographed at a distance great enough for the performance of the -lens to be truly representative of its behavior on an object at infinite -distance. This means in practice a chart of 4 or 5 meters side, to be -photographed at a distance 20 to 30 times the focal length of the lens. - -[Illustration: - - FIG. 15.—Photograph of a lens testing chart, showing failure in - defining power outside area for which the lens is calculated.] - -A typical photograph of such a chart is shown in Fig. 15. It reveals at -a glance the more conspicuous lens errors. At the sides and corners the -concentric circles show the lens's astigmatism, by the clear definition -of the lines radial to the center of the field and their blurring in the -tangential direction. The falling off in illumination with increasing -distance from the center is also exhibited; and the blurring of all -detail outside the rectangle for which the lens was calculated shows -that spherical, chromatic, and other aberrations have become -prohibitively large. - -But the only complete test of a lens is the quantitative measurement of -errors made on an optical bench. A point source of light, which may at -will be made of any color of the spectrum, is used as the object and its -image formed by the lens in a position where it can be accurately -measured for location, size, and shape by a microscope. A chart giving -the results of such a test is shown in Fig. 16. In the upper left-hand -corner is shown the position of the focus for the different colors of -the spectrum. Below this is recorded the lateral chromatism at 21 -degrees, in terms of the difference in focus for a red and a blue ray. -Below this again comes the distortion, or shift of the image from its -proper position, for various angles (plotted at the extreme right) from -the lens axis. To the right of this is the image size, at each angle, -and finally, to the right of the diagram, are plotted the distances of -the two astigmatic foci from the focal plane, together with the mean of -the two foci, which practically determines the shape of the field. - -An important point to notice is that these data are uniformly plotted in -terms of a lens of 100 millimeters focal length irrespective of the -actual focal length of the lens measured. Thus this particular chart is -for a 50 centimeter lens but would be plotted on the same scale for a 25 -or a 100 centimeter lens. Underlying this practice is the assumption -that all the characteristics of lenses of the same design and aperture -are directly proportional to their focal length. If this were so, then a -50 centimeter lens would give double the size of image that a 25 -centimeter does, and so on. As a matter of fact, test shows that the -size of the image does not increase so rapidly as the focal length; so -that while the image size for a 25 centimeter lens would be, say, .05 -millimeters per 100 millimeters focal length, it will be only .03 or .04 -millimeters per 100 millimeters focal length for a 50 centimeter lens. -The actual size of a point image will therefore be greater, though not -proportionately greater. - -[Illustration: - - FIG. 16.—Chart recording measurements of lens characteristics.] - -The chart presents tests on a good quality lens, and so gives a good -idea of the permissible magnitude of the various errors. In many ways -the most important figure is that for image size, including as it does -the result of all the aberrations. In the example given, this varies -from .075 to .15 mm. actual size. For the same type of lens of 25 -centimeters focus this range will be from .05 to .10 mm. Since these are -commonly used focal lengths, a good average figure for image size, -commonly used in aerial photographic calculations, is ⅒ mm. In regard to -astigmatic tolerances, the two astigmatic foci should not be separated -at any point by more than 6 to 7 millimeters, and the mean of these -should not deviate from the true flat field by more than ½ millimeter, -in each case the figures being based on the conventional 100 millimeters -focal length. Distortion should not be over .08 millimeter at 18° or .20 -millimeter at 24° from the axis (per 100 millimeters focal length). - -=Lens Aperture.=—In the simple lens the aperture is merely the diameter. -In compound lenses the aperture is not the linear opening but the -effective opening of an internal diafram. Photographically, however, -aperture has come to have a more extensive meaning. While in the -telescope the actual diameter of an objective is perhaps the most -important figure, and in the microscope the focal length, in photography -the really important feature is the amount of light or illumination. -This is determined by lens opening and focal length together; -specifically, by the ratio of the lens area to the focal length. The -common system of representing photographic lens aperture is by the ratio -of focal length to lens diameter, the lens being assumed to be circular. -Thus F/5 (often written F.5) indicates that the diameter is one-fifth -the focal length. - -Two points are to be constantly borne in mind in connection with this -system of representation. First, all lenses of the same aperture (as so -represented) give the same illumination of the plate (except for -differences due to loss of light by absorption and reflection in the -lens system). This follows simply from the fact that the illumination of -the plate is directly proportional to the square of the lens diameter, -and inversely as the square of the focal length. Secondly, the -illumination of the plate is inversely as the square of the numerical -part of the expression for aperture. That is, lenses of aperture F/4.5 -and F/6 give images of relative brightness (6/4.5)^2 = 1.78. - -What lens aperture, and therefore what image brightness, is feasible, is -determined chiefly by the angular field that must be covered with any -given excellence of definition. The largest aperture ordinarily used for -work requiring good definition and flat field free from distortion is -F/4.5. Anastigmatic lenses of this aperture cover an angle of 16° to 18° -from the axis satisfactorily, which corresponds to an 18 × 24 centimeter -plate with a lens of 50 centimeters focus. Lenses with aperture as large -as F/3.5 were used to some extent in German hand cameras of 25 -centimeters focal length, with plates of 9 × 12 centimeters. English and -American lenses of this latter focal length were commonly of aperture -F/4.5, designed to cover a 4 × 5 inch plate. - -As a general rule the greater the focal length the smaller the -aperture—a relationship primarily due to the difficulty of securing -optical glass in large pieces. Thus while 50 centimeter lenses of -aperture F/4.5 are reasonably easy to manufacture, the practicable -aperture for quantity production is F/6, and for 120 centimeter lenses, -F/10. This means that a very great sacrifice of illumination must be -faced to secure these greater focal lengths. As is to be expected from -the state of the optical glass industry, the German lenses were of -generally larger aperture for the same focal lengths than were those of -the Allies. Besides the F/3.5 lenses already mentioned, their 50 -centimeter lenses were commonly of aperture F/4.8, their 120 centimeter -lenses of aperture F/7, or of about double the illuminating power of the -French lenses of the same size. - -Demands for large covering power also result in smaller aperture. The 26 -centimeter lenses used on French hand cameras utilizing 13 × 18 -centimeter plates were commonly of aperture F/6 or F/5.6. The lens of -largest covering power decided on for use in the American service was of -12 inch focus, to be used with an 18 × 24 centimeter plate (extreme -angle 26°); the largest satisfactory aperture for this lens is F/5.6. - -Ordinarily the question of aperture is closely connected with that of -diaframs, whereby the lens aperture may be reduced at will. Diaframs -have been very little used in aerial photography. All the aperture that -can be obtained and more is needed to secure adequate photographic -action with the short exposures required under the conditions of rapid -motion and vibration peculiar to the airplane. Any excess of light, over -the minimum necessary to secure proper photographic action, is far -better offset by increase of shutter speed or by introduction of a color -filter. For this reason American aerial lenses were made without -diaframs. In the German cameras, however, adjustable diaframs are -provided (Fig. 43), controlled from the top of the camera by a rack and -pinion. In the camera most used in the Italian service an adjustable -diafram is provided, but this is occasioned by the employment of a -between-the-lens shutter of fixed speed, so that the only way exposure -can be regulated is by aperture variation, a method which has little to -recommend it. - -=The Question of Focal Length.=—In aerial photography the lens is -invariably used at fixed, infinity, focus. Under these conditions the -simple relationship holds that the size of the image is directly -proportional to the focal length and inversely proportional to the -altitude. If any chosen scale is desired for the picture the choice of -focal length is determined by the height at which it is necessary to -fly. This at least would be the case were there no limitation to the -practicable focal length—which means camera size—and were one limited to -the original size of the picture as taken; that is, were the process of -enlargement not available. But the possibility of using the enlarging -process brings in other questions: Is the defining power of a short -focus lens as good in proportion to its focal length as that of a long -focus lens? If so a perfect enlargement from a negative made by a short -focus lens would be identical with a contact print from a negative made -with a lens of longer focus. Is defining power lost in the enlarging -process with its necessary employment of a lens which has its own errors -of definition and which must be accurately focussed? - -Certain factors which enter into comparisons of this sort in other lines -of work, such as astronomical photography, play little part here. These -are, first, the optical resolving power of the lens, which is -conditioned by the phenomena of diffraction, and is directly as the -diameter; and, second, the size of the grain of the plate emulsion. The -first of these does not enter directly, because the size of a point -image on the axis of the lens, due merely to diffraction, is very much -less than that given by any photographic lens which has been calculated -to give definition over a large field, instead of the minute field of -the telescope. Yet it may contribute toward somewhat better definition -with a long focus lens because of the actually larger diameter of such -lenses. The second factor is not important, because, as will be seen -later, the resolving power of the plates suitable for aerial photography -is considerably greater than that of the lens. The emulsion grain is in -fact only a quarter or a fifth the size of the image as given by a 25 -centimeter lens, and enlargements of more than two or three times are -rarely wanted. - -A series of experiments was made for the U. S. Air Service to test out -these questions, using a number of representative lenses of all focal -lengths, both at their working apertures and at identical apertures for -all. With regard to lens defining power, as shown by the size of a point -image, the answer has already been reported in a previous section. -Lenses of long focus give a relatively smaller image than lenses of the -same design of short focus. In regard to the whole process of making a -small negative and enlarging it, the loss of definition is quite marked, -as compared to the pictures of the same scale made by contact printing -from negatives taken with longer focus lenses. - -This answer is clear-cut only for lenses calculated to give the same -angular field. Thus a 10 inch lens covering a 4 × 5 inch plate has about -the same angle as a 50 centimeter lens for an 18 × 24 centimeter plate. -When, however, it comes to the longer foci, such as 120 centimeters, the -practical limitation to plate size (18 × 24 cm.) has been passed, and -the angular field is less than half that of the 50 centimeter lens. The -120 centimeter lens need only be designed for this small angle, with -consequent greater opportunities for reduction of spherical aberration. -It is therefore an open question whether a 50 centimeter lens designed -to cover a plate of linear dimensions 50/120 times that used with the -regular 50 centimeter lens could not be produced of such quality that it -would yield enlargements equal to contacts from a 120 centimeter lens. -If so, lenses of larger aperture could be used, and a considerable -saving in space requirements effected. - -Focal lengths during the Great War were decided by the nature of the -military detail which was to be revealed and by the altitudes to which -flying was restricted in military operations. In the first three years -of the war the development of defences against aircraft forced planes to -mount steadily higher, so that the original three or four thousand feet -were pushed to 15,000, 18,000, and even higher. Lenses of long focus -were in demand, leading ultimately to the use of some of as much as 120 -centimeters (Fig. 41). In the last months of the war the resumption of -open fighting made minute recording of trench details of less weight, -while the preponderance of allied air strength permitted lower flying. -In consequence, lenses of shorter focus and wider angle came to the -fore, suitable for quick reconnaissance of the main features of new -country. At the close of the war the following focal lengths were -standard in the U. S. Air Service, and may be considered as well-suited -for military needs. Peace may develop quite different requirements. - - Focal length Aperture Plate size - 10 inch F/4.5 4 × 5 inch - 26 cm. F/6 13 × 18 cm. - 12 inch F/5.6 18 × 24 cm. - 20 inch F/6.3 to F/4.5 18 × 24 cm. - 48 inch F/10 to F/8 18 × 24 cm. - -The question of the use of _telephoto lenses_ in place of lenses of long -focus is frequently raised. Lenses of this type combine a diverging -(concave) element with the normal converging system, whereby the effect -of a long focus is secured without an equivalent lens-to-plate distance. -This reduction in “back focus” may be from a quarter to a half. Were it -possible to obtain the same definition with telephoto lenses as with -lenses of the same equivalent focus, they would indeed be eminently -suitable for aerial work because of their economy of length. But -experience thus far has shown that the performance of telephoto lenses, -as to definition and freedom from distortion, is distinctly inferior, so -that it is best to hold to the long focus lens of the ordinary type. - -=Lenses Suitable for Aerial Photography.=—Among the very large number of -modern anastigmat lenses many were found suitable for airplane cameras -and were used extensively in the war. A partial list follows: The Cooke -Aviar, The Carl Zeiss Tessar, the Goerz Dogmar, the Hawkeye Aerial, the -Bausch and Lomb Series Ic and IIb Tessars, the Aldis Triplet, the -Berthiot Olor. - -=The Question of Plate Size and Shape.=—Plate size is determined by a -number of considerations, scientific and practical. If the type of lens -is fixed by requirements as to definition, then the dimensions of the -plate are limited by the covering power. From the standpoint of economy -of flights and of ease of recognizing the locality represented in a -negative, by its inclusion of known points, lenses of as wide angle as -possible should be used. If the focus is long, this means large plates, -which are bulky and heavy. If the finest rendering of detail is not -required a smaller scale may be employed, utilizing short focus lenses -and correspondingly smaller plates. Thus a six inch focus lens on a 4 × -5 inch plate would be as good from the standpoint of angular field as a -12 inch on an 8 × 10 inch plate. This is apt to be the condition with -respect to most peace-time aerial photography, which may be expected to -free itself quickly from the huge plates and cameras of war origin. - -For work in which great freedom from distortion of any sort is -imperative, small plates will be necessary, for two reasons. One is that -the characteristic lens distortions are largely confined to the outlying -portions of the field. The other is that a wide angle of view inevitably -means that all objects of any elevation at the edge of the picture are -shown partly in face as well as in plan, which prevents satisfactory -joining of successive views (Fig. 128). In making a mosaic map of a -city, if a wide angle lens is employed with large plates, the buildings -lying along the junctions of the prints can be matched up only for one -level. If this is the ground level, as it would be to keep the scale of -the map correct, the roofs will have to be sacrificed. In extreme cases -a house at the edge of a junction may even show merely as a front and -rear, with no roof, while in any case the abrupt change at these edges -from seeing one side of all objects to seeing the opposite side is not -pleasing. - -The table in a preceding section gives the relation of plate size to -focal length found best on the whole for military needs. Deviations from -these proportions in both directions are met with. In the English -service the LB camera, which uses 4 × 5 inch plates, is equipped with -lenses of various focal lengths, up to 20 inches. The German practice, -as well as the Italian, was almost uniform use of 13 × 18 centimeter -plates for all focal lengths. Toward the end of the war, however, some -German cameras of 50 centimeter focal length were in use employing -plates 24 × 30 centimeters. - -It will be recognized that these plate sizes are chosen from those in -common use before the war. A similar observation holds with even greater -force on the question of plate _shape_. Current plate shapes have been -chosen chiefly with reference to securing pleasing or artistic effects -with the common types of pictures taken on the ground. These shapes are -not necessarily the best for aerial photography. Indeed the whole -question of plate shape should be taken up from the beginning, with -direct reference to the problems of aerial photography and photographic -apparatus. - -A few illustrations will make this clear, taking Fig. 17 as a basis. If -it is desired to do spotting (the photography of single objectives), the -best plate shape would be circular, for that shape utilizes the entire -covering area of the lens. If it is desired to make successive -overlapping pictures, either for mapping, or for the production of -stereoscopic pairs, a rectangular shape is indicated. If the process of -plate changing is difficult or slow, it is advisable, in order to give -maximum time for this operation, to have the long side of the rectangle -parallel to the line of flight (indicated by the arrow). If economy of -flights is a consideration, as in making a mosaic map of a large area, -it is advantageous to have as wide a plate as the covering power of the -lens will permit. Reference to Fig. 17 shows that this means a plate of -small dimensions in the direction of flight. If the changing of plates -or film is quick and easy, the maximum use of the lens's covering power -is made by such a rectangle whose long side approximates the dimensions -of the lens field diameter. This is in fact the choice made in the -German film mapping camera (Figs. 61 and 63), whose picture is 6 × 24 -centimeters. An objection to this from the pictorial side, lies in the -many junction lines cutting up the mosaic. Another objection, if the -plane does not hold a steady course, is the failure to make overlaps on -a turn. (Fig. 62.) Here as everywhere the problem is to decide on the -most practical compromise between all requirements. - -[Illustration: - - FIG. 17.—Possible choices of plate shape.] - -=Focussing.=—The process of focussing aerial cameras was at first deemed -a mystery, though undeservedly so. A belief was long current that -“ground” focus and “air” focus differ. In other words, that a camera -focussed upon a distant object on the ground would not be in focus for -an object the same distance below the camera when in the plane. Belief -in this mysterious difference went so far that certain instruction books -describe in detail the process of focussing a camera by trial exposures -from the air. - -Careful laboratory tests performed for the U. S. Air Service showed that -neither low temperature nor low pressure, such as would be met at high -altitudes, alter the focus of any ordinary lens by a significant amount, -and that the possible contraction of the camera body was of negligible -effect on the focus (not more than 1/200 per cent. per degree centigrade -with a metal camera). In complete harmony with these tests has been the -experience that if the ground focussing is done carefully, by accurate -means, then the air focus is correct. The whole matter thus becomes one -of precision focussing. - -The best method, applicable if the air is steady, is to focus by -_parallax_. The ground glass focussing screen is marked in the center -with a pencilled cross. Over this is mounted, with Canada balsam, a thin -microscope cover-glass. The camera is directed on an object a mile or -more away, and the image formed by the lens is examined by a magnifying -glass through the virtual hole formed by the affixed cover-glass. With -the pencil line in focus the head is moved from side to side. If the -image and pencil mark coincide they will move together as the head is -moved. If the image moves away from the pencil mark and in the _same_ -direction as the eye moves, the image is too near the lens. If the image -moves away in the _opposite_ direction to the motion of the eye, it is -too far from the lens. In either case the focus is to be corrected -accordingly. - -In place of a distant object, which may waver with the motion of the -air, we may use an image placed at infinity by optical means. The -_collimator_, an instrument for doing this, consists of a test object -(lines, circles, etc.) placed accurately at the focus of a telescope -objective. The camera lens is placed against this and focussed by -parallax, as with a distant object. Collimators are employed in camera -factories, and should be part of the equipment of base laboratories -where repairing and overhauling of cameras is done. - -=Lens Mounts.=—All that is required for the mounting of an aerial camera -lens is a rigid platform, with provision for enough motion of the lens -to adjust its focus accurately. As already explained, the lens works at -fixed, infinity, focus, and therefore needs no adjustment during use. It -must be held far more rigidly than would be possible by the bellows, -which is an almost invariable adjunct of focussing cameras. The use of -ordinary types of hand cameras on a plane is rarely successful just -because of the bellows, which is strained and rattled by the rush of -wind. - -The lens mountings thus far used have been simple affairs. In the French -cameras the lens is merely screwed into a flange which in turn is -fastened by screws to a platform in the camera body. Adjustment for -focussing is not provided; instead, the flange is raised on thin metal -rings or washers, cut of such thickness by trial as to bring the lens to -focus, once and for all. - -The U. S. Air Service method of mounting is to provide the lens barrel -with a long thread, which screws into a flange that in turn is mounted -on a platform in the camera cone, by means of thumb-screws. The lens is -focussed by screwing in and out, and then clamped by a screw through the -side, bearing on the thread. The whole mount may be quickly removed by -loosening the thumb-screws, and once focussed in one cone, can be -transferred to another similar, machine-made cone without change of -focus. Fig. 18 shows a 20 inch lens mounted in this manner. The -photograph shows as well the ring on the front of the lens by means of -which circular color filters may be held in place. This ring screws down -on the filter, and the catch is dropped into the nearest vertical groove -to the tight position. - -[Illustration: - - FIG. 18.—50 centimeter F/6 lens in U. S. standard mount, showing color - filter retaining ring and catch.] - -A somewhat different and better method of tightening the lens in the -flange, when focussed, has been adopted in the English lens mount, which -is in general similar to the American. The threaded part of the flange -is split by a slot cut parallel to the flange base, and a screw is run -into the flange from the front, through the split portion. By tightening -this screw, which is always accessible, the split part of the flange is -squeezed together, thus rigidly holding the lens barrel. - - - - - CHAPTER V - THE SHUTTER - - -=Permissible Exposure in Airplane Photography.=—A definite limitation to -the length of exposure in airplane cameras is set by the motion of the -plane. If we represent the speed of the plane by _S_, the altitude of -the plane by _A_, and the focal length of the lens by _F_, we obtain at -once from the diagram (Fig. 19), that _s_, the rate of movement of the -image on the plate, is given by the relation, - - _s_ _F_ - ——— = ——— - _S_ _A_ - -If we call the permissible movement _d_, then the permissible exposure -time, _t_, is given by the relation— - - _d_ _Ad_ - _t_ = ——— = ———— - _s_ _FS_ - -As a representative numerical case, expressing all quantities in -centimeters and in centimeters per second, let _F_ = 50, _S_ = -20,000,000/3600 (200 kilometers per hour), and _A_ = 300,000, then - - 50 × 20,000,000 - _s_ = ——————————————— = .9 centimeters - 300,000 × 3600 - -If we take for the permissible undetectable movement, .01 centimeter, -which is, as has been shown, a reasonable figure for lens defining -power, we have, then, that the _longest permissible exposure is .011 -second_—in round numbers, one-hundredth. - -In flying with a slow plane, or in flying against the wind, the exposure -can sometimes be increased to as much as double this length. Diminishing -_F_ would similarly extend the allowable exposure, but the ratio of _F_ -to _A_ approximates to a constant in actual practice; in other words, a -certain resolution and size of image have been found desirable. If -flying is forced higher, a longer focus lens is used; if lower flying is -possible, a lens of shorter focus. This relationship has, of course, -been derived from war-time experience. Probably much of the prospective -peace-time mapping work will impose substantially easier requirements as -to definition and will thus allow longer exposures. - -[Illustration: - - FIG. 19.—Relative motion of plane and photographic image.] - -For low oblique views the longest exposure is much less. Taking 45 -degrees as a representative angle for the foreground, and 500 meters as -a representative height, the value of _t_ becomes 1/600. - -These figures will illustrate two important points: they show how severe -is the limitation as to exposure, with the consequent heavy demand on -lens and sensitive material speed; and they show how important it is to -secure a shutter with the maximum light-giving power for a specified -length of exposure. This leads to a study of the characteristics as to -efficiency of the two common types of shutter, namely, _shutters at or -between the lens_, and _focal-plane shutters_. - -=Characteristics of Shutters Located at the Lens.=—Of the various -shutters located at the lens the most common is the type that is -clumsily but descriptively termed the “between-the-lens” shutter. This -is composed of thin hard rubber or metal leaves or sectors which overlap -and which are pulled open to make the exposure. It may require two -operations, one for setting and one for exposing, or it may, as in some -makes, set and expose by a single motion. Clock escapements, or some -form of frictional resistance, are depended on to control the interval -between opening and closing. This shutter is the one almost universally -employed on small hand cameras and on all lenses up to about two inches -diameter. It gives speeds sometimes marked as high as 1/300 second, -although usually not over 1/100 on actual test. - -Between-the-lens shutters have been used to some extent on the shorter -focus (up to 25 centimeter) aerial cameras, notably in the Italian -service. They suffer, however, from two limitations. In the first place -we have not yet solved the mechanical problems met with in trying to -make the shutter of large size (as for 50 centimeter _F_/6 lenses) at -the same time to give high speeds. In the second place the efficiency of -the type is low because a large part of the exposure time is occupied by -the opening and closing of the sectors. - -If we define the _efficiency_ of a shutter as the ratio of the amount of -light it transmits during the exposure to the amount of light it would -transmit were it wide open during the whole period, then the efficiency -of the ordinary between-the-lens shutter is of the order of 60 per cent. -This means 1.6 times the motion of the image for the same photographic -action that we should have with a perfect shutter. The accompanying -photographic record (Fig. 20) of the opening and closing process of this -type of shutter clearly illustrates its deficiencies. - -[Illustration: - - FIG. 20.—Effective lens opening at equal intervals of time: (_a_) - during focal plane shutter exposure; (_b_) during between-the-lens - shutter exposure.] - -=Characteristics of the Focal-Plane Shutter.=—Long before the days of -aerial photography the problem of a high-efficiency high-speed shutter -for photographing moving objects on the ground—railway trains or racing -automobiles—had already led to the development of the _focal-plane -shutter_. This is a type peculiarly adapted to the problems of the -airplane camera. It consists essentially of a curtain, running at high -speed close to the photographic plate, the exposure being given by a -narrow rectangular slot. - -If the focal-plane shutter is in virtual contact with the sensitive -surface the efficiency, as defined above, is 100 per cent., since the -whole cone of rays from the lens illuminates the plate during the whole -time of exposure. But if the curtain is not carried close to the plate -the efficiency falls off rapidly with distance, especially so for small -apertures of the slot. - -[Illustration: - - FIG. 21.—Calculation of focal plane shutter efficiency.] - -_The efficiency of the focal-plane shutter_ may be calculated as -follows: Let the focal length of the lens be _F_, its diameter be _F/N_, -the width of the slot be _a_, and the distance from plate to curtain _d_ -(Fig. 21). Now if the curtain is moving at a uniform speed, the time -taken for the slot to traverse the whole cone of rays, from the instant -it enters till the instant it leaves, will be directly proportional to - - _d_ (_F_) _d_ - ——— (———) + _a_ = ——— + _a_ - _F_ (_N_) _N_ - -If the curtain were in contact with the plate the time taken for the -same amount of light to reach the sensitive surface would be -proportional to _a_. Again defining shutter efficiency as the ratio of -the light transmitted to what would have been transmitted were the -shutter fully open for the total time of exposure, the efficiency, _E_, -is given at once by the expression— - - _a_ - _E_ = ——————————— - _d/N_ + _a_ - -As an example let the lens aperture be _F_/6, so that _N_ = 6; let _d_ = -1, and _a_ = 1, then _E_ = 6/7. In the French deMaria cameras, where _d_ -= 4 centimeters, _E_ = 60 per cent. for the aperture assumed, which is -representative. Fig. 22 exhibits diagrammatically the chief -characteristics of the focal plane shutter. - -[Illustration: - - FIG. 22.—Characteristics of focal plane shutter.] - -In view of the necessity for _some_ distance between shutter and plate -it is obviously important to keep _a_ as large as possible, depending -for the requisite shutter speed on the velocity of the curtain. Large -aperture and high curtain speed are also found to be desirable when we -consider the distortion produced by the focal-plane shutter. - -=Distortions Produced by the Focal-plane Shutter.=—While the time of -exposure of any point on the plate can, with the focal-plane shutter, -easily be made 1/100 second or less, the whole period during which the -shutter is moving is much greater than this. For instance, a 1 -centimeter opening which gives 1/100 second exposure takes ⅒ second to -move across a 10 centimeter plate, or nearly ⅕ second for an 18 -centimeter plate. With a moving airplane this means that the point of -view at the end of the exposure has moved forward compared to that at -the beginning, by the amount of motion of the plane in the interval. If -the shutter moves in the direction of motion of the plane the image will -be magnified; if in the opposite direction, it will be compressed along -the axis of motion. The amount of this distortion is calculated as -follows: - -Let the velocity of the plane be _V_, and that of the shutter be _v_. -Let the focal length of the camera be _F_, and the altitude _A_. If the -camera were stationary, a plate of length _l_ would receive on its -surface an image corresponding to a distance _A/F_ × _l_ on the ground. -Due to the motion of the shutter the end of the exposure occurs at a -time _l/v_ after the start. In this time the plane has moved a distance -_V_ × _l/v_; hence the point photographed at the end of the shutter -travel is _Vl/v_ within or beyond the original space covered by the -plate, depending on the direction of motion of the curtain. The -distortion, _D_, is given by the ratio of this distance to the length -corresponding to the normal stationary field of view: - - _V/v_ × _l_ _VF_ - _D_ = ——————————— = ———— - _A/F_ × _l_ _vA_ - -When _V_ = 200 kilometers per hour, _v_ = 100 centimeters per second, -_F_ = 50 centimeters, _A_ = 3000 meters, we have— - - 20,000,000 × 50 1 - _D_ = ———————————————————— = approximately ——— - 3600 × 100 × 300,000 100 - -Or if the actual distance error on the ground is desired, - - _Vl_ - ———— = 10.8 meters - _v_ - -As a percentage error this one per cent. is small compared with other -uncertainties, such as film shrinkage or the error of level of the -camera. As an absolute error in surveying, thirty feet is, of course, -excessive. - -The distortion is diminished for any specified shutter speed by making -the speed of travel of the curtain as large as possible and by -correspondingly increasing the aperture. In connection with film -cameras, another solution which has been suggested is to move the film -continuously during the exposure in the direction of the plane's motion. -The requisite speed of the film _v'_ to eliminate distortion is given by -the relation: - - _v'_ _F_ - ———— = ——— - _V_ _A_ - -For the values of _V_, _F_, and _A_ used above, _v'_ = .92 centimeters -per second. This speed is clearly that which holds the image stationary -on the film—a fact which suggests another object for such movement, -namely, to permit of longer exposures. - -The effect of focal plane distortion may be averaged out in the making -of strip maps, if the shutter is constructed so as to move in opposite -directions on successive exposures. The first picture will be magnified, -the second compressed, and so on, but a strip formed of accurately -juxtaposed pictures will be substantially accurate in over-all length. -Such a shutter is embodied in one of the German film cameras (Fig. 61). - -Distortion of the kind above discussed is absent with between-the-lens -shutters, which may conceivably be improved in efficiency and in -feasible size. If so they would merit serious consideration for aerial -mapping. - -=Methods and Apparatus for Testing Shutter Performance.=—With a -focal-plane shutter the desirable qualities in performance are three in -number: (1) _Adequate speed range_, which may be taken as from 1/50 to -1/500 second for aerial work, (2) _good efficiency_, which has already -been treated, and (3) _uniformity of speed_ during its travel across the -plate. Before the advent of aerial photography little attention was paid -to speed uniformity, differences of 50 per cent. in initial and final -speed being common in focal-plane shutters, and but little noticed in -ordinary landscape work because of the natural variation of brightness -from sky to ground. In the making of aerial mosaic maps the -non-uniformity of density across the plate results in a most offensive -series of abrupt changes of tone at the junction points of the -successive prints (Fig. 140), an effect which must be minimized by -manipulation of the printing light. - -Instruments for testing the speed and uniformity of action of -focal-plane shutters are an essential part of any laboratory for -developing or testing photographic apparatus and some simple device for -setting and checking shutter speed should be available in the field. -Every such speed tester must contain some form of time counting -element—pendulum, tuning fork or clock-work. Elaborate shutter testers, -suitable for determining all the characteristics of all types of -shutter, have been developed and used in certain of the photographic -research laboratories. For the study and setting of focal-plane shutters -(whose efficiency need not be measured, as it can be simply calculated -from linear dimensions), the following simple kinds of apparatus are -adequate: - -[Illustration: - - FIG. 23.—Apparatus for testing focal plane shutter speed throughout - the travel of the curtain.] - -_Clock dial type of shutter tester._ This consists essentially of a -black clock dial carrying a white pointer which makes its complete -revolution in one second or less. If this dial is photographed by the -camera under test, the width of the sector traced during the exposure by -the moving pointer shows the time interval. If the dial is photographed -at several points on the plate—beginning, middle and end of the shutter -travel—the complete characteristics of the shutter can be determined. - -_Interrupted light type of shutter tester._ For the study of uniformity -of shutter action alone the apparatus shown in Fig. 23 may be employed. -_A_ is a high intensity light source, such as an arc or a gas filled -tungsten lamp. _L_ is a convex lens, focussing an image of the light -source on a small aperture in the screen _E_. _D_ is a sector disc -which, driven by the motor _M_, interrupts the transmitted light with a -frequency determined by the number of openings of the sector and by the -speed of rotation, which must be measured by a tachometer. The light -diverging from the aperture in _E_ falls upon the shutter _S_, which for -this test is reduced to a narrow slit of one millimeter or less. Passing -through the shutter opening the light falls upon the photographic plate -_P_. The principle is simple: If the light is uninterrupted, the plate -_P_ is exposed at all points; due to the interruptions, a series of -parallel lines of photographic action result, and their distance apart -gives a measure of the speed of the shutter at any chosen point in its -travel. A performance curve of the French Klopcic shutter is shown in -Fig. 24. The variation in speed lies over a range of two to one. So -serious is this defect in these shutters that diaframs are sometimes -inserted in the French cameras to cut off part of the light from the -lens on the most exposed end of the plate. This expedient produces -uniformity of photographic action, but does not overcome the movement of -the image, which is one of the chief faults of excessive exposure. - -[Illustration: - - FIG. 24.—Performance of Klopcic shutter.] - -[Illustration: - - FIG. 25.—Optical system of shutter tester for Air Service, U. S. Army.] - -A more complete apparatus, adapted both to absolute speed determinations -and to the study of uniformity of action, is that worked out and used in -the United States Air Service (Fig. 25). At _A_ is a high intensity -light source, an image of which is focussed by the lens _L__{1} upon a -slit _E_, in front of which stands a tuning fork _T_, of period 1024 or -2048 per second. The light diverging from the slit is received by a -second lens, _L__{2} which is arranged either to focus the slit image -upon the shutter curtain or to render the rays parallel, so that an -entire camera may be inserted. In the latter case the camera lens -_L__{3} serves to focus the slit image on the curtain _C_. After passing -through the curtain aperture the light is focussed by the lens _L__{4} -on the rotatable drum _D_, which carries a strip of sensitive film. - -The operation of testing a shutter consists in focussing the slit image -on the portion of the shutter whose performance is required, striking -the tuning fork to set it vibrating, rotating the drum rapidly and -setting off the shutter. There is thus obtained on the sensitive film an -exposed strip resembling in appearance the edge of a saw, the number of -teeth showing the time interval in vibrations of the tuning fork. Three -exposures usually give all the points necessary for a practical -knowledge of the shutter's uniformity of action. A point of some -importance, learned from numerous shutter tests, is that a focal-plane -shutter should be tested in the position in which it is to be used. -Aerial camera shutters should be tested in the horizontal position. - -=Types of Focal-plane Shutters.=—A variety of means have been utilized -for securing the necessary variation in speed in focal-plane shutters. -Their success is to be measured by the actual speed range and by the -uniformity of speed attained. In aerial cameras at present in use we -find _variable tension_ of the curtain spring, the aperture being fixed; -_variable opening_ with fixed tension; _multiple curtain openings_ with -fixed spring tension; and combinations of two or all of these methods of -speed control. The problem of covering the aperture during the operation -of winding up or setting the shutter has led to further elaborations of -shutter mechanism. These take the form of _lens or shutter flaps_, -_auxiliary curtains_, and shutters of the _self-capping_ type. Shutters -embodying all these features are briefly described below. - -=Representative Shutters.=—The Folmer variable tension shutter is used -on the United States Air Service hand-held and hand-operated plate -camera and on some of the film cameras. It consists of a fixed aperture -curtain wound on a curtain roller in which the spring can be set to -various tensions, numbered 1 to 10. The range of speeds attainable is at -best about three to one, or from 1/100 to 1/300 second, considerably -shorter than the range indicated as desirable. Its uniformity of travel -is variable with the tension, as shown by representative performance -curves in Fig. 30. Lacking any self-capping feature the shutter is -provided either with an auxiliary curtain, or in the hand-held camera -with flaps in front of the lens, opened by the exposing lever before the -curtain is released (Fig. 39). This shutter is made a removable unit in -the 18 × 24 centimeter hand-operated camera, but is built into the -hand-held and film cameras. - -[Illustration: - - FIG. 26.—Removable four-slit shutter of German (Ica) camera, showing - flaps.] - -_The Ica shutter_ used on the standard German aerial cameras is a good -example of the multiple slit curtain (Fig. 26). Four fixed aperture -slits are provided, with a single tension, the openings roughly in the -ratio 1, ½, ¼, ⅛, which when the spring tension is properly adjusted -give exposures of 1/90, 1/180, 1/375, 1/750 second. To pass from one -exposure time to another the setting milled head is wound up to -successively higher steps or else exposed one or more times without -resetting, depending on the direction it is desired to go. Capping -during setting, or during exposure, in order to change the opening, is -provided for by a pair of flaps on the shutter unit, which open into the -camera body. The mechanical work on these shutters is of excellent -quality, the curtain running with exceptional smoothness. Provision is -made for adjusting the tension until the marked speeds are attained; -this is presumably done in a repair laboratory to which the shutter only -need be sent, as it is a removable unit. Tests made on one of these -shutters wound to its highest tension are shown in Fig. 30. The marked -speeds are not attained, and there is considerable lack of uniformity -from start to finish of the travel. - -_L camera variable-aperture shutter._ The shutter of the L type camera -(Fig. 27) is representative of one of the most primitive methods of -varying aperture. The two jaws of the slit are held together by a long -cord passing completely around the aperture, fastened permanently at one -end and attached at its other end by a sliding clasp or saddle. As this -saddle is forced in one direction the slit is closed, in the opposite -direction the cord becomes slack, and after the shutter is released once -or twice the slit assumes a wider opening. A chronic trouble is the -breaking of the cords. Its opening can be changed only after the plate -magazine is removed. - -[Illustration: - - FIG. 27.—“L” type camera showing open negative magazines and shutter - mechanism.] - -_U. S. Air Service variable-aperture shutter._ This shutter is -incorporated in the American deRam and in other late American cameras -(Fig. 28). Its characteristic feature is the introduction of an idler, -whose distance from the main curtain roller can be varied. Tapes whereby -the following curtain is attached to the spring roller pass over this -idler, and by changing its position the aperture or distance between the -two curtain elements is altered over a large range. Tests of this -shutter are shown in Fig. 30. A speed of 1/50 second is provided for by -a slit width of five centimeters, and the highest speed is fixed only by -the practical limit of approach of the jaws. Experiment shows great -uniformity of rate of travel to be attainable by combining careful -choice of spring length and tension with good workmanship in the -mechanical features. Variable-aperture fixed-tension shutters have a -definite advantage over the variable-tension type in that they can -utilize for all speeds that tension which gives uniform action. The -capping feature of this shutter is provided in the American deRam by -flaps, in the automatic film camera by an auxiliary curtain. The shutter -is removable in the deRam, but built into the other camera. - -[Illustration: - - FIG. 28.—Variable aperture curtain developed in U. S. Air Service, and - used in American deRam, and “K” type automatic film cameras.] - -_The Klopcic_ variable-tension, variable-aperture, self-capping shutter -is an example of an attempt to meet all shutter requirements with an -entirely self-contained mechanism. It is shown diagrammatically in Fig. -29. Tapes _G__{1}, _G__{2} are used to connect the following curtain _B_ -directly to the spring roller _T_, at a fixed distance, while the -leading curtain, _A_, may be slid along the tapes by small friction -buckles, _C__{1}, _C__{2}, auxiliary springs _R__{1}, _R__{2} serving to -keep it taut in any position. When the shutter is being set the buckles -are arrested against stops while the winding-up continues for what is to -be the following half of the curtain in exposing. When released the -curtain moves across with an aperture fixed by the point of setting of -the buckle stops. At the end of the travel the buckles are arrested by -other stops, while the following portion of the curtain continues its -travel to the end. On re-winding, therefore, the aperture is closed. -Variable tension as well as variable aperture is provided, although -little used. In the French cameras a lens flap is also inserted behind -the lens, but this is not needed if the self-capping feature functions -properly. On the hand cameras this flap is said to be necessary in order -to prevent a curious kind of accident: if the camera is held on the -knee, pointing upward, an image of the sun may be formed on the curtain -and burn a hole through it. - -[Illustration: - - FIG. 29.—Mechanism of Klopcic variable aperture self-capping shutter.] - -The performance of the French shutter in respect to uniformity has -already been shown in Fig. 24. It leaves very much to be desired. -Besides non-uniformity of action during its travel it exhibits another -common defect of variable-tension shutters, namely, the curtain must be -released several times after a change of tension before the new speed is -established (Fig. 30, tensions 5 and 5´). - -[Illustration: - - FIG. 30.—Performances of various shutters used on aerial cameras. - Speeds expressed in reciprocals of fractional parts of one second.] - -The French shutter as made for the deMaria cameras is a removable unit. -The small size (13 × 18 cm.) sets by the straight pull of a projecting -pin, the larger (18 × 24 cm.) by winding up a milled head. The former is -the more convenient motion for an aerial camera. Care must be taken with -either type that the motion of setting is not stopped when the first -resistance is encountered; this occurs when the tape buckles strike -their stop and the slit begins to open. - - - - - CHAPTER VI - PLATE-HOLDERS AND MAGAZINES - - -In the earlier days of airplane photography the ordinary plate-holder or -double dark slide was used to some extent, but it is ill-suited to the -purpose because of the considerable time and attention required for its -operation. It has nevertheless the merit of adding little to the length -of the camera, and it works in any position. For these reasons it has -remained in occasional use for the taking of oblique views with long -focus cameras in a cramped fuselage. - -Next in order of progress rank the simple box magazines, for holding a -dozen, eighteen or twenty-four plates, as used in the English C, E, and -L type cameras. These are little more than boxes with sliding lids which -when open permit the introduction or removal of the plates. Figs. 45 and -46 illustrate the magazine of this type as made for the English C and E -cameras. It is constructed of wood, grooved to fit tracks on the camera, -and is furnished with a sliding door or lid hinged in the middle to fold -down out of the way when open. The eighteen plates are carried in metal -sheaths, both to provide opaque screens between them, and to protect -them from injury in the mechanism of the camera. Fig. 27 shows the -all-metal magazine made for the American model L camera. This differs -from the English in material of construction, plate capacity (24 instead -of 18) and manner of operating the slide, which is built up of three -thicknesses of phosphor bronze and draws out through metal guides bent -into semicircular form. A snap catch holds this slide at either end of -its travel. The leather strap introduced in the American model for -carrying and handling is a distinct improvement. These magazines contain -no springs or other mechanism, as the cameras with which they are used -depend upon the action of gravity for emptying the upper (feeding) -magazine, and filling the lower (receiving) one. - -[Illustration: - - FIG. 31.—Aerial hand camera (U. S. type A-2).] - -Next in order of complexity may be ranked the _bag magazine_ (Figs. 31 -and 44). In this the exposed plate is pulled out of the magazine proper -by a metal slide or rod into a leather bag. The rod is then pushed back, -the plate in its metal sheath is grasped through the leather bag, lifted -to the back of the magazine, and forced in behind the other plates. The -number of plates exposed is indicated either by numbers on the backs of -the sheaths, visible through a red glazed opening in the back, or else -by a counter actuated by the metal slide rod. Usually twelve are carried -in a magazine. For aerial work the common design of this magazine as -used for ground work must be modified by providing extra large easily -grasped hooks both on the draw rod and on the dark slide, which must be -drawn before making the first exposure and replaced after the last. The -small rings and grips of the standard commercial magazine are almost -impossible to handle through heavy gloves. - -The next type of magazine is represented by three designs, the _Gaumont_ -and _deMaria_, used very generally by the French during the war, and the -_Ernemann_, used almost universally in the German air service (Figs. 32, -40 and 42). In all of these the operation of plate changing is the same: -the end of the magazine is pulled out and thrust back, a more simple -operation than the bag manipulation just described. The internal -workings are different according to size. In the smaller French -magazines (13 × 18 cm.) the camera is first pointed upward, all the -plates are drawn out except the one to be changed, and this, with the -aid of springs, drops to the bottom, after which the other plates push -back over it. The plates pull out in the direction of their long -dimension. In the larger French magazine (18 × 24 cm.) only the exposed -plate pulls out. The pull is in the direction of the shorter dimension -of the plate, which is lifted up by heavy springs and slides back over -the top of the pile. In the Ernemann magazine only six plates are -carried, which there is good reason to believe represent the maximum -feasible number, judging by the reports of jambs and breakages in the -twelve-plate French magazines. In all of these magazines laminated wood -slides pull out and in at each operation, and while satisfactory if made -and operated in one climate, experience indicates that if made in -America and sent abroad swelling of the wood may be expected to prevent -their successful operation. - -[Illustration: - - FIG. 32.—Various plate magazines used on aerial cameras.] - -Alternative forms of magazine, somewhat more practical from the -standpoint of manufacture and export, are several designs embodying _two -compartments_ (Fig. 32). In the most simple of these the plates are -moved, immediately before or after exposing, from the unexposed to the -exposed side. Illustrative of this type are the Folmer designs, in which -the to-and-fro motion is imparted by a rack geared to a pinion actuated -either by a lever, in the hand camera, or by the power drive, in the -automatic design (Figs. 33 and 53). Another illustration is afforded by -the Piserini and Mondini magazine, in which the operation of changing is -performed by a back-and-forth motion of a hand-grip, which also sets the -camera shutter (Fig. 47). - -[Illustration: - - FIG. 33.—U. S. Air Service hand camera, with two-compartment magazine.] - -[Illustration: - - FIG. 34.—Film type hand camera.] - -In these magazines the center of gravity changes as the exposed plates -are moved over, and only half the inside space is occupied with plates. -These objections are overcome in the Chassel form, where both -compartments are always full. Transfer of the bottom exposed plate from -one compartment to the other is compensated for by the simultaneous -shift of the top plate in the receiving compartment, to the feeding -side. In a modification of this idea by Ruttan the exposing position is -when the plates are half-way through the shifting process, whereby the -magazine may be symmetrically mounted on the camera body. - -[Illustration: - - FIG. 35.—Apparatus for straightening plate sheaths.] - -Other more complicated magazines have been designed, some of which are -shown in the diagrammatic _ensembles_ of Figs. 32 and 48. In the -Jacquelin, the main body of plates is raised while the bottom (exposed) -plate is folded against the side. The main body of plates then drops -back to place, the exposed plate is carried on upward and folds down on -the back of the pile. The Bellieni magazine system uses three, a central -feeding one and two below for receiving, one on each side of the camera -body. These were made solely for attachment to captured German cameras. -In the Fournieux magazine the plates are carried in an interior rotating -box. The plate to be exposed is dropped off the front of the pile, down -to the focal plane, and after exposure is picked up and placed at the -back of the pile, which has turned over in the meanwhile. The deRam -rotating magazine is described in connection with the camera of which it -is an essential part (Fig. 52). - -[Illustration: - - FIG. 36.—Training plane equipped for photography, showing “L” camera - in floor mount and magazine rack forward of the observer.] - -For the protection of the plates during their manipulation, and in the -camera, all plate magazines thus far developed carry them in thin _metal -sheaths_. These add greatly both to the weight and to the time necessary -to handle the plates, but no means have as yet been found for dispensing -with them. Fig. 35 shows a representative sheath or septum, as used in -the L camera. On three sides the edge is bent up and turned over, -forming a ledge for the plate to press against. The fourth side is left -open for inserting the plate, which is then held in by a small upward -projecting lip, and kept close against the ledges by narrow springs at -the sides. To insert or remove the plate the projecting lip is -depressed, either by springing the sheath by pressure from the sides or -by using an appropriate tool. - -_Care of sheaths._ Unless systematically taken care of, plate sheaths -become bent or dented. They are then a menace to camera operation, -catching or jamming in the plate changing process, breaking plates and -damaging camera mechanisms. In order to maintain them flat and true, -steel forms are necessary on which the sheaths may be hammered to shape -with a mallet (Fig. 35). - -_Magazine racks._ Reconnaissance and mapping call for a number of -exposures much greater than the capacity of one 12, 18, or 24 plate -magazine. Additional magazines must therefore be carried. These should -be in racks convenient to the observer (Fig. 36), securely held yet -capable of quick removal and insertion. In the rack designed to carry -two of the metal magazines for the American L Camera, the magazines -slide into loose grooves formed by a metal lip. They are prevented from -slipping out by a spring catch, past which they slide when inserted but -which is instantly thrown aside by pressure of the thumb as the hand -grasps the magazine handle for removal. - - - - - CHAPTER VII - HAND-HELD CAMERAS FOR AERIAL WORK - - -=Field of Use.=—The first cameras to be used for aerial photography were -hand-held ones of ordinary commercial types. Indeed the idea is still -prevalent that to obtain aerial photographs the aviator merely leans -over the side with the folding pocket camera of the department store -show window and presses the button. But the Great War had not lasted -long before the ordinary bellows focussing hand camera was replaced by -the rigid-body fixed-focus form, equipped with handles or pistol grip -for better holding in the high wind made by the plane's progress through -the air. Even this phase of aerial photography was comparatively -short-lived. The need for cameras of great focal length, and the need -for apparatus demanding the minimum of the pilot's or observer's -attention, both tended to relegate hand-held cameras to second place, so -that they were comparatively little used in the later periods of the -war. - -Yet for certain purposes they have great value. They can be used in any -plane for taking oblique views, and for taking verticals, in any plane -in which an opening for unobstructed view can be made in the floor of -the observer's cockpit. They can be quickly pointed in any desired -direction, thus reducing to a minimum the necessary maneuvering of the -plane, a real advantage when under attack by “Archies” or in working -under adverse weather conditions. - -For peace-time mapping work the hand-held camera, when equipped with -spirit-levels on top, and when worked by a skilful operator, possesses -some advantages over anything short of an automatically stabilized -camera. For experimental testing of plates, filters and various -accessories, the ready accessibility of all its parts makes the -hand-held camera the easiest and most satisfactory of instruments. - -The limitations of the hand-held camera lie in its necessary restriction -to small plate sizes and short focal lengths, and in the fact that it -must occupy the entire attention of the observer while pictures are -being taken—the latter a serious objection only in war-time. - -=Essential Characteristics.=—In addition to the general requirements as -to lens, shutter and magazine, common to all aerial cameras, the hand -camera must meet the special problems introduced by holding in the -hands, especially over the top of the plane's cockpit. An exceptionally -good system of handles or grips must be provided whereby the camera can -be pointed when pictures are taken, and held while plates are being -changed and the shutter set. The weight and balance of the camera must -be correct within narrow limits; the wind resistance must be as small as -possible; the shutter release must be arranged so as to give no jerk or -tilt to the camera in exposing. - -As to the method of holding the camera, a favorite at first among -military men was the pistol grip, with a trigger shutter release (Fig. -37). Because of the size and weight of the camera the pistol grip alone -was an inadequate means of support and additional handles on the side or -bottom had to be provided for the left hand. Small (8 × 12 cm.) pistol -grip cameras were used to some extent by the Germans (Fig. 42), and a -number of 4 × 5 inch experimental cameras of this type were built for -the American Air Service (Fig. 37). But the grasp obtained with such a -design is not so good as is obtained with handles on each side or with -flat straps to go over the hands. The camera balances best with the -handles in the plane of the center of gravity. As to weight, no set -rules are laid down, but experience has shown that a fairly heavy -camera—as heavy as is convenient to handle—will hold steadier than a -light one. The American 4 × 5 inch cameras described below weigh with -their magazines in the neighborhood of twelve pounds. - -[Illustration: - - FIG. 37.—Pistol-grip aerial hand camera.] - -=Representative Types of Hand-held Cameras.=—French and German hand-held -cameras are essentially smaller editions of their standard long-focus -cameras, and a description of them will apply to a considerable extent -to the large cameras to be discussed in a later chapter. The English and -American hand-held cameras are generally quite different in type from -the large ones, which are used attached to the plane. - -[Illustration: - - FIG. 38.—Diagram of French (deMaria) 26 cm. focus hand camera, using - 13 × 18 cm. plates.] - -_The French hand-held camera_ uses 13 × 18 centimeter plates, carried in -a deMaria magazine, and has a lens of 26 centimeters focus. The shutter -is the Klopcic self-capping type already described, and is removable. -The camera body, built of sheet aluminum, takes a pyramidal shape. In -Fig. 38, _A_ is the shutter release and _B_ the rectangular sight, of -which _C_ is the rear or eye sight. The complete sight may be placed -either on the top or on the bottom of the camera. At _D_ are the -handles, sloping forward from top to bottom; _E_ is a catch for holding -the magazine; _F_ is a door for reaching the back of the lens and the -lens flap; _G_ is a snap clasp for holding the front door of the camera -closed; _H_ is a ring for attaching a strap to go around the observer's -neck; _I_ is the lever which opens the flap behind the lens and releases -the focal-plane shutter; _J_ is a snap catch for holding the front door -of the camera open. - -The operations with this camera are three in number. Starting -immediately after the exposure, the camera is pointed lens upward and -the plate changed by pulling the inner body of the magazine out and then -in; next the shutter is set; then the camera is pointed, and finally -exposed by a gentle pull on the exposing lever. - -_The English hand-held camera_ (Fig. 186). This differs from the French -in the size of plate (4 × 5 inch), in the shape of the camera body, -which is circular, and in the type of shutter, which is fixed-tension -variable-opening. In the longer focus camera (10 to 12 inch) the shutter -is self-capping, and the aperture is controlled by a thumb-screw at the -side. In the smaller (6 inch) a lens flap is provided in front of the -lens and the shutter aperture is varied by a sliding saddle and cord. -The handles of the camera are placed vertical, instead of sloping as in -the French. The shutter is released by a thumb-actuated lever. Double -dark slides are used, as the multiple plate magazine has not found favor -in the English service. - -_The German hand-held camera_ (Fig. 42). The German hand-held camera is, -like their whole series, built of canvas-covered wood, the body having -an octagonal cross-section. It is equipped with the Ica shutter and uses -the Ernemann six plate (13 × 18 cm.) magazine. The excellent system of -grips by which the camera is held and pointed is an especially -commendable feature. On the right-hand side is a handle similar to the -French type, but carefully shaped to fit the hand. The left-hand grip -consists of a long, rounded block of wood running diagonally from top to -bottom of the side, with a deep groove on the forward side for the -finger tips, while over the hand is stretched a leather strap, the whole -aim being to provide an absolutely sure and comfortable hold on the -camera during the plate changing and shutter setting operations. - -[Illustration: - - FIG. 39.—Front view of U. S. aerial hand camera, showing lens flaps - partly open, and details of tube sight.] - -_United States Air Service hand cameras._ The hand camera developed for -the United States Air Service and manufactured by the Eastman Kodak Co. -is made in three models, using the bag magazine, a two-compartment -magazine, and roll film, respectively. The shutter is of the fixed (one -or two) aperture variable tension type, built into the camera. A -distinctive feature is the double lens flap, in front of the lens -actuated by the thumb pressure shutter release (Fig. 39). In the bag -magazine camera the shutter is set, as a separate operation, by a wing -handle, and a similar handle controls the tension adjustment. In the -two-compartment type (Fig. 33) the shutter wind-up is geared to the -plate changing lever, so that but one operation is necessary to prepare -the camera for exposure. In the film type (Fig. 34) a single lever -motion sets the shutter and winds up the film ready for the next -exposure. After the last exposure of all the film is wound backward on -its own (feeding) roller before removing from the camera. The film is -held flat by a closely fitting metal plate behind, and by guides at the -edges in front, an arrangement which with small sizes works fairly well -although the exquisite sharpness of focus attainable with plates is not -to be expected. The saving in weight made possible by the use of film in -place of plates in metal sheaths is about three pounds per dozen -exposures. - -In all these cameras the sight—a tube with front and back cross wires—is -placed at the bottom. This position has been found the most convenient -for airplane work, as it necessitates the observer raising himself but -little above the cockpit, a matter of prime importance when the -tremendous drive of the wind is taken into account. - - - - - CHAPTER VIII - NON-AUTOMATIC AERIAL PLATE CAMERAS - - -The ideal of every military photographic service has been an automatic -or at least a semi-automatic camera, in order to reduce the observer's -work to a minimum. Yet as a matter of fact almost all the aerial -photography of the Great War was done with entirely hand-operated -cameras. The primary reason for this was that no entirely satisfactory -automatic cameras were developed, cameras at once simple to install and -reliable when operated. Even the propeller-drive semi-automatic L type -of the British Air Service was very commonly operated by hand, for many -of the pilots and observers regarded the propeller merely as another -part to go wrong. - -Any automatic mechanism in the airplane must work well in spite of -vibration, three dimensional movements, and great range of temperature. -The requirements were well recognized when the war closed, but had not -yet been met. Careful study of the conditions and needs by competent -designers of automatic machinery may be expected to result at an early -date in reliable cameras of the automatic type, but the description -below of hand-operated cameras really covers practically all the cameras -found satisfactory in actual warfare. - -=General Characteristics of Hand-operated Cameras.=—As distinguished -from the hand-held cameras the larger hand-operated cameras are -characterized by the greater focal length of their lenses, the size of -plate employed, and the manner of holding—by some form of anti-vibration -mounting attached directly to the fuselage. - -Except for the early English C and E type cameras which called for 10 -inch lenses and 4 × 5 inch plates, the general practice at the close of -the war by agreement between the French, English and American Air -Services, was for the use of 18 × 24 centimeter plates and for lenses -with focal lengths of approximately 25, 50 and 120 centimeters. The -English also made use of a 14 inch (35 centimeter) lens, and never made -a regular practice of anything larger than 50 centimeters. The Germans -and Italians restricted themselves to the 13 × 18 centimeter size of -plate, while a lens of 70 centimeters focal length was standardized with -the Germans, in addition to the 25, 50, and 120 centimeter. - -The particular focal length was determined by the nature of the -photographic mission. Where large areas were to be covered at low -altitudes or without the demand for exquisite detail, the shorter focus -lenses suffice. The most commonly used lens in the French Service was -the 50 centimeter, while the 120 was employed when high flying was -necessary or when minute detail was required. As already mentioned, the -common practice was to keep cameras of all focal lengths available, but -the ideal at the close of the war was to have the camera nose and lens a -detachable unit, so that any focal length desired could be secured with -the same camera body. - -_The standard French camera._ The hand-held form of French camera has -already been described. The cameras for larger plate sizes and longer -focus lenses differ only in the addition of a Bowden-wire distance -release for the shutter and in the use of the Gaumont magazine which -operates without the necessity of pointing the exposed side of the -magazine upward. Fig. 40 illustrates the 50 centimeter camera, and Fig. -41 the 120. - -[Illustration: - - FIG. 40.—50 centimeter deMaria hand operated camera on tennis ball - mounting.] - -[Illustration: - - FIG. 41.—120 centimeter deMaria camera.] - -_The German Ica cameras._ These are larger editions of the light wood -hand camera already described, but with the addition of a Bowden-wire -shutter release. The body of the larger cameras carries a distinctive -feature in the distance control of the lens diafram, worked by means of -a lever which actuates racks, pinions and connecting rods leading to the -lens. On the side of the camera body a shallow box is provided for -carrying the color filter in its bayonet joint mount to fit on the lens -(Figs. 42 and 43). - -[Illustration: - - FIG. 42.—German aerial cameras.] - -[Illustration: - - FIG. 43.—Diagram of German 50 centimeter camera.] - -[Illustration: - - FIG. 44.—U. S. hand-operated aerial camera (type M) with 10 and 20 - inch cones.] - -_The hand-operated bag-magazine camera_ of the United States Air Service -(Type M) is similar to the small hand-held camera, but differs in three -respects: a removable shutter (of the variable-tension fixed-aperture -type) embodying an auxiliary curtain for capping during the setting -operation; a Bowden-wire shutter release; and the employment of a set of -standard interchangeable cones to hold lenses of several focal lengths. -The 20 inch and 10 inch cones are shown in Fig. 44. The operation of -this camera is similar to the French standard cameras, but not so simple -because of the number of motions required in manipulating the bag. Its -chief objection for war work lies in fact in the magazine, which should -be superseded by a two-compartment or other satisfactory type of plate -changing chamber. The camera alone, with 20 inch cone, weighs -approximately 40 pounds; the loaded magazine, with its plates in metal -sheaths, 15 pounds. - -[Illustration: - - FIG. 45.—English C type aerial camera.] - -_The English C and E type cameras._ The C and E type cameras have now -chiefly an historic interest. They were the first used in the English -service, fixed to the fuselage, and were later used in training work in -England and in the United States. They were never built for plates -larger than 4 × 5 inch nor for lenses of more than 12 inch focus, a -limitation set by the lenses available at the time of their design. - -[Illustration: - - FIG. 46.—English type “E” hand-operated plate camera.] - -In several respects the mode of operation of the two types is the same. -The unexposed plates are held in a magazine lying above the camera, in -the axis of the lens (Fig. 32). After exposure the bottom plate is -carried to one side and allowed to fall by the action of gravity into -the receiving magazine. In the C type (Fig. 45) an opaque slide is drawn -between the lens and the (variable-opening) shutter during the setting -operation. During the exposure period this slide projects into a -compartment on the opposite side of the camera from the receiving -magazine, thus making the camera mechanism three plates wide. In the E -type (Fig. 46), a flap over the lens makes it possible to dispense with -the sliding screen, and reduces the camera to about the width of two -plates. In the C type the plates are changed by a handle on top of the -camera; in the E type provision is made for distance control by cords, -and for shutter release by a Bowden wire. In both cameras the operation -of plate changing also sets the shutter, a definite advance over the two -preparatory motions in the French apparatus. The C type was constructed -of wood, the E of metal. - -[Illustration: - - FIG. 47.—Italian (Piserini and Mondini) two compartment magazine - hand-operated camera.] - -_Italian two-compartment magazine camera._ A camera designed by Piserini -and Mondini was used to some extent by the Italian service toward the -close of the war (Fig. 47). This has the desirable feature just noted in -the C and E cameras: the operations of plate changing and shutter -setting are performed in a single motion. Unlike those cameras, however, -the plates are changed from one compartment to another of the magazine -already described, without dependence on gravity, by an entirely -positive shifting action. The setting of the self-capping focal-plane -shutter is accomplished by a projecting finger engaging the shutter -mechanism. Cameras of this general type, built for 18 by 24 centimeter -plates, with interchangeable lens cones, removable shutters, and -preferably magazines in which the center of gravity does not shift as -the plates are changed, represent the next step in advance of the French -practice, and may indeed prove all that is necessary or desirable in -camera complexity for peace-time photography from the air. - -_The standard Italian camera and similar types._ The camera (Lamperti) -which the Italian Air Service used almost exclusively during the war -exemplifies a type quite different from anything as yet described (Figs. -48 and 49). Plates to the number of twenty-four (13 × 18 cm.) are loaded -into a chamber at the top of the camera. Each plate is held in a septum -furnished with projecting lugs at one end. A lever acting through a -Bowden wire, exposes the bottom plate, which then swings downward about -these lugs as pivots, and is forced by a pair of fingers into a -compartment at the side. The between-the-lens shutter has a single speed -of 1/150 second, and variation of exposure is achieved by altering the -lens aperture. - -[Illustration: - - FIG. 48.—Various plate changing devices.] - -The great advantage of this camera is its simplicity, a single motion -performing all the operations. Its disadvantages are its dependence on -gravity for operation, its between-the-lens shutter, the limitation set -to the number of exposures, and the necessity for removing the whole -camera to take out the plates for developing. In actual practice the -camera has worked out well. The better light found in the Italian as -contrasted with the northern theatre of war makes the between-the-lens -shutter at high speed adequate, while the limitation to the number of -exposures has been met by carrying several complete cameras in each -plane. Because of the Bowden-wire operation these cameras need not be -accessible to the observer or pilot, so that the practice of carrying -them in single-seaters was common. Attempts at standardization of Allied -practice through the adoption of standard lens cones were, of course, -out of the question with this camera. With its limitations of shutter -efficiency and plate size it is doubtful whether it would have been -satisfactory outside the service for which it was developed. - -[Illustration: - - FIG. 49.—Italian (Lamperti) single-motion plate camera, on - anti-vibration tray.] - -The limitations set by the between-the-lens shutter in this type have -been overcome in an experimental camera along similar lines made by the -Premo Works of the Eastman Kodak Company, and in the French Aubry model -(Fig. 48). These employ focal-plane shutters which swing out of the way -and are set as the exposed plate swings or drops to the receiving -chamber. The dependence on gravity in this type could doubtless be -avoided by positive finger mechanisms. If so, the resultant cameras, set -and exposed by a single motion, would acquire a highly desirable -simplicity of operation. They would have peculiar merit because of the -very short interval required between exposures—a characteristic needed -for making low stereo-oblique views. The cameras just mentioned have, -however, departed far in form from the lines of standardized practice -and have not been followed up. - - - - - CHAPTER IX - SEMI-AUTOMATIC AERIAL PLATE CAMERAS - - -In the hand-operated camera the limit to progress is set when the number -of operations is reduced to a minimum. In cameras using the larger sizes -of plates a reduction in the number of operations almost inevitably -results in inflicting considerable muscular labor upon the operator. -Furthermore, distance operation becomes difficult to arrange for, -because the common reliance—the Bowden wire—is unfitted for heavy loads. -Consequently, for setting the shutter and changing the plates we must -resort to some other source of power than the observer's arm. Air-driven -turbines or propellers have been used on aerial cameras, as well as -clock-work, and also electric power, the latter derived either from a -generator or from storage batteries. The relative merits of these -sources of power form the subject of a separate chapter. Mention only is -here made of the form of drive actually employed in connection with the -various cameras. - -The term _semi-automatic camera_ is best used to designate that type in -which the observer (or pilot) has merely to release the shutter, after -which the mechanism performs all the operations necessary to prepare for -the next exposure. There has been some difference of opinion as to -whether it is ever advisable to go further than this with plate cameras. -The English Service holds that completely automatic exposing, in -addition to plate changing, is apt to encourage the making of many more -pictures than necessary, involving carrying an excessive weight of -plates. The French Service has rather generally favored entirely -automatic cameras in theory, although during the war practically all the -work of the French army was done by the hand-operated cameras already -described. - -=The English L Type Camera.=—The L, a modification of the earlier C and -E models, differs from its predecessors chiefly in the addition of a -mechanism which when connected with a suitable source of power can be -used whenever desired for changing the plates and setting the shutter. -As in the C and E types, all unexposed plates are carried in a magazine -above the camera, while the exposed plates are shifted in a horizontal -direction to one side and fall thence to a receiving magazine. - -[Illustration: - - FIG. 50.—American model, English “L” type semi-automatic camera.] - -[Illustration: - - FIG. 51.—Mechanism of “L” camera.] - -Fig. 50 shows the American model, which is a copy, with modifications, -of the original English design. Its weight with one loaded magazine is -about 35 pounds. Its manner of functioning may be studied from the -picture of the mechanism (Fig. 51). The part of the mechanism to the -left is inoperative during hand operation, and the large toothed wheel -is locked by the removable pin shown hanging on its chain in Fig. 50. To -change a plate and set the shutter the projecting lever (Fig. 50) is -thrown over and back. This causes a sliding tray, in which the exposed -plate rests, to travel to the right, over the receiving magazine, where -the plate is dropped. After this the tray returns to the left exposing -position. Simultaneously the shutter is wound up. Exposure is made -either by pressing down upon the plunger, or better, by using a Bowden -wire. Provision for both methods of exposing, one for the pilot and one -for the observer, is shown in Fig. 81. The shutter is the -variable-aperture type already described, provided in addition with a -tension adjustment on the back of the camera. A flap behind the lens -does the capping during the setting operation. - -For power operation the camera is connected through a flexible shaft -with a wind driven propeller (Figs. 50, 83 and 84). The locking pin is -now moved over from the toothed wheel to the lever arm, so that the -rotation of the worm driving the large toothed wheel forces the lever -through its plate changing motion. To prevent repetition, a part of the -periphery of the toothed wheel is cut out, so that it stops when its -cycle is run. When the Bowden wire actuates the shutter release it -forces the toothed wheel around into engagement (aided by one spring -tooth) and so starts the cycle once more. - -When connected with the air propeller the worm is rotated continuously. -Other sources of power—an electric motor, for instance—can be attached -through the same kind of flexible shaft. If an electric motor is -employed it may be run continuously or it may be operated with an -insulated sector introduced into the large toothed wheel so that the -electric circuit is broken and the motor stops until the wheel is once -more forced around by the exposing lever. - -_Faults of the L camera._ The L camera was the mainstay of the English -Air Service. In fact for the last two years of the war it was -practically the only camera the English used, and they thought highly of -it. It is, of course, subject to the limitation of small plate size and -short focus lens. It is in many ways an inconvenient camera to handle. -For instance, the upper magazine cannot be closed or removed until all -the plates are passed through. Its dependence upon gravity for the plate -changing operation is a fundamental weakness, responsible for its -frequent tendency to jam in the air. Experience made the English -observers very expert in relieving these jams. Sometimes they would turn -the propeller backward (mounting it in an accessible position to provide -for this contingency), sometimes they would shake or thump the camera. -But while these makeshifts would serve to secure pictures—the chief -object, of course, of the photographic service—they can scarcely be said -to render the camera satisfactory. - -Moreover, the propeller drive has not been universally approved, as it -furnishes an additional mechanism to make trouble. Since it is not -feasible to change from power to hand operation while in the air, the -camera is put out of commission whenever the propeller or shaft is -disabled. Bowden-wire controls for both plate changing lever and shutter -release were common in the British service, which considered the extra -operation or the extra muscular exertion unimportant when compared with -the greater assurance of reliable action. - -=The English LB and BM Cameras.=—During the closing months of the war an -improved L type camera was constructed, the LB. This differs from the L -in a number of detail changes, dictated by experience. The shutter is -now made removable and self-capping. Pivoted lugs are provided to hold -the exposed plate horizontal until the very instant it drops, in an -effort to prevent jams caused by the plates piling up at an angle in the -receiving magazine. The chief addition, however, is the provision of -several interchangeable cones and cylinders, for carrying lenses of -focal lengths from 4 to 20 inches. Fig. 95 shows the LB with 20 inch -lens cylinder mounted on a bell crank support in the camera bay of an -English plane. - -The BM camera is but a larger edition of the LB, for 18 × 24 centimeter -plates. It also carries several interchangeable lens cones. - -=The American model deRam camera.=—The rotary changing box devised by -Lieutenant deRam of the French army and incorporated in his entirely -automatic plate camera, has been adapted by the American Air Service to -a very successful semi-automatic camera. Fig. 52 shows the principle of -this changing box. The pile of fifty plates, each in its sheath, is -carried in a rectangular box open at top and bottom. The lower plate -next the focal-plane shutter is first exposed; the pile then rotates -about a horizontal axis through a complete turn. When the exposed plate -arrives in a vertical position it is allowed to drop off, by the opening -of cam actuated fingers, and lodges against the side of the enclosing -camera box proper. Still further along in the cycle the plate is thrown -off from its lodging place into a “scoop” on the top of the rotating -container and laid on the top of the plate pile. Meanwhile the curtain -of the focal-plane shutter winds up, at the same time that it is -depressed out of the way of the revolving plate container. Although the -plate changing operation depends on gravity, it nevertheless functions -satisfactorily up to 30 degrees from the vertical. - -The shutter in this model is the variable-aperture fixed-tension type, -adjusted by pivoted idlers (Fig. 28). In the exposing position it runs -within three millimeters of the plate surface, and is therefore of high -efficiency for all openings. Capping during the operation of setting is -performed by flaps at the bottom of the camera body. Interchangeable -cones are supplied for lenses of various focal lengths. - -For hand operation the changing box is turned over by means of a handle, -which rotates four times for the complete cycle (Fig. 90). For -semi-automatic operation an additional mechanism is provided on the side -of the rectangular camera body, copied with some necessary modifications -after the L camera power drive. From the observer's standpoint the -operation of the whole camera is the same as in the L camera, with the -important exception that power operation in no way interferes with hand -operation. Indeed, the hand can help out if the power flags or fails. - -[Illustration: - - FIG. 52.—Diagram of automatic plate camera movements.] - -This camera is most satisfactorily driven by a 12 volt ⅒ HP electric -motor working through a flexible shaft attached to a swivel connection -at the front of the semi-automatic drive box. A change once every four -to five seconds is possible, but greater speed is apt to throw the -changing plate too violently for safety. - -The chief practical objection to this camera is its bulk. Its great -height makes it impractical for many planes. Its weight of nearly a -hundred pounds is a formidable load for a plane to carry, but this is no -more and probably less than that of any other camera when taken up with -the same number of plates in magazines. The price paid for economizing -in magazine weight is that the whole camera body, excluding the lens -cone, must be carried to and from the plane for both loading and -unloading. - - - - - CHAPTER X - AUTOMATIC AERIAL PLATE CAMERAS - - -=General Characteristics.=—The ideal in the automatic plate camera is to -provide a mechanism which will not only change the plates and set the -shutter, as does the semi-automatic, but make the exposures as well, at -regular intervals under the control of the operator. Such a wholly -automatic camera would leave the observer entirely free for other -activities than photography and it is to meet this tactically desirable -aim that the war-time striving for automatic cameras was due. - -It is obvious that the one essential difference between the automatic -and semi-automatic types lies in the self-contained exposing mechanism -with its device for the timing of the exposures. There is no difficulty -in arranging for the driving power to trip the shutter, but it is no -easy matter to design apparatus which will space the exposures equally, -and at the same time permit of a variation of the interval. It is indeed -the crux of the problem of automatic camera design to provide for the -easy and certain variation of the interval from the two or three seconds -demanded for low stereoscopic views to the minute or more that high -altitude wide angle mapping may permit. This problem is one intimately -bound up with the question of means of power drive and its regulation, -and will be treated in part in that connection. It is to be noted, -however, that there are in general two modes of exposure interval -regulation. One is by variation in the speed at which the whole camera -mechanism is driven. The other is by the mere addition to a -semi-automatic camera of a time controlled release which affects in no -way the speed of the plate changing operation. In many respects the -latter is the best way to make an automatic camera. - -While the advantages of automatic cameras are great it must not be -overlooked that a camera which can only be operated automatically is of -limited usefulness. It is not suited for “spotting” at any definite -instant, as, for illustration, at the moment of explosion of a bomb. It -should, therefore, be the aim of the automatic camera designer to so -build the apparatus that it can, at will, be used semi-automatically. In -addition, to meet the contingency of any break-down in the source of -power, the camera should be capable of hand operation, as in the case of -the American semi-automatic deRam. In short, the automatic camera should -not be a separate and different type; it should merely have an -additional method of operation. - -Certain desirable mechanical features of all aerial cameras have already -been enumerated. Some of these may be repeated here with the addition of -others peculiar to automatic cameras. As a general caution, mechanical -motions depending on gravity or on springs should be avoided. Movements -adversely affected by low temperatures (20 to 30 degrees below zero, -Centigrade), are unsuitable. All adjustments called for in the air must -be operable by distance controls whose parts are large, rugged, and not -dependent on sound or delicate touch for their correct setting. The -center of gravity of the camera should not change during operation -(important in connection with the problem of suspension). The camera -should work in the oblique as well as in the vertical position. The -power required for operation must not exceed that available on the -plane. Electrical apparatus, for instance, should not demand more than -100 watts. - -Any devices which diminish the weight of the camera are particularly -desirable in automatic plate cameras, because of the large number of -exposures which such cameras encourage. For instance, if the plates -could be handled without placing them in metal sheaths we should gain a -substantial reduction in weight (the sheaths weigh nearly as much as the -plates) as well as in the time necessary for handling. - -=The Brock Automatic Plate Camera.=—This camera is somewhat similar to -the same designer's film camera, both in shape, in size, and in its -employment of a heavy spring motor for the driving power. It uses 4 × 5 -inch plates, and carries a 10 to 12 inch lens. - -The plate-changing operation is unique. As shown diagrammatically in -Fig. 52, the unexposed plates are carried in a magazine on top of the -camera, the exposed ones in a magazine inserted in the body of the -camera, directly below the unexposed magazine. The bottom plate of the -exposed pile drops into a sliding frame and is carried along the top of -the camera to the exposing position. After exposure, the plate is -carried back and drops into the receiving magazine. In order for the -plate to fall only the proper distance at each stage of the cycle, -special plate sheaths are necessary. These are cut away to form edge -patterns which clear or engage control fingers so as to ride or fall -through the sliding frame as required. - -The camera is entirely automatic in operation. Regulation of the -exposure interval is by a special form of variable length escapement -controlled through a Bowden wire, in a manner parallel to that in the -Brock film camera, described elsewhere. These plate cameras were never -produced in quantity. - -=Folmer 13 × 18 Centimeter Automatic Camera.=—This camera, also never -manufactured in quantity, is shown in Fig. 53, and a sketch of its -manner of operation is included in the _ensemble_ of automatic camera -diagrams (Fig. 52). Its most distinctive feature is perhaps the use of a -two compartment magazine. This is similar in form to the one already -described in connection with the hand-held cameras, but larger, to hold -eighteen 13 × 18 centimeter plates. The unexposed plates are placed in -one compartment, and after exposure are shifted to the other. The -transfer is effected by the motion of a rack, which is part of the -magazine and which is driven by a toothed pinion, also part of the -magazine, which in turn engages in a toothed wheel projecting upward -from the camera body. This toothed wheel is turned first in one -direction and then in the other by an arrangement of gears and levers -driven by the source of power, which as shown in Fig. 53 is a wind -turbine connected through a flexible shaft. Operation is either -automatic or semi-automatic as desired, and the camera can be put -through its cycle by hand if necessary. - -[Illustration: - - FIG. 53.—Folmer 13 × 18 centimeter automatic and semi-automatic plate - camera.] - -[Illustration: - - FIG. 54.—French model deRam automatic plate camera.] - -As with several other designs, the completion of the working model of -this camera occurred after agreements had been reached by the Allies, as -to plate size, standard lens cones, and other features, not easily -incorporated in it, thus making manufacture inadvisable. The validity of -the design for peace-time work is, of course, not affected by this fact. - -=The deRam Camera.=—The only completely automatic plate camera actually -produced commercially before the end of hostilities was the French model -deRam (Fig. 54). Its plate-changing action has already been described in -connection with the American semi-automatic model (Figs. 52, 90 and 91). -It differs from the American model in the shutter, which is of the -self-capping variety, carried on a rising and falling frame; and in the -exposing mechanism. The latter embodies a clutch whose point of -attachment to a uniformly rotating disc in the camera is governed -through a Bowden wire, whereby the interval between the plate-changing -operation and the shutter release is varied. The intervals are indicated -by figures on the dial to which the observer's end of the Bowden wire is -attached. The source of power for the camera is a constant speed -propeller. Complete semi-automatic operation is not possible, as an -interval of 1 to 2 seconds elapses between the time a single exposure is -called for and its occurrence. No arrangement is provided for hand -operation. - -It will be noted that while this camera is a true automatic apparatus it -does not meet even a majority of the requirements listed above as found -desirable by experience. There exists a great opportunity for designing -and developing an entirely satisfactory automatic plate camera—provided -it is agreed that anything more than semi-automatic operation is ever -advisable for plates. - - - - - CHAPTER XI - AERIAL FILM CAMERAS - - -The weight of the glass and the sheaths in the plate camera forms its -most serious drawback. This weight must be reckoned at least three -quarters of a pound for each 18 × 24 centimeter plate. Consequently, -with the use of these large plates, and with the demands for ever -increasing numbers of pictures to be taken on long reconnaissance -flights, a serious conflict arises between the weight of the -photographic equipment and the carrying capacity of the plane. Among -plate cameras probably the most economical in weight is the deRam. It -carries fifty 18 × 24 centimeter plates, and has a total weight of -approximately 100 pounds. An advance to 100 or 200 plates—not feasible -in the deRam construction—even if we assume the lightest possible -magazines, would bring the weight of camera and plates to 150 or 200 -pounds, which would be detrimental to the balance and would seriously -infringe on the fuel carrying capacity and ceiling of any ordinary -reconnaissance plane. - -Early and persistent attention was therefore paid to the possibilities -of celluloid film in rolls, as used so widely in hand cameras and in -moving picture work. The two great advantages of film would be its -practically negligible weight (approximately one-tenth that of plates, -not including sheaths) and its small bulk, which would permit the -greatest freedom in the development of entirely automatic cameras to -make exposures by the hundreds instead of by the dozens. Certain -disadvantages were foreseen at the outset: the difficulty of holding the -film flat and immune from vibration in the larger sizes; the difficulty -of quickly developing and drying large rolls; the question whether as -good speed or color sensitiveness could be obtained in sensitive -emulsions when flowed on a celluloid base as on glass. Early trials -revealed a further problem to solve: how to eliminate the discharge of -static electricity occurring at high altitudes, especially when the -weather is cold. - -As far as camera construction is concerned the chief problems are to -hold the film flat, and to eliminate static. - -=Methods of Holding Film Flat.=—Several means have been proposed and -used for holding the film flat. Disregarding mere pressure guides at the -side, which are suitable only for small area films (up to 4 × 5 inch), -the successful means have taken three forms: _pressure of a glass -plate_, _pressure of the shutter curtain_, and _suction_. A glass -pressure plate can be used in either of two ways; the film may be in -continuous contact with it or may be pressed against its surface only at -the moment of exposure. The advantage of this first method lies solely -in its mechanical simplicity; its disadvantage in the likelihood of -scratches or pressure markings on the film. Where a glass plate is used -there is always the chance of a dust or dirt film accumulating, or of -the condensation of moisture, to impair the quality of the negative. -There is, moreover, an inevitable loss of light (about 10%), together -with some slight distortion, due to the bending of the marginal oblique -rays through the thickness of the glass. In cases where a filter would -normally be employed, the loss of light is minimized by using yellow -glass for the plate, so that it serves for filter and film holder as -well. - -_Pressure of the shutter curtain_ is utilized in the Duchatellier film -camera by furnishing the edges of the curtain aperture with heavy velvet -strips, whose light and gentle pressure during the passage of the -shutter holds the film against a metal back. In many ways this is the -simplest film-holding device; it occasions no loss of light, and needs -no mechanical movements or external accessories, such as are called for -in the suction devices next described. There is always danger of -markings on the film, if the velvet is not of the right thickness and -softness, and the operation and speed control of the shutter are -necessarily complicated by the additional frictional load. - -_Suction_ of the film against a perforated back plate has been found a -very successful means of securing flatness. Suction at the moment of -exposure may be produced by the action of a bellows, which has been -compressed beforehand by the camera-driving mechanism. Continuous -suction can be produced either by a continuously driven pump, or by a -Venturi tube placed outside the fuselage. The Venturi tube (Fig. 55) -consists of a pipe built up of two cones, placed vertex to vertex, to -form a constriction. When air is forced through this at high velocity -suction is produced in a small diameter tube taken off at the -constriction. A suction of two centimeters of mercury, acting through -holes about one centimeter equidistant from each other in the back -plate, has been found adequate to hold flat a film 18 × 24 centimeters. - -One merit of suction applied only at the moment of exposure is that the -film-driving mechanism does not have to work against the drag of the -suction. Continuous suction, on the other hand, gives a longer -opportunity for flattening out kinks in the celluloid, and easily -permits movement of the film during the exposure, either for the purpose -of permitting a longer exposure or for the purpose of preventing -distortion due to the focal-plane shutter. A disadvantage of continuous -suction is the production of minute scratches on the celluloid surface -as it drags over the suction plate. These are ordinarily too small to -cause trouble, but may show up when printing is done in an enlarging -camera. - -[Illustration: - - FIG. 55.—Venturi tube on side of plane.] - -=Static discharges= are produced by the friction of the celluloid -against the pressure back or other surfaces with which it comes into -contact. They show in the developed film as branching tree-like streaks -(Fig. 56) and in cold dry weather may be numerous enough to ruin a -picture. The discharges are noticeably less frequent with film coated on -the back with gelatine (“N.C.”), but the extra gelatine surface is -extremely undesirable. When handled by developing machines, as large -rolls must be, this back gelatine surface becomes scratched and bruised -in a serious manner. Plain unbacked film is much to be preferred if the -static can be obviated. - -To avoid static, it is necessary to provide for the immediate -dissipation of all acquired electrical charges. Experiments made by the -United States Air Service have shown that nothing is so good as rather -rough cloth, thoroughly impregnated with graphite, held in close contact -with the celluloid during as great a portion of its travel as possible. -In the United States Air Service film camera which uses suction through -a perforated back plate, the plate has been covered with thin graphited -cloth, and similar cloth sheets are pressed against the film rolls by -sheets of spring metal (Fig. 65). In cameras with this equipment no -trouble has been experienced with static. - -[Illustration: - - FIG. 56.—Print from film camera negative, showing static discharge, - and (upper left-hand corner) record of altitude and compass - direction made by Williamson film camera auxiliary lens (Fig. 58).] - -=Representative Film Cameras.=—_The English F type_ (Williamson). This -is one of the earliest cameras designed for film, as is indicated by the -nature of the power drive, which presupposes that the camera is to be -carried on the outside of the fuselage. Its essential features are shown -in Figs. 57 and 58. It consists of a rectangular box with a cone at the -front on which is mounted a propeller, intended to be rotated by the -wind made by the motion of the plane. This drives, through a governor -controlled friction clutch, a train of gears which draws the (5 × 4 -inch) film across the focal plane, sets and exposes the shutter at -regular intervals. - -[Illustration: - - FIG. 57.—English type “F” (Williamson) automatic film camera.] - -Above the camera, supported on a tripod, are a compass and altimeter, -both recording on a single dial, illuminated from below by the light -reflected from a circular white disc painted on top of the camera. An -image of the dial is thrown on a corner of the film by a lens, whose -shutter is actuated in synchronism with the main focal-plane shutter. No -special means are provided for holding the film flat. Special film with -perforated edges is used. - -The camera was designed for mapping work on the Mesopotamian and other -fronts where no maps at all existed. - -_The Duchatellier_ camera is essentially a film magazine to fit on the -standard French deMaria camera bodies, of the 18 × 24 centimeter size. -In its simplest form it embodies a shutter (the regular focal-plane -shutter of the camera being removed) and a film-moving mechanism, both -actuated by a single motion of the hand. Automatic and semi-automatic -operation are accomplished by an auxiliary mechanism to which Bowden -wires from the hand lever are attached. The motive power is an air -propeller. Variation of speed is obtained by changing the point of -contact of a roller on a friction disc, the disc being directly -connected to the propeller shaft, the roller to the camera drive shaft. - -[Illustration: - - FIG. 58.—Interior of type “F” camera, showing lens for photographing - compass and altitude readings.] - -The most distinctive features of the Duchatellier camera is its use of -the focal-plane shutter to hold the film flat during the exposure. As -already explained, this is accomplished by pressure, velvet strips on -the shutter edges keeping the film close against the back plate. The -return of the shutter curtain to the “set” position is accomplished by -locking it to the film by perforating points, so that it is pulled -across as the film is wound. This introduces between each pair of -pictures a strip of tremendous over exposure, as wide as the curtain -opening. A fixed-aperture variable-tension shutter is used. The magazine -carries a roll of film long enough for 200 exposures, feeding the long -way of the picture. When film needs to be changed in the air, this is -done by changing the entire magazine, including its shutter. - -[Illustration: - - FIG. 59.—G. E. M. automatic film camera.] - -_The G. E. M. camera_ (Fig. 59) is a very light self-contained -clock-work-drive camera taking 36 pictures six inches square. The film -is unrolled from a small-diameter feeding roller on to a large-diameter -receiving roller to which the driving mechanism is attached. By this -means approximately equal spacing of pictures on the film is assured. -The film is held flat by continuous contact with a glass plate, which is -made of yellow glass, so that it serves at the same time as a color -filter. The lens—of 8 to 12 inch focus—is equipped with a single speed -between-the-lens shutter. The operation of the camera is entirely -automatic. The interval between pictures is controlled by varying the -clock-work speed, through a lever on the outside of the camera box. -Protection of the camera from vibration is sought by supporting it on -four spring cushions mounted on a solid frame, to which the camera is -held by spiral springs attached to its sides. - -[Illustration: - - FIG. 60.—Brock automatic film camera.] - -_The Brock Film camera_ (Fig. 60) is an entirely automatic, very compact -self-contained camera, taking one hundred 4 × 5 inch pictures. The -motive power is clock-work, regulated in speed by an escapement -controlled by a flexible shaft carried to a dial which may be fastened -to the instrument board or to some other convenient part of the plane. -The lens is 6, 12, or 18 inch focus. The shutter is of the -fixed-aperture variable-tension type, of long travel, and with a flap -behind the lens for covering during the setting period. None of the -special means above described for holding the film flat are provided. A -metal plate resting on the back, and a flat metal frame in front with a -4 × 5 inch aperture, are considered sufficient check on the excursions -of the small-sized film. A ball bearing double pivoted frame serves to -support the camera in a pendulous manner, permitting it to assume a -vertical position after tilting. Damping of oscillations and vibration -is arranged for by two pneumatic dash pots. - -The _German film mapping camera_, shown in Fig. 61, is distinguished by -a number of special features. The size of the pictures, 6 × 24 -centimeters, is unusual. It has its advantages, however. Since the short -dimension is in the line of flight, the maximum width of field covered -by the lens is utilized (Fig. 17). This of course necessitates a larger -number of exposures to complete a strip, which is perhaps an added -advantage, since the narrower the individual pictures the better the -junctions will be, especially if large overlaps are made. This proved to -be the case with captured German mosaics. Difficulty is experienced in -making overlaps on a turn (Fig. 62), but this is not a vital objection. -The shutter has a fixed aperture, narrower at the center than at the -ends, to compensate for the falling off in illumination away from the -center of the lens. No safety flap is needed because the curtain moves -in opposite directions on successive exposures, thereby also -compensating for shutter distortion, as has already been discussed. -Shutter speed is controlled by varying the tension of the actuating -spring. - -[Illustration: - - FIG. 61.—German automatic film camera.] - -The camera is driven by an electric motor, connected to a set of gears, -whose shifting provides for speed variation. The film is moved by rubber -rollers which are cut away for part of the circumference, allowing the -film to stand still until they bite again. A yellow glass pressure plate -holds the film during the exposure and serves as color filter also (Fig. -63). An electric heater is provided near the shutter, as in all the -later German cameras. - -[Illustration: - - FIG. 62.—Method of joining and printing film from German camera.] - -_United States Air Service automatic film camera_—_Type K_ (Figs. 64, -65, 92, 93, 98, 99). This is an entirely automatic camera, manufactured -by the Folmer and Schwing Division of the Eastman Kodak Co., taking 100 -pictures of 18 × 24 centimeter size at one loading. As with all the -American cameras of this size, it uses the standard lens cones of any -desired focal length. The camera proper consists of a compact chamber in -which the film rollers are carried at each end forward of the focal -plane, the shutter lying between. In consequence of this arrangement the -vertical depth of the camera is the absolute minimum—short of decreasing -the length of the optical path by mirror arrangements—making it possible -to suspend the camera diagonally in the American and British planes, for -taking oblique pictures. - -Flatness of the film is secured by a suction plate covered with -graphited cloth and connected with a Venturi tube. The top cover is -removed for re-loading. The shutters on the first cameras of this type -are of the variable-tension fixed-aperture design, though later ones -have the variable-aperture curtain controlled by an idler, as used in -the American deRam. An auxiliary curtain shutter serves to cap the true -shutter during setting. - -The operation of the film driving mechanism is comparatively simple. It -consists of a train of gears, driven by a flexible revolving shaft -attached to some separate source of power capable of speed variation. -The action of the gears is to move the film, set the shutter and then -expose it; in the earlier cameras with the film continuously moving. In -the first cameras constructed the space between the pictures varies as -the film rolls up, due to the increasing diameter of the roll. In later -cameras the film roller is disengaged from the gears just before the -shutter is tripped, so that the film stands still during the exposure, -and is then re-engaged at a new point on a ratchet wheel governed by the -diameter of the receiving roll, whereby the pictures are equally spaced. -In all the cameras, punch marks made at the time of exposure enable the -limits of the picture to be detected in the dark room by touch. - -[Illustration: - - FIG. 63.—Film winding and exposing mechanism in German film camera.] - -Variable speed is arranged for in any one of several ways. For -peace-time uses a turbine attached to the side of the plane is simple -and positive, and, provided it is made of sufficient size—which is not -the case with the one shown in the Figure—will give adequate speed -regulation upon varying the aperture through which the air enters. The -Venturi tube may be carried upon the same mount, or a small rotary pump -can be attached on the same shaft. Where the high wind resistance of the -turbine is an objection the camera is driven electrically, by a motor -acting through the intermediary of a variable speed control described in -the next chapter (Fig. 68). - -The camera weighs complete about forty pounds, and the film rolls about -four pounds. The latter can be changed in the air without great -difficulty provided the camera is mounted accessibly and so that the top -may be opened. - - - - - CHAPTER XII - MOTIVE POWER FOR AERIAL CAMERAS - - -As long as circumstances permit, hand operation still remains the most -reliable and satisfactory method of driving a camera. It is always -available, can be applied to just the amount desired, and at the time -and place needed. For instance, in a magazine of the Gaumont type (Fig. -40), what is needed is the periodic application of a very considerable -force rather quickly, and while this can be done quite simply by hand, -no mechanism has even been attempted to go through this same operation -automatically. Instead, the fundamental design of automatic magazines -has been made along other lines calculated to utilize smaller forces -more steadily applied. - -It must be granted, however, that for war planes, and particularly for -single seaters, cameras should be available which are capable of -operating semi-automatically or automatically. This necessarily means -the employment of artificial power, whose generation, transmission to -the camera and control as to speed present a mechanical problem of no -small difficulty. - -=Available Sources of Power.=—The sources from which power may be drawn -on the plane are four, although the various combinations of these -present a large number of alternative approaches to the problem. These -sources are: - - 1. The engine of the plane. - 2. Wind motors. - 3. Spring motors. - 4. Electric motors. - -These may first be considered largely from the descriptive standpoint, -leaving questions of performance and efficiency for separate treatment. - -Power may be derived directly from the engine through a flexible shaft, -similar to that used for the revolution counter. This source of power is -inherently the most direct and efficient, since the engine is the seat -of all the lifting and driving energy of the plane. There is no loss -through transformation into other forms of energy, such as electrical; -or by the use of more or less inefficient intermediary apparatus, such -as wind propellers. Against the direct drive of the camera from the -engine may, however, be urged that the usual distance between engine and -camera is too great for reliable mechanical connection, as by flexible -shafting. Objections also arise from the standpoint of speed. This -cannot be controlled by the camera operator; and varies over too wide a -range, as the engine changes from idling to full speed, to fit it for -automatic camera operation. The first objection may be met by that -combination of methods of power drive which consists in transmitting the -power electrically; that is, by letting the engine operate a generator -from which cables run to a motor close to the camera. This method, of -course, sacrifices efficiency, and it breaks down when the engine speed -drops below the speed necessary to generate the requisite voltage. This -defect may in turn be met by floating in storage batteries, which brings -up the whole question of electrical drive, to be treated presently. -While use of the engine for direct drive or for generating electric -current has not been adopted in the American service, it is known that -some German planes were supplied with electric current in this way. - -Coming next to the wind motors, these possess one very great merit: they -utilize a motive power that is always present as long as the plane is in -motion through the air. On the other hand, the process of using the main -propeller of the plane to pull another smaller propeller through the air -appears a roundabout way to utilize the driving power of the airplane -engine. Yet on the whole it is probable that some form of propeller or -wind turbine is the simplest and most convenient device we have for the -operation of airplane auxiliaries. As long as the amount of power -required is small, such inefficiency as is inherent in its use is offset -by its convenience and reliability. An advantage of the propeller is -that its speed is almost directly proportional to that of the plane -through the air, a desirable feature in automatic cameras provided the -proportionality is under control. Yet it is just in this matter of -varying the speed at will that the propeller presents difficulties, to -be met only by additional mechanisms for gearing down or governing. -Propellers have the practical disadvantage that they present an easily -bent or broken projection to the body of the plane (Figs. 83 and 84). -The strength of small propellers for operating auxiliaries is never so -much in question with reference to their resistance to whirling and -thrust of air as it is to their ability to withstand the inevitable -knocks and careless handling that will fall to their lot. The propeller -bracket is just what the pilot is looking for to scrape the mud off his -boots before climbing in. - -The wind turbine has the advantage over the propeller that its speed can -be varied rather simply by exposing more or less of its face to the -wind. A turbine fitted with an adjustable aperture for admitting the -wind is shown in Fig. 64, in connection with the type K automatic film -camera. The turbine has the advantage of being compact and lying close -against the body of the plane. In the form figured, altogether too much -head resistance is offered—just as much for low as for high speeds—but -with proper design this need not be the case. It is, moreover, quite too -small to give the needed speed regulation, as it only begins to operate -near its full opening. - -[Illustration: - - FIG. 64.—U. S. Type “K” (Folmer) automatic film camera, with wind - turbine and Venturi tube.] - -[Illustration: - - FIG. 65.—Type “K” camera, open, showing suction plate.] - -Spring motors have the very real advantage that by their use the camera -can be made entirely self contained. The simplest application of the -spring motor would be to the semi-automatic camera, where no close -regulation of speed is required. In such a camera the operation of -exposing the shutter would release the spring, which would then change -the plate or film and re-set the shutter, repeating this operation as -long as the spring retained sufficient tension. Small film hand-cameras -of this type, using self-setting between-the-lens shutters, have been -designed, though not for aerial work. The possibilities of using springs -as motive power in semi-automatic cameras have not apparently been -seriously considered. - -When a spring motor is used for automatic camera operation it at once -becomes necessary to add to the motor an elaborate clock mechanism for -controlling and regulating its speed of action. Springs are much better -fitted for giving power by quick release of their tension than by slow -release, and the necessary clock mechanisms for their regulation become -very heavy, as well as complicated and delicate, when they are made -large enough to do any real work. For their repair they require the -services of clock makers rather than the usual more available kind of -mechanic. - -Coming next to electric motors, we meet with a source of power of very -great flexibility both in its derivation and in its application. If a -source of electric current is already provided for heating and lighting -as it is on the fully equipped military plane, and if it has sufficient -capacity to handle the camera, its use is rather clearly indicated, -irrespective of how efficiently or by what method it is produced. -Especially is this the case, from the standpoint of economy and -simplicity, if a propeller-driven generator is the source of current, -and the alternative power drive is an additional propeller for the -camera. If, on the other hand, the camera must have its own source of -electric power, the advantages and disadvantages must be closely -scrutinized. In this case either a generator must be provided, or resort -be made to storage batteries, or a combination of the two. - -Ruling out a special propeller-driven generator, we are left with either -the generator driven from the engine or the storage battery. Inasmuch as -storage batteries are practically indispensable with generators, in -order to maintain the voltage constant at all speeds, it is on the whole -advisable to rely upon batteries alone. An advantage of their use is -that the power plant is entirely within the plane: All projections such -as propellers are avoided. Another merit is that the power is drawn upon -only as needed. Against storage batteries is their weight, the need for -frequent charging, and their loss of efficiency at low temperatures—a -loss so serious with those of the Edison form as to preclude their use. - -When once the source of electrical energy is decided upon, its method of -application needs to be considered. Here we meet at once the peculiar -merit of electrical energy, namely, the ease and convenience with which -it may be transmitted. All we need is a pair of wires, led to any part -of the plane by any convenient route and connected by simple binding -posts. It may with equal ease be turned on or off by merely making or -breaking a contact with a switch. For operating semi-automatic cameras -this feature may be utilized in the interest of economy, if the power is -automatically turned off as soon as the plate-changing operation is -finished. Exceptionally reliable make and break contacts are necessary -to insure the success of this latter scheme. - -Two methods of transforming the supply of electrical energy into -mechanical motion are available. The first is by the use of a solenoid -and plunger. This is a device practically restricted to semi-automatic -cameras, in which the operation consists of a straight to-and-fro -motion, initiated at the will of the operator. It has been used little -if at all. The second motion is the continuous rotary one secured by the -use of an electric motor. This motion is the most practical one for the -continuous operation of any mechanism, but on the other hand requires -that the imposed load be reasonably uniform at all times through the -cycle of operations. Assuming that the camera mechanism is of this -character, the motor may be attached directly to the camera, or if it -must be so large as to cause danger by vibration, it may be connected -through a flexible shaft. This use of an electric motor is very -practical for semi-automatic cameras such as the “L” or the American -deRam, in planes supplied with a suitable source of current. - -When it comes to entirely automatic cameras, where uniform and -regulatable speed is required, as in making overlapping pictures for -mapping, the electrical drive is not so convenient. The shunt-wound -motor runs at nearly constant speed, while the series-wound motor in -which the speed can be regulated by the interposition of resistance, has -nothing like a sufficient range of variation for the purpose (at least -five to one is imperative) before it fails to carry the load. Hence we -must either incorporate in the camera some mechanism for varying the -interval between exposures while the speed of the motor remains -constant, or introduce an auxiliary device to effect the required -transformation in speed. If we do use an auxiliary device the train of -apparatus, consisting of battery (or generator), motor, speed control -and camera, is altogether too long; it is apt to cause annoying delays -in connecting up in an emergency, and it offers an excessive number of -chances for break-down. - -=Performance and Efficiency Data.=—The first step in deciding upon -methods of power drive, and indeed in deciding whether power drive is -feasible at all, is to assemble definite data as to the power required -to drive representative cameras. Approximate figures for some of the -cameras described in previous chapters are: - - L camera, 26 watts, - deRam, 60 watts, - “K” film, 30 watts. - -These requirements—not exceeding ⅒ horse power—are insignificant in -comparison with the total of 100 to 400 horse power available for all -purposes from the plane's engine. - -_Propeller characteristics._ Data on the performance of small propellers -are somewhat meagre. However, the results of the rather extensive -researches on large ones, suitable for driving planes, may be applied, -with proper reservations, to give a fair guide to the study of the -application of small propellers for driving plane auxiliaries. - -The first factor to be considered is the thrust or _head resistance_ -offered by a propeller to motion through the air. This varies as the -_square of the velocity_, as the _density of the medium_, and as the -_area of the body_ projected normally to the wind, the formula being - - _T_ = _cdaV_^2 - -where _T_ = thrust, _d_ = density, _a_ = area, _V_ = velocity. Data on -the L camera propeller are shown in Fig. 66, where its thrust both when -free and when loaded with the camera is given, as well as that of a -solid disc of the same diameter as the propeller. For this propeller, -which is double-bladed, and six inches in diameter, _cda_ = .000275 with -the load on. The total thrust amounts to only about three pounds when -the plane velocity is 100 miles per hour. The head resistance of the -whole plane is a matter of hundreds of pounds, so that the propeller -resistance is quite negligible. - -[Illustration: - - FIG. 66.—Wind propeller data.] - -The next factor is the speed of revolution of the propeller, expressed -in revolutions per minute. This varies with the design—the number of -blades, their area, and pitch. For a given design the speed of -revolution is _directly proportional to the speed of motion through the -air_, and to _the density of the air_. Representative data for the L -camera propeller are shown in Fig. 67. It will be noted that the speed -goes up to 8000 for 120 miles per hour air speed. This illustrates the -necessity for great strength to withstand centrifugal force. Propellers -should be constructed of tough material, and subjected to whirling tests -up to speeds considerably in excess of any the plane will attain in any -maneuver. At low speeds the linear relationship fails, as a critical -velocity is reached—about 3500 r. p. m. for this propeller—where it -refuses to turn. - -[Illustration: - - FIG. 67.—Relation between air speed and propeller revolutions.] - -The fact that the speed of the propeller depends on the density of the -air has an interesting corollary, which is that a propeller adequate at -low altitudes will fail at high ones. The density of the air varies with -altitude according to the following figures: - - At 3000 meters, 72 per cent. of sea level - 5000 meters, 59 per cent. of sea level - 6000 meters, 52 per cent. of sea level - -If we take the r. p. m. at 90 miles per hour at sea level as 6000, then -at the above altitudes the speeds will be 4300, 3500, and 3000, -respectively. The last figure is below that for which this size of -propeller stalls with its normal load, as noted in the last paragraph. -Consequently, if flying is to be done at these altitudes a larger -propeller must be carried, which will still deliver enough power at the -lower density. - -The next factor to be considered is the _power furnished by the -propeller_. As a representative figure may be quoted the performance of -the L propeller. This gives 27 watts at 3600 revolutions per minute (56 -miles per hour). From this figure the performance of other propellers -may be deduced from the basic laws, which are: that the _power varies as -the density of the medium_ and as the _cube of the velocity_ (assuming -constant efficiency). Since the power delivered by the six inch diameter -L propeller is already adequate at 60 miles per hour, the necessary -dimension to function satisfactorily at 100 miles per hour would need to -be only a little more than three inches, except for the desirability of -a safety factor for high altitudes and low air densities. - -The _efficiency_ of the propeller is defined by the relation— - - power delivered by the propeller - Efficiency = ———————————————————————————————— - power supplied to the propeller - -The denominator of this fraction is the thrust times the velocity, for -which the curves of Fig. 66 supply us data for the L propeller. Using -the figures 3600 r. p. m., 56 miles per hour, and 27 watts, we find the -efficiency to be about 50 per cent. This increases with the velocity, -with a possible upper limit of 70 to 80 per cent. Since the main -propeller of the plane is not over 80 per cent. efficient we have at -most an efficiency of 64 per cent. in using a propeller drive, as -compared with taking the power directly off the engine. - -In considering the use of _spring and clock-work motors_ we meet at once -with the problem of comparing the effect on the performance of a plane -of a carried weight, as against a head resistance. The efficiency of a -spring motor is measured in terms of its weight, that of a propeller in -terms of its head resistance. The general answer to this question is -given by the relation that _a pound of dead weight is equivalent to ⅕ -pound head resistance_. - -In order to apply this relation to the study of spring motors for -driving cameras, data are necessary on the power delivery per pound -weight of such mechanisms. Such data are not easily accessible, largely -because clock-work has not generally been seriously considered as a -motive power for large apparatus. To arrive at an approximate figure we -may take the fact that in an 8 × 10 inch film camera designed by one of -the manufacturers who have utilized clock-work, the motor weighed 30 -pounds. This is equivalent to six pounds head resistance. Now the type -K, 18 × 24 centimeter film camera is operated, even with the addition of -a friction drive speed control, by means of the L camera propeller. As -shown in Fig. 66, at 100 miles per hour the head resistance of this -propeller is still less than three pounds. Consequently, it appears that -from the efficiency standpoint the clock mechanism is quite outclassed -by the wind propeller. - -Coming next to the _electric motors_, the L camera and the K are both -operated satisfactorily with a 1/20 horse power motor, weighing 6 -pounds. For the deRam a ⅒ horse power motor has been adopted. - -Taking up efficiency considerations, we have, if the current is supplied -by a generator from the engine, a transformation factor of 70 to 80 per -cent. from mechanical to electrical energy and a similar factor in using -a motor for the camera. When batteries are employed the matter of weight -_versus_ head resistance again arises. The batteries found most -satisfactory for operating the K and deRam cameras are of the six-cell -12 volt lead type. Their capacity is 40 ampère hours at three ampères or -36 at five ampères—more than is necessary for a single reconnaissance, -but a practical figure when economy of charging and replacement are -considered. The weight of this unit is 27 pounds. To this must be added -the weight of the motor—6 lbs.—making a total of 33 pounds, equivalent -to a head resistance of nearly 7 pounds. This is more than twice the -propeller head resistance invoked to do the same work. - -These considerations of efficiency have been gone into because they are -usual in studying any engineering problem and because of the insistent -demand from the plane designer that every ounce of weight and head -resistance be saved. Actually, as already stated, the load imposed by -any method of power drive is trivial in comparison with the whole load -of the plane. There is, however, an important reservation to be made, -which applies against clock-work and batteries: This is, that while the -equivalent head resistance of any camera motive power carried as dead -weight is small, its effect on _balance_ may not be so. While the use of -a propeller need not disturb the plane's balance, the weight of the -camera alone, without any driving apparatus, is already seriously -objected to on this score. The merely mechanical superiority of the -propeller as a source of motive power is on the whole rather marked. - -=Control of Camera Speed.=—In the semi-automatic camera the only control -required on the speed of the operating motor is at the upper and lower -limits. It must not go so fast as to anticipate the completion of any -steps in the cycle of camera operation, such as the fall of plates or -pawls into position, which would jam the camera. On the other hand, it -must not be so slow that pictures cannot be obtained with the requisite -overlap for maps or stereoscopic views. In the American deRam camera the -cycle of operations cannot safely be put through in less than four -seconds, a short enough interval for most purposes. It is also highly -desirable in the semi-automatic camera to have the motive power capable -of stopping completely. This saves wear and tear on both motor and -camera mechanism. - -In the automatic camera an extreme range of speed is called for by the -several problems of mapping, oblique photography, and the making of -stereoscopic views. For mapping alone, the shortest likely interval may -be taken as that required for work at approximately 1000 meters -altitude, for a plane speed of 150 kilometers per hour, which demands an -interval of six seconds with a ten inch lens on a 4 × 5 inch plate. For -vertical stereos at the same altitude and speed this interval is divided -by three, and low oblique stereos need even quicker operation. Hence a -range of from 1 to 30 pictures per minute should be provided for. This -requirement is difficult to meet with any simple mechanism. - -From the standpoint of simplicity in speed regulation the wind turbine -of adequate vane surface has much to recommend it. It is only necessary -to present more or less of its vane area to the wind in order to secure -a considerable range of speed. The method of doing this by a shutter -interposed in front is uneconomical, but it is probable that the design -can be so altered that more or less of the turbine is exposed beyond the -side of the plane, possibly by varying the angle, to secure the same -result without introducing useless head resistance. A serious practical -objection to the turbine lies in the large vane surface necessary to -give adequate power combined with proper speed variation. In the -automatic film camera (Type K) this area should be as much as 40 to 50 -square inches. - -The wind propeller does not lend itself at all well to speed variation. -It cannot be partially covered from the air stream, as can the turbine, -because of the resulting strain on its mount. A possible form of -variable speed propeller, one which, however, has not yet been -practically developed, is a propeller with controllable variable pitch. -If this could be made mechanically sound it would be well-suited for -camera operation. That such a propeller could be worked out is indicated -by the good performance of a constant speed propeller developed for -radio generators and used on the French deRam camera (Fig. 54). -Parenthetically, it maybe questioned whether a constant speed propeller -is really desirable with an airplane camera. What is required is not -exposures at a definite time interval—although most of the data are in -that form—but exposures at definite intervals with respect to the motion -of the plane, which practically means with reference to its air speed. -Rather than build a camera calculated to give exposures at intervals of -so many seconds when it is attached to a constant speed propeller, we -would do better to use a propeller which responds to the speed of the -plane, in conjunction with some form of tachometer to show the rate at -which exposures are being made. This in turn should be coördinated with -the indications of a proper camera-field indicating sight. - -One solution of the problem of speed control with a propeller of -practically fixed speed, is to use a governor and slip clutch as in the -English Type F film camera (Fig. 57). Here the propeller shaft and the -camera driving axle are connected by two friction discs. That on the -camera mechanism is forced against the other by a spiral spring, whose -tension is controlled by a ball governor. If the camera speed becomes -too high the governor reduces the tension on the spiral spring and the -discs slip over each other. The point where this slipping occurs is -determined by the position of the governor as a whole, and this is -controlled by a lever on top of the camera. - -Another speed control device, perhaps more positive but certainly more -complicated and wasteful of power, consists of a large flat disc, driven -by the propeller or electric motor, and from which the camera is driven -by a shaft from a smaller friction disc which may be pressed against any -point from the center to the periphery of the larger disc. The speed -range attainable in this way is limited only by the size of the large -disc. An application of this idea is shown in the speed control (Fig. -68), designed for the American Type K camera when operated on an -electric motor or on a simple propeller. The same idea is utilized in -the Duchatellier film camera, in connection with the constant speed -propeller already described. - -[Illustration: - - FIG. 68.—Friction disc speed control.] - -On the whole it is eminently desirable from the standpoint of power -operation that the automatic camera should embody its own means for -altering the interval between exposures, so that all the external -attachment needed is a single connection to a source of power either of -constant speed, as an electric motor, or of speed proportional to that -of the plane, as with a simple wind propeller. This makes the camera -largely independent of the nature of the power supply, whereas a camera -designed for a special variable speed device is of little use on a plane -where this is not available. - -=Transmission of Power to the Camera=—It has already been pointed out -that the ease of transmission of electrical energy makes it particularly -convenient for use in a plane. All other sources of power, except -clock-work incorporated in the camera, require flexible shafting, so -that the question of bearings and connections becomes a serious one, -especially when the shaft runs continuously for long periods at very -high speeds. - -The shafting found most suitable is the spirally wound form commonly -known as dental shafting. This must be encased in a smoothly fitting -sheath, flexible enough to permit of easy bends. The ends of the shaft -should be equipped with square or rectangular pins to fit into -corresponding slots in the motor and camera shafts. The ends of the -shaft casing may be fitted either to attach by bayonet joints or by -smoothly fitting screw collars. At the point of attachment to the camera -it is desirable to have some form of junction adjustable as to the -direction from which the shaft may be connected, so that it need be bent -as little as possible. A right angle bevel gear offers one means of -doing this. Bearings, such as those of the propeller, should be of the -ball variety, while heavy lubrication, such as vaseline, should be -freely used, both in the bearings and in the shaft casing. - -An important feature of any power drive system should be a safety -device, so that the power will race in case of any jam or stoppage in -the camera. This will often prevent serious damage through the breakage -of some relatively weak part of the camera mechanism on which the whole -force of the driving apparatus is suddenly thrown. The “L” camera -propeller is fitted with a spring friction clutch with the idea that if -the camera refuses to operate the propeller will slip instead of -wrenching the shaft to pieces. - - - - - CHAPTER XIII - CAMERA AUXILIARIES - - -=Distance Controls and Indicators.=—All operations connected with the -exposing and changing of plates (except the changing of whole magazines) -should be arranged for accomplishment at a distance. Other operations, -such as changing the shutter speed or the interval between exposures in -an automatic camera, which are usually done on the ground, may sometimes -be satisfactorily left for performance at the camera. Conditions of -extreme inaccessibility may, however, make it necessary to carry even -these controls to a distance. Indicators of the number of exposures -already made, and of the readiness of the camera for the next exposure, -may be attached to the camera, but often are more profitably placed at a -distance. Distance control and indication are especially necessary if -the pilot makes the exposures—a common English practice in two seaters, -and the only recourse in single seaters. - -When electric power is available, electrical distance control devices -are perhaps the simplest kind, as they transmit motive power without -displacing or jarring the camera. Solenoids suffice for the simple -pressing of releases or for counting mechanisms, while small service -motors may be utilized for operations involving more work. A standing -practical objection to electrical control lies in the necessity for -using contacts, which are apt to be uncertain under conditions that -involve vibration. - -_The Bowden wire_—a wire cable carried inside a heavy non-extensible but -flexible sheath—constitutes the most satisfactory mechanical means for -transmitting straight pulls. By means of “the Bowden” a pull may be -transmitted so as to be made entirely relative to two parts of the same -body, calling forth no tendency of the body as a whole to move. Thus in -the L camera shutter release (Fig. 50), the releasing lever with its -attached counter is several feet distant from the camera. If the plate -bearing the lever and sheath end is rigidly fastened down, the pressure -exerted on moving the lever acts between the lever and the end of the -sheath. This pressure passes immediately to the other end of the sheath, -while the pull on the wire is transmitted to its farther end on the -camera. In this way the conditions at the lever are reproduced, but with -the advantage that, due to the flexible cable and sheath, any vibration -of the lever support is damped out. - -Due to its stretching, there is a pretty definite limitation to the -feasible length of the Bowden wire. This length is about four feet. -Where according to English practice the pilot makes the exposure, a -considerably longer wire and sheath are called for. In this case the -effective length of the release is increased by giving the pilot a -second releasing lever, connected to the first by a rigid rod (Fig. 69). -The releasing lever, wire, and all mechanical parts of the Bowden -release should be made much stronger than would be indicated by bench -tests of the camera. In the air it is impossible to decide either by -sound or by delicacy of touch whether the mechanism has acted, so that -the observer is apt to pull much harder than necessary and to strain or -break the release if it is weak. - -The Bowden wire is used in the American service only for shutter -release. In the English service it has been used for plate changing with -the L camera. - -[Illustration: - - FIG. 69.—Bowden wire release in rear cockpit, with rod connected to - similar release for the pilot.] - -[Illustration: - - FIG. 70.—Bowden wire release with stop watch attached, for use in - timing for overlaps.] - -=Sights.=—In airplane photography the need for a finder or sight is -fully as great as in everyday work. A new condition, however, prevails, -for except with hand-held cameras, and even to some extent with them, -the operation of pointing the camera involves pointing the whole vehicle -that carries the camera. The pointing of airplane cameras is therefore -akin to the sighting of great guns. While the observer may perform the -actual operation of taking the picture, the responsibility for covering -the objective rests with the pilot. Teamwork counts equally with tools. -Airplane camera sights may accordingly be divided into two classes: -sights attached to the camera, for use principally with hand-held -apparatus, and sights attached to the plane, for the use of pilot, of -observer, or of both. - -=Sights for Hand-held Cameras.=—The simplest form of sight attached -directly to the camera is modeled on the _gun sight_, consisting of a -forward point or bead and a rear V. This sight of course serves merely -to place the objective in the center of the plate and gives no -indication of the size of field covered. Another simple sight of rather -better type is the _tube sight_—a metal tube of approximately one inch -diameter and three inches length, carrying at each end pairs of wires -crossed at right angles. The camera is in alignment when the front and -back cross wires both exactly match on the object to be photographed. -The best way to mount the cross-wires is with one pair turned through 45 -degrees with respect to the other, so that it is at once apparent which -is the front and which the rear pair (Figs. 31 and 39). - -_Sights to indicate the size of the field_ are usually less needed on -hand cameras than on fixed vertical cameras. Yet certain circumstances -make them most desirable, for instance in naval work where a complete -convoy must be included on the plate. A sight of this kind can be made -up of two wire or stamped metal rectangles, a large one in front and a -smaller one behind, of such relative sizes and separations that the true -camera field is outlined when the eye is placed in position to see the -two rectangles just cover each other. The dimensions should be so chosen -that the correct position of the eye is approximately its natural -location with respect to the camera when this is held in the hands in -the plane. It is usual to provide the rectangular sights with -cross-wires to indicate the center of the field. Alternative rear sights -are simple beads or peep-holes—the use of the bead assuming that the -camera is held at about the right distance from the eye for the -rectangle to indicate the field. The peep-sight is not a desirable form, -as it is hard to hold the camera as near the face as is necessary. These -various types of rectangle sights are well illustrated in the cameras -shown in Figs. 38, 40 and 186. They are all made so as to fold down flat -on the camera and to snap quickly open when needed. The springs to -support the sights must be fairly strong, and the surface presented to -the wind as small as possible. Wire frames give very little from the -pressure of the wind, but flat metal frames are apt to be bent back. - -The position of the sight on the camera is important. If the observer -can stand, or if he sits up well above the edge of the cockpit, the -conventional position of the sight on a pistol, namely, on top, is -unobjectionable. But if the observer sits very low, as he usually does, -then the sight should be on the bottom of the camera, thereby avoiding -any need for the observer to raise his head unduly into the slip stream. -Similarly, if the camera is used over the side for verticals, as it is -in flying boats, a sight on the top is impractical, since it requires -the observer to lean out dangerously far (Fig. 185). - -=Sights Attached to the Plane.=—Any of the sights just described can be -attached to cameras fixed in the plane, but they would be useless in the -positions ordinarily occupied by the camera. It has therefore become -common practice to attach the camera sight to some accessible part of -the plane. The most primitive method of sighting is merely to look -downward over the side—a method in general use to the very end of the -Great War. One step in advance of this is to mark a large inverted V on -the side, with its vertex at a point where the observer can place his -eye and so see the fore and aft extension of the field of view covered -by the camera. This kind of sight was common on the French “photo” -planes. On some of the English planes the tube sight was carried on the -outside of the cockpit. Any of the sights described can be carried on -the inside of the fuselage, provided a hole is cut in the floor. For -satisfactory sighting a hole in the floor is really necessary, as it -enables the terrain on both sides of the vertical to be seen. One -drawback to the simple hole, however, is that it cannot be made large -enough to show the whole field from the ordinary height of the -observer's eye, thus forcing him to bring his head down near the floor. -This difficulty is gotten over in a very beautiful way by the use of the -_negative lens sight_ shown diagrammatically in Fig. 71. - -[Illustration: - - FIG. 71.—Diagram of negative lens sight.] - -Let _F_{1}_ be the distance at which the edge of the hole (or a -rectangle marked on the lens) appears the size of the camera field (if -the hole is the size of the plate, _F_{1}_ is the focal length of the -camera lens). Let _F_{2}_ be the distance from the floor to the -observer's eye. What is desired is a concave lens which will diverge the -rays from their normal meeting point at _F_{1}_ to a new meeting point, -_F_{2}_. The focal length of lens required is given at once by the -simple lens formula— - - 1 1 1 - ——————— - ——————— = ——— - _F_{1}_ _F_{2}_ _F_ - -Thus if _F_{1}_ is 12 inches, and _F_{2}_ is 36 inches, _F_ will be 18 -inches. The lens is to be marked with a rectangle showing the shape and -size of the camera field, and a central mark such as a cross. An upper -rectangle, or a bead, or a pair of cross wires a few inches below the -lens, may be used for the other sight. For precision work the sight -above or below the lens should be adjustable in position, especially -where the camera suspension permits the camera to be adjusted for the -angle of incidence of the plane. - -A negative lens sight should be placed in the observer's cockpit, if he -takes the pictures, and also in the forward cockpit, so that the pilot -may be accurately guided in his part of the task. In addition, it is -advisable to place a negative lens well forward in the pilot's cockpit, -to enable him to see the country some distance ahead. The lenses should -be planoconcave with the flat side upward; otherwise, all the loose dirt -in the airplane settles in the middle of the concave depression. A -negative lens sight in a metal frame forming a completely self-contained -unit ready for mounting in the plane is shown in Figs. 72 and 73. - -=Devices for Recording Data on Plates.=—_Numbering devices._ The number -of the camera is impressed on negatives taken with the American L camera -through the agency of a small transparent corner of celluloid. It would -be entirely possible to incorporate a rotating disc which should turn by -the operation of plate changing and carry a series of numbers, so that -each exposure could be numbered serially. Numbering of individual plates -is more commonly done by holes, notches, or even numerals, in the turned -over portion of the sheaths, which are then recorded photographically -when a picture is taken (Fig. 75). The chief objection to this method is -the difficulty of keeping the sheaths together in sets, especially as -individual ones become damaged or lost. In practice there is also danger -of the sheaths being carelessly loaded in wrong order. - -[Illustration: - - FIG. 72.—Negative lens and mount, viewed from above.] - -The more ambitious idea of recording on the plate all the information -given by the instrument board of the plane occurs independently and -spontaneously to all aerial photographic map makers. These ideas vary -from attempts to photograph the actual instrument board on every plate—a -difficult task indeed with the instruments and camera placed as they are -in the ordinary plane—to the incorporation of compass, altimeter, and -inclinometer in the camera itself. - -Figure 58 shows the plan adopted in the English F type film mapping -camera already described, for photographing a compass and an altimeter -on the film. Here the combined compass and altimeter dial is above the -camera, and is mounted in a cell with a glass bottom. Below it is a lens -focussing the needles and compass points on the plane of the film. The -light for photography is furnished by a diffusely reflecting white -surface on top of the camera, illuminated by the sky. (The camera was -carried outboard.) In Fig. 56 is shown a picture with the compass image -impressed upon it. - -[Illustration: - - FIG. 73.—Negative lens and mount, side view.] - -Figure 74 shows a type of inclination indicator found in some captured -German cameras. It consists essentially of two small pendulums or -plumb-bobs; one to indicate lateral, the other longitudinal inclination, -arranged to be photographed in silhouette on the plate, as shown in the -lower part of the diagram and in the print from a captured negative -(Fig. 75). - -[Illustration: - - FIG. 74.—Diagram of inclinometer used in some German cameras.] - -[Illustration: - - FIG. 75.—Photograph made with German camera, showing inclinometer - record, four points for locating diameters and center of plate, and - (upper right-hand corner) number of the plate sheath.] - -Both these devices suffer from the deficiencies of the instruments they -photograph. The compass and the inclinometer, as already mentioned in -the discussion of airplane instruments, only behave normally in -straight-away flying, failing to indicate correctly when the plane is -subject to accelerations in any direction. In general all attempts to -record directional data in the camera are of little promise, unless -either the instruments or the camera are automatically held level by -some gyroscopic device. If the instruments are so controlled, rather -elaborate means for photographing them are necessary. If the camera is -stabilized, the inclinometers are unnecessary, and the compass behaves -rationally. - -Another scheme for indicating inclinations, which is not subject to the -above objections, is to photograph the horizon either on a separate film -or on the same sensitive surface, simultaneously with the principal -exposure. The difficulty here is the practical one that it is only -feasible in localities of great atmospheric clearness. Ordinarily, -especially anywhere near the sea-coast, the horizon is too rarely seen -to be a reliable mark (Fig. 4). It is possible, however, that this -objection could be overcome by the use of specially red sensitive plates -and suitable color filters, as discussed in the chapter on “Filters.” -The method would in any case be useless in mountainous country. - -The difficulties discussed with reference to direction indicating -instruments of course do not hold with the altimeter. Ordinarily, -though, the altitude changes slowly enough to permit of sufficiently -accurate records being made by pencil and pad. For high precision map -making a photographic record of altimeter readings has a legitimate -claim. As we have seen, a small altimeter is incorporated in the English -F camera, but the bulk which a really precision altimeter would assume -would be a bar to its use in this way. A time or serial number record on -the plate or film, synchronized with a similar record on the film of an -auxiliary camera which photographs the altimeter and other instruments, -may be the simplest way to preserve the majority of the desired data. - -=Devices for Heating the Camera.=—Parts of the camera mechanism which -depend on the uniformity of action of springs or upon adequate -lubrication are susceptible to change with variation of temperature. At -high altitudes low temperatures are met which may freeze ordinary -machine oils or may cause springs to seriously alter their tension, even -to break. To meet this difficulty, and probably also to dispel the -occasional condensation of moisture on the optical parts, the German -cameras are equipped with an electrical heating coil placed just below -the shutter, and arranged to connect with the general heating and -lighting current of the plane. Two contacts are ordinarily provided, for -offsetting the effects of temperatures of -15 and -30 degrees -centigrade. An additional function of this heating coil is perhaps to -maintain the sensitiveness of the plates or film. - - - - - III - THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS - - - - - CHAPTER XIV - THEORY AND EXPERIMENTAL STUDY OF METHODS OF CAMERA SUSPENSION - - -=General Theory.=—In addition to the limitation of exposure set by the -ground speed of the plane another limitation is set by the _vibration_ -of the camera. This may be caused either by the motor, or by the elastic -reactions of the plane members to the strains of flight. Unlike the -movement of the image due to the simple motion of the plane, movements -due to vibration may be eliminated by proper anti-vibrational mounting -of the camera. - -The effect of vibration may show as an indistinctness of the whole -image—this is its only effect with a between-the-lens shutter—or as a -band or bands of indistinctness parallel to the curtain opening (Fig. -76). These are due to shocks or short period vibrations during the -passage of the focal-plane shutter. - -The obvious remedy for vibration troubles is to mount the camera on some -elastic, heavily damping support, like sponge rubber or metal springs. -Such a mounting should, however, be designed on sound principles derived -from a proper analysis of the nature and effect of the possible motions -of the camera. Otherwise, the vibrational disturbances may be increased -rather than diminished by the camera mount. Such an analysis, based -merely on general mechanical principles, shows that all motions of the -camera are resolvable into _six_. These are three _translational_ -motions, namely, two at right angles in one plane such as the -horizontal, and one in the plane at right angles to this (vertical); and -three _rotational_ motions, one about each of the above directions of -translational motion as an axis (Fig. 77). - -[Illustration: - - FIG. 76.—Captured German photograph, showing zones of poor definition - due to vibration during passage of focal plane shutter aperture.] - -Brief consideration will show that only the latter—the rotational -motions—are of any importance when the small displacements due to -vibration are in question. To illustrate the negligible effect of -vibrations which merely move the camera parallel to itself in any -direction it is only necessary to imagine the camera moved parallel to -the ground through a large distance, such as 10 centimeters. Now 10 -centimeters motion of the camera at 3000 meters elevation means, with a -25 centimeter camera lens, - - .25 1 - ———— × 10 = ———— centimeter - 3000 1200 - -motion on the plate, which would be only a tenth the distance separable -by a good lens. If we reduce this motion to the small fraction of a -centimeter which vibration would actually produce, it is evident that -such vibration is of absolutely no importance. Similarly, if we imagine -the camera, under the same conditions, moved vertically with reference -to the ground by ten centimeters, the scale of the picture would merely -be changed by 1/12000 or by 1/1000 centimeter on a 12 centimeter plate, -again quite negligible. - -When we consider motions of rotation, however, the case is quite -different. If the camera is mounted so that the effect of any vibration -is to rotate it around a horizontal axis, this is exactly equivalent to -rotating the beam of light from the lens so that it sweeps across the -plate. Thus a millimeter displacement of the lens of the camera with the -plate remaining fixed gives approximately a millimeter motion of the -image. Consequently, a rotation producing only a fraction of a -millimeter's relative motion of lens and plate during the period the -curtain aperture is over a given point would cause fatal blurring—and -the visible vibration of plane longerons and cross members is easily of -half millimeter amplitude or more. Reduced to angular units it is easily -shown that a rotation of one degree per second—which is quite slow as -plane oscillations go—is beyond the limits of toleration. Translational -motions of large amplitude may be allowed, but the mounting of the -camera must not permit these translations to be at all different for -different parts of the camera. - -[Illustration: - - FIG. 77.—Diagram showing possible motions of the airplane camera: - three of translation and three of rotation, and their combinations.] - -The proper way to eliminate vibrational effects is to devise a mounting -that will transmit only the translational shocks or that will transform -the rotational ones into translations. Platforms pivoted and -cross-linked so as to be free to move only parallel to themselves -(described in the next chapter) represent one attempt to reach this -result. Quite the simplest and most scientific form of mounting to -achieve this end is to support the camera solely _in the plane of the -center of gravity_. The principle here involved is easily grasped if we -note that when we jar a camera supported above or below its center of -gravity, the effect is to start the camera vibrating with the center of -gravity oscillating pendulum-like about the point of support. The closer -the center of gravity to the center of support, the smaller the moment -of the body about the latter point. - -=Experimental Study of Methods of Camera Support.=—Conclusive evidence -as to the merits of any system of camera mounting can be obtained only -under conditions that eliminate the effect of other variables which may -be equally efficacious in diminishing the effects of vibration, but -which have only limited application. Very brief exposures—1/500 second -and less—will, for instance, result in good pictures with almost any -condition of vibration. Hence a sharp picture offers no proof of the -merits of a camera mounting unless it is known that the exposure was no -shorter than the limit set by the ground speed of the plane. In fact it -may be said that the chief object of studying methods of camera -suspension is to increase the allowable exposure to a maximum, thus -lengthening the working hours and multiplying the useful working days -for aerial photography. - -The most satisfactory method of test yet developed is to fly over a -light or a group of lights on the ground with the camera shutter open. -In the first use of this method, which originated in the English -Service, such flights were made at night, but later it was found that -good results could be got by placing the lights in a forest and making -the tests when the sun was fairly low. One of the group of lights must -be periodically interrupted, at a known rate, to furnish the time -intervals. - -[Illustration: - - FIG. 78.—Tests of camera mounting, made by flying over a bright light - against a dark background. (_a_) Rigid fastening on side of plane; - (_b_) held in the hand, inexperienced observer; (_c_) held in the - hand, experienced observer; (_d_) camera mounted at center of - gravity on gimbals bedded in sponge rubber.] - -Some characteristic “trails” obtained by this method of test are shown -in Fig. 78. The first trail is that given by a camera rigidly fastened -to the fuselage. The second and third show hand camera trails, made by -an inexperienced and by an experienced observer, respectively. They show -by comparison with the other figures that the human body is an excellent -block to vibration, but in unskilled hands a poor check to rapid erratic -(probably rotational) motions of the camera. The fourth is the trail -given by a camera supported by gimbals bedded in sponge rubber -accurately in the plane of the camera's center of gravity. Other trails -are shown in the next chapter in connection with the description of -practical camera mountings. Clearly the best suspension is that giving -the smallest amplitude of displacement during the interval of time -covered by an average exposure. It is, in fact, possible to determine -from these trails the permissible exposure for any assumed permissible -blurring of the image. The rigid mounting trail indicates very bad -conditions, calling for literally instantaneous exposures. The center of -gravity trail, at the other extreme, shows practically no limitation of -exposure in so far as vibration is concerned, thus bearing out the -theoretical conditions above discussed. An interesting conclusion from -these experiments is that a rapidly running motor gives less harmful -vibration than a slow one, although in the war it was a common practice -to throttle the motor before exposing. As might be expected, the greater -the number of cylinders, the shorter the period and the smaller the -amplitude of the vibration. - -=Pendular Camera Supports.=—The design of the camera support may be -approached from a different standpoint, namely, with the aim of carrying -the camera so that it will tend to hang always vertical. In mapping this -is of fundamental importance. It is, indeed, a question whether aerial -mapping will ever be worthy of ranking as a precision method unless the -camera can be mounted so that its pictures are taken in the horizontal, -undistorted position. - -The simplest way to hold the camera vertical is to mount it on gimbals, -with its center of gravity below the point of support. When so mounted -the camera swings as a pendulum. Delicacy of response to variation of -level is obtained by leaving a considerable distance between the center -of gravity and the center of support. Oscillation about the vertical -position is to be prevented by some system of dash pots or other -damping. A suspension of this kind is furnished with the Brock film -camera (Fig. 60). - -It will be seen at once that the relation of center of gravity to center -of support called for here is in direct contradiction to the -requirements for eliminating vibration. Either one requirement or the -other must be sacrificed, or else a compromise made in which neither -delicate response to inclination of the plane nor fully satisfactory -freedom from vibration is attained. This is a very serious objection to -the pendular support. But the really vital objection to the pendular -support is that it performs its function only very partially. It is -entirely satisfactory only under conditions of steady flying, as in a -uniform climb or glide, with the plane tail or nose heavy, or in flying -with one wing down. As soon as we introduce any acceleration, as in -making a turn, the camera follows the plane and not the earth. - -It is true that mapping photography is done from a plane flying as level -as possible, and that except under bad air conditions it holds its -course with very little turning, if handled by a skilled pilot. -Nevertheless, a surprisingly small deviation from straight flying causes -quite serious variations from the vertical. It is of interest to -calculate how large may be the horizontal accelerations that accompany -swervings from a straight course which one might think insignificant. -For instance, consider the horizontal acceleration due to a turn having -a radius of a kilometer when the plane is moving at 100 kilometers per -hour. If _a_ is the acceleration, _v_ the velocity of the plane, and _r_ -the radius, we have from elementary dynamics that - - _v_^2 - _a_ = ————— - _r_ - -Substituting the values chosen, we have— - - 100,000^2 meters - _a_ = ————————————— = .77 —————— - 3600^2 × 1000 sec^2 - -The acceleration of gravity being 9.80 meters/sec^2 we have that the -ratio of the horizontal acceleration to the vertical is - - .77 - ———— = .078 - 9.80 - -This is the tangent of the angle of deviation from the vertical, from -which the angle turns out to be about 4½ degrees, a very considerable -error, rapidly multiplied as the speed of the plane is increased. It is, -indeed, open to question whether the average deviations from the -vertical are not apt to be less with the camera rigidly fixed to the -plane, if guided by a skilled pilot who will hold the ship level at the -expense of “skidding” the slight turns he must make to hold his -direction. - -=Gyroscopic Mountings.=—The ideal support for the aerial camera will -undoubtedly be one embodying gyroscopic control of the camera's -direction. By proper utilization of the principles of the gyroscope it -is to be expected that not only can the camera be maintained vertical, -but it may be supported anti-vibrationally as well. At the present time -the problem of gyroscopic control is in the experimental stage, so that -only the elements of the problem and the possible modes of solution can -be laid out. - -The gyroscope consists essentially of a heavy ring or disc rotating at a -high speed on an axis free to point in any direction (Fig. 79). If -mounted so that the axes of the supporting gimbals pass through the -center of gravity of the rotating disc, the result is a _neutral_ -gyroscope. Its characteristic is that its axis maintains its _direction -fixed_, but this fixity is with respect to space and not with respect to -the gravitational vertical. Consequently, as the earth revolves the -inclination of the gyroscopic axis changes with respect to the earth. In -latitude 45° this change is approximately a degree in five minutes. -Furthermore, the action of friction in the supports, which can never be -entirely eliminated, also acts to slowly alter the direction of the -gyroscopic axis. Therefore, unless some _erector_ is applied even the -gyroscope will not perform the task required of it. - -[Illustration: - - FIG. 79.—Diagram of simple gyroscope.] - -Before discussing possible forms of erectors it may be noted in general, -first, that these must depend upon gravity; second, that such being the -case, they must respond to the resultant of gravity and any -acceleration, that is, to the _apparent_ or _pseudo-gravity_. As already -seen, this pseudo-gravity, during a turn, is exactly what limits the -usefulness of the pendular support, and necessitates recourse to the -gyroscope. The problem thus becomes one of making an erector-gyroscope -combination which will respond to true gravity and not to -pseudo-gravity. - -In general this problem would be insoluble, since there is no difference -in the nature of the acceleration of gravity and that due to centrifugal -force. A way out is offered, however, by the fact that true gravity acts -continuously and at a small angle to the axis of the gyro, while the -components which cause the pseudo-gravity are of short duration, liable -to rapid changes of direction, and, on a turn, act at a large angle. -What we require, therefore, is an erector which will respond slowly but -surely to the _average_ acceleration, which is downward, but too -sluggishly to be affected by the shorter period accelerations due to -turns or rolls. Slowness of response is a matter of the erecting forces -being small and of the mass and angular velocity of the gyro disc being -large. The success of the compromise called for depends on the relative -times taken for the gyroscope to tilt seriously from the true vertical, -due to the causes above mentioned, and for the average turn or roll. -Fortunately the former is a matter of minutes, the latter of seconds or -at the worst of fractions of a minute. More than this, since the roll or -turn is apt to be of much greater angle than any normal deviation of the -gyroscopic axis from the vertical in the same time, we are offered the -possibility of some device for filtering out the deviations which alone -are to effect the erector. For instance, by shunting the restoring force -whenever it is called upon to act through more than a predetermined -small angle. - -As to the method of erecting the gyroscope, its characteristic property -must be kept in mind. This is that the axis does not tilt under an -applied force in the direction it would if the gyro were not rotating, -but around an axis at right angles to that of the applied couple. Thus -in Fig. 79, if a weight is attached as shown, the disc does not incline -downward toward the weight, around the axis _Y_, _Y´_, but _precesses_ -about the vertical axis Z, Z´. Some means is therefore needed to -translate the pull which any gravitational control, such as a freely -swinging pendulum, would give, into a pull with _at least a component_ -at a finite angle to this. - -In the Gray stabilizer several metal balls are slowly rotated in a tray -above the center of gravity of the gyroscope. Specially shaped grooves -or compartments limit the freedom of motion of these balls so that when -the gyro is inclined the balls travel at different distances from the -center on the ascending and descending sides. By this scheme a couple is -produced about the axis through the center and the low point of the -disc, which tilts the apparatus to the gravitational vertical. In an -alternative form the balls are carried past the low point by their -momentum and are prevented from returning by the walls of the containing -compartment, which have meanwhile been advanced by the rotation of the -erector as a whole. The net result is to shift the center of gravity of -the system of balls in the proper direction to erect the gyro. The -rectifying action is purposely made quite slow so that the displacements -of the balls due to pseudo-gravity will be averaged out. - -In a design due to Lucian, small pendulums work through electric -contacts to actuate solenoids which in turn move small weights in the -appropriate directions to give the desired tilt. Response is made fairly -quick and delicate, and pseudo-gravity, due to turns and rolls, is -rendered inoperative by the contacts breaking whenever the pendulums -swing more than three or four degrees. This can only happen if they move -too quickly for the erecting forces to act, reliance being here placed -on the characteristic differences of action in respect to time of real -and pseudo-gravitational forces. - -Besides the neutral gyroscope as just considered there is the pendular -or top type, in which the center of gravity is not in the plane of the -supports. In general this type depends on a couple resulting from the -gravitational pull and the inevitable friction of the supports to slowly -tilt the axis to the gravitational vertical. This type is slower to -respond than the designs in which a definite couple in the proper -direction is provided and it reaches the true vertical only through a -circuitous path. - -[Illustration: - - FIG. 80.—Diagram of camera linked to gyroscopic stabilizer.] - -Three methods of controlling a camera by a gyroscope are suggested. One -is to fasten the gyroscope rigidly to the camera and mount the whole -system on gimbals. A second is to mount both camera and gyro side by -side on gimbals, linking the two so that the camera is moved parallel to -the gyro (Fig. 80). A third method is to utilize the gyro to make -electric contacts to operate motors which in turn move the camera. - -Considerable weight and space are required for a gyroscope capable of -stabilizing a camera. The rotating disc should be about half the weight -of the camera, and with its mounting may be expected to double the room -required for the camera alone. Motive power for maintaining the gyro in -continuous rotation may be supplied by an air blast, or the gyro may be -made up as an induction motor—the latter necessitating an alternating -current supply. - -In view of the space and weight limitations in a plane it is a question -still to be decided whether it is more economical to stabilize the -camera or to stabilize an inclinometer and photograph its indications -simultaneously with the release of the shutter which takes the aerial -picture. - - - - - CHAPTER XV - PRACTICAL CAMERA MOUNTINGS - - -=General Considerations.=—Camera mountings as used during the war were -far from being developed on the basis of scientific study or test. At -first the need for special supporting apparatus was not realized, and -the suspensions later in use were largely field-made affairs, often -dependent on adjustments made according to individual taste. Through -lack of accurate methods of test and of conclusive evidence on the -subject, it was quite common to find extremists who, on the one hand, -denied the efficacy of suspensions in general, and on the other ardently -supported crazily conceived supporting arrangements which accurate -comparative test show to be even worse than useless. - -In the French service, despite numerous types of suspension available, -the very general practice was to lift the camera from its support and -hold it between the knees. Or else the hand was pressed on the top of -the camera during exposure, more reliance being placed on the damping -qualities of the body than on any of the rubber or spring mechanisms. - -As is clearly shown by the experimental data described in the last -chapter, a correctly designed supporting device, carrying the camera -accurately in the plane of its center of gravity, accomplishes -practically perfect elimination of vibrational troubles. So important is -the use of a mount and so important is it that the mount should be -correctly dimensioned and adjusted for the camera, that an entirely -different attitude should be adopted from the prevalent one which -focuses attention on the camera and regards the mounting as a mere -auxiliary to be left more or less to chance. _The mounting should be -considered an integral part of the camera._ The man in the field should -receive camera and mount together, leaving as his only problem the -attachment of the complete camera—and—mount unit to the plane. This may -be arranged, by proper designing, to be a simple matter of rigid bolting -or strapping, requiring ingenuity perhaps but not the scientific -knowledge which is required for mounting design. - -[Illustration: - - FIG. 81.—“L” camera mounted outside the fuselage. Observer using - exposure plunger, pilot using Bowden wire release.] - -=Outboard Mountings.=—In the English service the camera was first -attached to the plane outside the fuselage by a rigid frame, to which -the camera was strapped or bolted (Fig. 81). Obvious objections exist to -placing the camera in this position, such as the resistance of the wind -and the difficulty of changing magazines. However, in the earlier -English planes with their fuselages of small cross section no other -accessible place for the camera was to be found. Vibrational -disturbances with the rigid outboard mounting are quite serious, as is -so clearly indicated by the trace shown in Fig. 78. Extremely short -exposures are alone possible, and a very large proportion of the -pictures are apt to be indistinct. - -=Floor Mountings.=—A step in advance of the outboard mounting is to -support the camera snout in a padded conical frame on the floor of the -plane (Fig. 82). This mounting avoids the objection on the ground of -wind resistance that holds with the outboard, and has possibilities of -being worked out as an entirely satisfactory support. Yet to be -satisfactory, the point of support must lie in the plane of the center -of gravity of the camera, and the camera must be of a type that -preserves its center of gravity unchanged in position as the plates are -exposed. Unless these conditions are fully met the floor mounting gives -results little better than does the outboard. - -=Cradles or Trays.=—Floor space in the cockpit being unavailable in the -battle-plane, due to duplicate controls, bomb sights, etc., the English -service was driven to the practice of carrying the camera in the -compartment or bay behind the observer. Here it was attached either to -the structural uprights or longerons, or to special uprights and -cross-pieces built into the plane to serve photographic ends. As an -intermediary between the camera and the supporting cross-pieces there -was introduced the camera _tray_ or _cradle_. This is essentially a -frame carrying sponge rubber pads into which the camera is more or less -deeply bedded. Figs. 83 and 84 show an American L camera cradle based on -the design of the English L camera tray. Thick sponge rubber pads -support the two ends of the camera top plate, and additional pads are -provided to hold the nose of the camera. Careful tests show this cradle -to be superior to the outboard mounting, but still leave much to be -desired. Its performance is better with the nose of the camera left -free. - -[Illustration: - - FIG. 82.—“L” camera in floor mounting.] - -[Illustration: - - FIG. 83.—“L” camera and cradle mount in skeleton DeHaviland 4 - fuselage, side view.] - -=Tennis-ball Mounting.=—A very simple mount used by the French consists -of a frame enclosing the nose of the camera, and carrying four tennis -balls, on which the whole weight rests (Fig. 40). If the center of -support is in the plane of the center of gravity and if the four balls -are of uniform age and elasticity, this form of support is good. As -provided by the camera manufacturer, the tennis ball frame fits much too -far down on the camera. Another application of the tennis ball idea was -frequently made in the French service, in which the balls were close up -under the shutter housing (Fig. 85). Additional support was, however, -given to the camera nose by flexible rubber bands, the success of the -whole being largely a matter of the adjustment of the tension on the -bands. - -[Illustration: - - FIG. 84.—“L” camera and cradle mount in skeleton DeHaviland 4 - fuselage, front view.] - -=Parallel Motion Devices.=—A form of suspension favored by the French -consists of parallel bell cranks, rigidly linked together and held up by -springs. Mountings of this sort are illustrated in Figs. 86, 87, 88 and -96. The guiding principle is that any sort of shock will be transformed -into a straight up-and-down or side-wise motion of the camera, which is -harmless. The mounting as adapted by the English surrounds the camera -body, making the plane of support somewhere near the center of gravity. -In certain of the French suspensions employing this principle the whole -camera is hung below the bell cranks (Fig. 86), and then the nose is -restrained by heavy rubber bands. The net result is largely a matter of -adjustment. - -[Illustration: - - FIG. 85.—Tennis ball suspension, assisted by elastic bands attached to - nose of camera.] - -[Illustration: - - FIG. 86.—French spring and bell crank suspension.] - -[Illustration: - - FIG. 87.—U. S. hand-operated 18 × 24 centimeter plate camera on bell - crank mount with rotating turret.] - -[Illustration: - - FIG. 88.—Same camera in plate changing position.] - -[Illustration: - - FIG. 89.—Tests of camera mountings: (_a_) deRam camera on - bell-crank-and-spring mount, below the center of gravity; (_b_) - same, at center of gravity; (_c_) type “K” film camera on universal - mounting (Fig. 88).] - -Tests on the English design made in the United States Air Service appear -to show that the chief virtue of the mounting lies in the approximation -of the point of support to the center of gravity in the English cameras. -A deRam camera supported by its cone, so that its center of gravity was -considerably above the center of support gave rather poor results (Fig. -89_a_), but when the bell cranks were attached near the center of -gravity, highly successful results were obtained (Fig. 89_b_). The -French deRam camera as ordered for the American Expeditionary Force was -fitted with a bell crank supported in this position. - -Figures 90 and 91 show a bell crank mounting furnished with a rotating -turret. This was designed to facilitate the changing of magazines in the -English B M camera, which is swung around through 90 degrees from the -exposing position to bring the magazine near the observer. The camera -shown in the mounting is the American hand-operated model (type M), in -which there is the same necessity for turning in order to manipulate the -bag magazine easily. The camera is shown in both exposing and plate -changing positions. An important detail of these mounts is a _safety -catch_, which must be fastened before the plane lands, in order to -prevent the shocks of landing from producing oscillations sufficient to -throw the camera out of the mount. - -=Center of Gravity Rubber Pad Supports.=—Given a camera whose center of -gravity does not change during operation, a simple and entirely adequate -anti-vibration support is furnished by a ring of sponge rubber in the -plane of the center of gravity. But if provision has to be made for -oblique views or for adjusting the camera to the vertical, something -more elaborate is necessary. - -[Illustration: - - FIG. 90.—U. S. model deRam camera on anti-vibration mounting - adjustable for the angle of incidence of the plane.] - -[Illustration: - - FIG. 91.—U. S. deRam camera and mount installed in photographic - DeHaviland 4 (Fig. 100). Viewed from above the observer's cockpit.] - -[Illustration: - - FIG. 92.—U. S. type “K” film camera on universal mounting, vertical - position.] - -[Illustration: - - FIG. 93.—U. S. type “K” film camera on universal mounting, oblique - position.] - -Mountings for the American deRam and for the Air Service film camera, -embodying the results of complete study of the anti-vibration problem, -are shown in Figs. 90, 92 and 93. Trusses carrying the cameras on pivots -rest on four pads of sponge rubber which are mounted on frames correctly -spaced ready for attachment to the cross-pieces of the airplane camera -supports. In the deRam (Fig. 90) the pivots, attached to the camera -body, permit it to be leveled fore and aft, to compensate for the -inclined position of the fuselage assumed at high altitudes or in some -conditions of loading. This will sometimes amount to as much as 11 or 12 -degrees, which is very serious, since one degree causes (with an angular -field of 20 degrees) about one per cent. difference of scale at the two -sides of the plate. The film camera mounting carries the camera in a -conical ring, and is pivoted not only for vertical adjustment, but for -the taking of obliques as well (Fig. 93). These mounts transmit -practically no vibration. - -[Illustration: - - FIG. 94.—Tests on two types of camera mount: (_a_) Support at bottom - of camera; (_b_) support above center of gravity.] - -A caution must be noted with regard to center of gravity mountings. Any -change in the camera, in particular the substitution of a short for a -long lens cone, must be made so as to cause no alteration of the -relative positions of the center of support and the center of gravity. -Either the short cone must be weighted, or additional supporting pivots -must be provided in the plane of the new center of gravity. - -=The Italian and G. E. M. Mountings.=—These mounts (Figs. 49 and 59) are -similar in that the protection from vibration is furnished by an elastic -support at the bottom of the camera. Tests show that these two cameras -give very similar results, of the unsatisfactory sort to be expected -from this kind of mounting in view of the lessons of the last chapter on -the proper point of support. Fig. 94, _a_, shows a trace given by the -Italian mount. The permissible exposure, on the criterion adopted, is -very short with either mount, about 1/200 second. - -=The Brock Suspension.=—This consists of a pair of frames into which the -camera is fitted by ball bearing pivots, so that it is free to move in -any direction (Fig. 60). In order to permit gravity to control the -direction of the camera, the point of support is made considerably (ten -inches) above the center of gravity. Air dash pots are provided for -damping the swings. As already explained, the pendular method of support -is in basic contradiction to the requirements for vibration elimination. -Tests of the Brock suspension, shown in Fig. 94, _b_, indicate it to be -of low efficiency in damping out the short period vibrations which are -responsible for poor definition. - - - - - CHAPTER XVI - THE INSTALLATION OF CAMERAS AND MOUNTINGS IN PLANES - - -=Conditions to Be Met.=—The characteristic difficulty in installing the -airplane camera is that there is no place for it. After the gasoline -supply, the armament, the wireless, the oxygen tank, the bombs, and -other necessities are taken care of there is neither space available nor -weight allowable. Where space may be found it will be inaccessible, or -accessible only through a maze of tension and control wires; or it will -be in a position where any weight will endanger the balance of the -plane. Plane design has in fact been more or less of a conflict between -the aeronautical engineer, who is designing the airplane primarily as a -machine to fly, and the armament and instrument men, who look upon it as -a platform for their apparatus. Lack of appreciation of the extreme -importance of aerial photography resulted, during a large part of the -war, in the camera installation being neglected until the plane was -supposedly entirely designed, and even in production. At that stage the -installation could be but a makeshift. Only in the later stages of the -war, when plane design became a matter of coöperation between all -concerned, were fairly convenient and satisfactory arrangements made for -the camera. Always, however, the rapid succession of new plane designs, -with various shapes of fuselage and details of structure, made camera -installation in the war plane a matter calling for the greatest -ingenuity. - -The problem was met in part by constructing both cameras and mountings -in sections, to be laboriously wormed in through inadequate apertures, -in part by later structural changes in the planes, such as the -substitution of veneer rings or frames for the tension wires. In certain -cases the rear cockpit controls were omitted, thereby freeing accessible -and often adequate space for the larger cameras. Rear controls were -never used in the German planes, so that their standard practice was to -carry the camera forward of the observer. This, together with the -general restriction to the 13 × 18 centimeter size plate, made the -installation problem less difficult in the German aircraft than in the -Allied. - -=Practical Solutions.=—An important feature of camera installation has -already been mentioned, but may well be repeated for emphasis. The -camera and its anti-vibration mounting should always be considered as a -unit, and should be so designed that simple bolts or straps will suffice -to fasten it in its place in the plane. Even should the spacing of the -structural parts of the plane not correspond to that anticipated by the -mounting design, the ingenuity of the man in the field may be depended -upon to make the necessary alterations or additions to the plane. The -design of the camera suspension itself cannot, however, be left to -uneducated ingenuity. - -Assuming the camera and mounting supplied, the next step—a very -difficult one—is to insure uniformity in the structures to be built into -the planes for the purpose of supporting the camera mountings. With this -uniformity must, however, be combined the greatest possible flexibility -to provide for various designs of cameras. - -In the English service the standard camera installation consists of -wooden uprights with cross bars athwart the plane, adjustable as to -height (Fig. 95). A distance between the cross bars of 13¼ inches has -been standardized, and all camera cradles and mountings are notched or -otherwise spaced to fit this dimension. The installation adopted in the -American planes is similar, but with a distance of 16 inches between -cross bars. These uprights and cross bars are ordinarily situated in the -bay behind the observer, but can be placed in any available space. Fig. -83 shows, in a model bay, the arrangement of uprights and cross bars in -the American DH 4, with the L camera in place in its cradle. It is just -possible to introduce camera and cradle separately from the observer's -cockpit through the tension wires, and, by uncomfortable reaching, -magazines may be changed. - -[Illustration: - - FIG. 95.—“LB” camera with 20-inch lens, mounted on bell-crank - suspension in skeleton fuselage. Stream-lined hood below to cover - projecting end of lens cylinder. Propeller and Bowden release in - place.] - -A step in advance is made when the top tension wires and superstructure -are replaced by a rigid frame with an opening large enough to admit the -entire camera and mounting. When this is done considerably larger -cameras may be accommodated in the same sized bay, as shown in Fig. 96. -A further advance, from the standpoint of accessibility and convenience -of installation, follows when the tension wires between observer's and -camera bay are replaced by a ply-wood ring, as shown in Fig. 97. Here -the only serious limitations are those due to the vertical height of the -camera, and of course its weight. - -Openings for the lens to point through are simply provided in the fabric -covered aircraft, by cutting through the canvas and stiffening the edge -of the hole by wire. Tension wires are often in the way. They may either -be disregarded, since they merely cut off a little light, or replaced in -part by metal rings, as shown in Fig. 96. In veneer covered fuselages -the hole must of course go through the wood. This may be undesirable, -since the veneer is depended on to furnish structural strength, a point -which further emphasizes the importance of the photographic requirements -being thoroughly considered while the plane is being designed. - -Single seater or scout planes do not lend themselves to the insertion of -such standardized uprights and cross-pieces, because of their small size -and the common utilization of all space inside the fuselage for gasoline -tanks and control wires. Some French scouts, whose fuselages are very -wide, due to the rotary engines, have been fitted with compartments for -contemplated automatic film cameras. The most commonly used camera in -the single seater was, however, the Italian 24-plate single-motion -apparatus (Fig. 49). This camera and its carrying tray occupy very -little lateral space and have in actual practice been carried beneath -the seat or pushed up through an opening in the bottom of the fuselage -under the gasoline tank. Whatever criticism may be made of the adequacy -of the mounting, it must be said that the camera, as used, is perhaps -the most eminently practical of all developed in the war, as its use on -scouts testifies. - -[Illustration: - - FIG. 96.—U. S. type “K” film camera on bell-crank mount, in camera bay - of DeHaviland 4. Veneer frame at top of bay in place of usual - cross-wires.] - -[Illustration: - - FIG. 97.—Section of fuselage of veneer construction affording superior - accessibility to camera.] - -=Special Photographic Planes.=—As cameras grew in size, the difficulty -of installing them in planes built without regard to photographic -requirements greatly increased. Few planes could carry even the 50 -centimeter focus camera obliquely without the necessity of poking its -nose through the side where it would catch wind and oil; while the 120 -centimeter camera could be carried obliquely only in the fore and aft -position. Even vertical installation of the latter camera was really -feasible in but few planes; sometimes the camera was carried to the -exclusion of the observer—and, in fact, this size was never used by the -English, whose fuselages were small in cross-section. - -This situation led, late in the war, to steps toward producing planes -designed primarily for photographic reconnaissance. In these the camera -would be entirely accessible, and cameras of any size could be carried -in any desired position. One scheme which properly belongs under this -heading was the provision of a special removable photographic cockpit, -for the front or nose of a twin-motored three seater. Other noses, for -bombing and heavy machine guns, were also planned, all to be -interchangeable. Since the regular photographic bay with uprights and -cross-pieces was also provided to the rear, this special photographic -ship could on occasion do two classes of work, such as long focus -spotting and short focus mapping. - -The most completely worked out photographic plane was probably the model -designated P1 by the United States Air Service. This is a modified -DeHaviland 4 in which the rear controls have been removed and the -cowling raised and at the same time made squarer in cross-section. The -space formerly occupied by the rear controls provides ample room for all -types of camera. These are carried on uprights at the standard distance -apart, 16 inches, with cross-pieces adjustable as to height. The camera -space is accessible not only from the observer's cockpit, but from -above, upon folding back the metal cover. Doors at the bottom and at -each side permit not only vertical but oblique exposures. The latter are -not interfered with by the wings, as they would be in some designs of -plane if the camera occupied the same position relative to the cockpits. -Fig. 91 shows the deRam camera in place, as seen from the rear. Figs. 98 -and 99 show the 18 × 24 centimeter film camera, set both for vertical -and oblique views. - -Negative lenses are provided for both pilot and observer, the one for -the pilot permitting him to see from a point far ahead to directly -underneath, while the observer's is furnished with cross wires below and -etched rectangles of the camera field sizes on the upper surface. -Windows of non-breakable glass assist in sighting obliques. The -accompanying picture (Fig. 100) of the plane showing an oblique camera -in position gives an excellent idea of its appearance. Its special -features are worthy of copying in peace-time photographic aircraft. - -=Installation of Auxiliaries.=—It is quite necessary that the camera -lens be protected from splashing mud and often from oil spray due to the -motor. For this purpose an easily opened and closed door is essential, -unless the camera is carried well up in the plane. An alternative, -possessing certain advantages, is to incorporate into the camera -protecting flaps operating in front of the lens, which open only when -the exposure is made. If the camera projects beyond the fuselage, -_stream lined hoods_ (Fig. 95) must be provided to protect the camera -nose with the minimum of air resistance. - -[Illustration: - - FIG. 98.—20-inch focus automatic film camera mounted obliquely in - photographic DH-4.] - -[Illustration: - - FIG. 99.—20-inch focus automatic film camera mounted vertically in - photographic DH-4.] - -[Illustration: - - FIG. 100.—DeHaviland 4 re-constructed as a special photographic plane, - showing 20-inch camera mounted for oblique photography.] - -The mounting of the regular camera auxiliaries—releases, sights, -propellers, speed controls, motors—is usually a great bother, due to -lack of space and to the severe restrictions on methods of fastening. -Screws in longerons or uprights are taboo. Metal straps to go around -structural parts are the approved device, but with variations in the -size of these members, the holes, straps, bolts and nuts provided are -very apt not to fit. Changes of construction, such as that from -skeletons covered with fabric to veneer bodies, also interfere with any -standard means of attachment, and leave this, like many other problems -in war-time aerial photography, to the resourcefulness of the man in the -field. - -Magazine racks must be tucked away in any available space. Under the -seat is a position frequently utilized. Especially with plates is it -desirable to carry the extra magazines in a position to interfere as -little as possible with the balance of the plane. In the DH 4 this means -that they should be carried if possible forward of the observer, even -though he must turn completely around to get and insert each magazine. - - - - - IV - SENSITIZED MATERIALS AND CHEMICALS - - - - - CHAPTER XVII - THE DISTRIBUTION OF LIGHT, SHADE AND COLOR IN THE AERIAL VIEW - - -The general appearance of the earth as viewed from above has already -been described and illustrated (Figs. 10 and 11). It remains to deal -with the earth's appearance in a more analytic and quantitative manner, -in order to decide upon the characteristics to be sought in our -photographic sensitive materials. - -=Range of Brightness.=—The absence of great contrasts so apparent in the -view of the earth from a plane is confirmed by photometric observations. -These show that the average landscape, as seen from the air, rarely -presents a range of brightness of more than seven to one, even when seen -without the presence of veiling haze. It is to be remembered that -shadows constitute no important part of the aerial landscape. Vertical -walls in shadow, which form a substantial part of the surfaces seen by -an observer on the ground, are invisible or greatly foreshortened from -the air. Moreover, they are never contrasted against the sky, which is -photographically often the brightest part of the ordinary picture. To -the aviator's eye shadows on the ground are only of any length at early -and late daylight hours. Even at these times they cover but a small -area, since the number of high vertically projecting objects in a -representative landscape is small. Lacking shadows, the brightness range -is only that between various kinds of earth, water, and vegetation. -Chalk (from freshly dug trenches), reflected sunlight from water, or -marble buildings, furnish almost the only extensions to the brightness -scale as above given. - -_Diurnal and seasonal changes._ During the winter months on the Western -Front photography from the air was only possible for two or three hours -around noon, on clear days. This calls attention to another factor of -prime importance, namely, the large variation in the intensity of -daylight during the course of the day and during the course of the year. - -[Illustration: - - FIG. 101.—Variation of average daylight intensity during the day.] - -Measurements showing typical variations from morning to night are -exhibited in Fig. 101, from which it appears that there is an increase -in illumination of four to five times from 8 o'clock—when it would be -considered full daylight for purely visual observation—until noon, while -there is a corresponding decrease by four o'clock. Fig. 102 shows sets -of measurements by two different authorities which give the average -intensity of daylight for each month throughout the year. From December -to July there is an increase of approximately ten times. From both sets -of data it therefore appears that—neglecting the frequent occurrence of -clouds which reduce the illumination to a half or a quarter or even -less—a variation in illumination of forty or fifty times occurs between -mid-day in summer and morning in winter. In the photography of -stationary objects on the ground this range of intensities is easily -taken care of by selection of lens stop and shutter speed. On the -airplane it is quite otherwise, because the shutter speeds called for at -the lower illuminations are much slower than the motion of the plane -will allow. - -[Illustration: - - FIG. 102.—Variation of intensity of daylight through the year; two - different sets of measurements.] - -=Haze.=—At low altitudes the brightness range is substantially that -which would be obtained by photometric measurements of soil and -vegetation made at the earth's surface. At higher altitudes, especially -above 2000 meters, this brightness range is materially decreased by -atmospheric haze. The significance of this lies in the fact that for -safety from anti-aircraft guns, war-time aerial photography must be -carried out at very great elevations. Toward the end of the Great War -photographic missions traveling at from 5000 to 7000 meters were the -rule. At these heights, even in very clear weather, a veil of -bluish-white haze reduces the already small contrasts still more. Some -means for overcoming the effect of this haze becomes imperative, -therefore, in order to secure in the picture even the normal contrast of -the object. - -Haze is to be sharply distinguished from clouds or fog. Clouds and fog -consist of globules of water vapor of large size, opaque to light. Haze, -on the contrary, is more opaque to some colors than to others, or is -_selective_ in its veiling effect. Its scattering action on light is -greatest in the violet and blue of the spectrum, decreasing rapidly -through the green, yellow, and red, the exact relation being that the -scattering is inversely as the fourth power of the wave-length. It is, -consequently, possible to pierce or cut haze by using yellow, orange, or -red color screens. It is this possibility which has led to the extensive -use of yellow or orange goggles for shooting and for naval lookout work. -In aerial photography the equivalent is to be found in _color filters_, -used with color sensitive (orthochromatic or panchromatic) plates, which -have been found essential for all high altitude work. - -=Color.=—Visual observation from the airplane is aided in no -inconsiderable degree by the differences of _color_ that exist between -various objects of nearly the same brightness. This means of -distinguishing differences of character fails in the photographic plate, -which is color-blind; that is, it reproduces all objects as grays of -varying brightness. It is color-blind in another sense as well, in that -it evaluates colors as to brightness differently from the way the eye -does, overrating blues and violets and underrating yellows and reds. -This first kind of color-blindness is a positive disadvantage, for it -leaves available for differentiating objects only their brightness -differences. The second kind of color-blindness may on occasion actually -be an advantage. For it may happen by accident, or by design (through -the skilful use of color filters), that objects appearing nearly the -same to the eye appear different in the plate. More will be said about -this in connection with the use of filters for the detection of -camouflage. - -The range of hues seen in the aerial landscape is not large. Greens -(grass and foliage) predominate, followed by browns (earth), neither -color being bright or saturated. Over towns or cities we find that grays -(roads) and redder browns (brick) are conspicuous. Blues are practically -never seen, although it is to be noted that a fair share of the -illumination of the ground is by blue sky light and that the haze itself -is bluish. Consequently, the general tone of a landscape is much bluer -than one would be apt to imagine it from consideration of the general -green and brown character of the constituent objects. A color photograph -from the air would greatly resemble a pastel in its low range of tones -and the absence of bright colors. - -=The Photographic Requirements Dictated by Brightness and Color -Considerations.=—Considering only the demands made by the character of -the view presented to the airplane camera, and leaving out of account -other limitations to photographic operations in the plane, certain -requirements as to sensitized materials may be outlined. First of all, -the photographic process must not reduce, but should rather be capable -of exaggerating, the range of brightness of the object. Preferably the -seven-to-one range of the object photographed should be lengthened out -to the full range of the printing paper, which may be two to three times -this. With such an increase of range, those minute differences of -brightness are accentuated, on which the detection of many objects -depends. - -Next, the plate or film must be sensitive to the portion of the spectrum -transmitted by a yellow or orange filter which will cut out the effect -of haze. This calls for orthochromatic or panchromatic plates, depending -on the depth of filter required. Next, if the objects to be photographed -differ little in brightness but are different in color composition, we -may have to rely on color filters of peculiar transmissions, capable of -translating these color differences into brightness differences. These -will, in general, call for fully color sensitive, or panchromatic -plates. - -In conclusion it may be pointed out that the endeavor in ordinary -orthochromatic photography—to reproduce the visual brightness of colors -in the photographic print—has no real justification in aerial work. -Neither in respect to color values nor in respect to brightness range is -it the object of aerial photography, especially for war purposes, to -present a truthful tone reproduction. Its aim is rather the adequate -differentiation of detail, by whatever means necessary. - - - - - CHAPTER XVIII - CHARACTERISTICS OF PHOTOGRAPHIC EMULSIONS - - -The purely photographic problem in aerial photography, as distinct from -the instrumental one, is the selection of photo sensitive materials -which will yield useful results under the conditions peculiar to -exposure from the air. After such materials have been found by extensive -field tests, it is preeminently desirable to determine their -characteristics in such terms that the kind of plate or film may -thereafter be specified and selected on the basis of purely laboratory -tests. Specification must be made in terms of the ordinary sensitometric -constants of the photographic emulsion—its speed, contrast, fog, -development factor, its color sensitiveness, its ability to render fine -detail, and its grosser physical properties such as hardness and -shrinkage. - -=Sensitometry.=—The most generally used system of sensitometry is that -of Hurter and Driffield, commonly referred to as the “H & D.” By this -system, in order to determine the characteristics of a given -photographic plate, it is necessary to take a series of graduated -exposures, a standard illumination of the plate being varied in known -amount by a rapidly rotating disc cut to a series of different openings, -or by some other suitable means. The negative thus obtained is developed -in a standard developer for a definite time, at a fixed temperature, and -is then measured for transmission on a photometer. The following terms -are defined and used in plotting the results: - - intensity of light transmitted _I_ - Transparency = _T_ = —————————————————————————————— = ——————— - intensity of incident light _I__{O} - - intensity of incident light _I__{O} 1 - Opacity = _O_ = —————————————————————————————— = ——————— = ——— - intensity of transmitted light _I_ _T_ - - Density = _D_ = -log_{10}_T_ = log_{10}_O_ - -Hurter and Driffield pointed out that a negative would give a true -representation of the differences in the light and shade of the object -if it reproduced these differences by equivalent differences in opacity. -This is equivalent to stating that if the densities are plotted against -the logarithms of the corresponding exposures, a straight line should be -obtained at 45 degrees to the axis of exposure times. If the line is at -another angle the opacities of the negative will be _proportional_ to -the brightness of the object photographed, but the _contrast_ will be -different. - -A typical H & D plot is shown in Fig. 103. It will be noted that two -curves are shown. These are obtained with different developments, and -illustrate the fact that the contrast or proportionality between -exposure differences and opacity differences is a matter of time of -development. Each of these curves exhibits certain characteristics which -are common to all made in this way. There is primarily a _straight line -portion_, where opacities are proportional to illumination. This is -commonly called the region of _correct exposure_. The slope of this -straight line portion—the ratio of density/(log exposure)—is the -_development factor_, commonly denoted by “γ,” a gamma of unity denoting -exact tone rendering. Below the region of correct exposure is a “toe,” -or region of smaller contrast, called the region of _under exposure_. -Above the correct exposure region is another where the opacity -approaches constancy (afterwards decreasing or “reversing”), called the -region of _over exposure_. - -The _speed_ of a plate on the H & D scale is given by the intersection -of the straight line portion of the characteristic curve when produced, -with the exposure axis. This intersection point, called the _inertia_, -is the same irrespective of the time of development, as is shown in Fig. -103. The numerical value of the speed is obtained by dividing 34 by the -inertia, when the exposure is plotted in candle-meter-seconds. - -If a plate is developed until no more density and contrast can be -obtained, its development factor is then γ_{∞}, (gamma infinity), and -the larger this is the more a plate can be forced in development. If the -plate fogs in its unexposed portions this fog is measured and recorded -in density units along with the other constants. The speed of -development is represented by the _velocity constant_, commonly -symbolized by κ. - -[Illustration: - - FIG. 103.—Typical characteristic curves of photographic plate.] - -The length of the straight line portion determines the _latitude_ of the -plate, or the range of permissible exposures to secure a “perfect -negative.” Thus if we assume that an object has a range of brightness of -1 to 30, then a plate with a straight line characteristic extending over -a range of 1 to 120 would have a latitude of 120/30 or 4. That is, the -exposure could be as much as four times the necessary one, and still -give the same result on a sufficiently exposed print. If the latitude of -the plate is too small, the shadows will fall in the under exposure -region, the high-lights in the over exposure portion of the -characteristic curve, with consequent poor rendering of contrasts. - -=Criteria of Speed.=—In airplane photography _speed_ is of paramount -importance, but great care must be exercised to insure that all the -factors are considered which can contribute toward yielding the -desirable pictorial quality in the brief exposure which alone is -possible from the moving plane. A “fast” plate on the H & D scale is not -necessarily suitable for aerial work, when we remember that accentuation -of natural contrast is desirable, particularly under hazy conditions. -For, as is shown in Fig. 104, it is a common characteristic of “fast” -plates to have comparatively small latitude and low contrast at their -maximum development. - -It is to be noted that the Hurter and Driffield measure of speed is -bound up with the idea of correct tone rendering and with the use of the -straight line portion of the characteristic curve. Other criteria of -speed exist. For instance, the exposure necessary to produce a just -noticeable action (threshold value); and the exposure necessary to give -a chosen useful density in the high-lights when development is pushed to -the limit set by the growth of fog. - -As has already been pointed out, correct tone rendering is not necessary -or even indicated as desirable in aerial views. It is, moreover, a -matter of experience that the majority of aerial exposures with existing -plates fall in the “under exposure” period, where contrasts with normal -development are less than in the subject. This being the case, the -problem is to select not necessarily a fast plate, by the H & D -criterion, but a plate which will develop up workable densities in the -under exposure region. A plate of medium speed will sometimes develop to -greater densities in the short exposure region, if development is -forced, than will a fast plate. The contrast in the normal exposure -region will be excessive, but this is of no significance if no exposure -falling in this region is present on the plate. - -[Illustration: - - FIG. 104.—Characteristic curves of fast and slow plates, developed to - maximum contrast.] - -In addition to its capacity for developing density, the plate should -have as low a threshold as possible, thus meeting to some extent the -requirements of both the alternative criteria of speed given above. At -the same time it is true that low threshold and good density for short -exposures are not to be found in really slow plates. Consequently, while -high speed, as ordinarily understood, is undoubtedly the first -requirement, we may expect the complete specification for the best -aerial plate to be a rather complicated thing, describing the -characteristics of a workable “toe” of the curve, in terms of which -several (_e.g._, contrast and speed) are derived from another and quite -different exposure region. - -=Effect of Temperature on Plate Speed.=—It has been found by Abney and -Dewar that very low temperatures materially decrease the speed of -photographic emulsions. This decrease may amount to as much as 50 per -cent. in the temperature range from 30 degrees Centigrade above zero to -30 degrees below zero, which is the range over which aerial photographic -operations will have to be carried on in war-time. This effect has not -been at all fully studied, and it is not known whether it is general or -only found in certain kinds of plates. The remedy indicated is to -provide means for heating the plates or films when low temperatures are -encountered. This is fairly easy in film cameras, or in plate cameras -like the deRam, where the entire load of plates is carried in the camera -body. Plates carried in magazines present a more difficult problem. The -heating coil incorporated in the German cameras is perhaps partly for -this purpose. - -=Color Sensitiveness.=—Complete specifications for an aerial plate -cannot be made solely on the basis of its speed, contrast, latitude, -threshold, and other sensitometric values which have to do only with the -intensity of the light acting on it. These in general apply to -photography from low altitudes, where the illumination and natural -contrast of the subject are the only factors to consider. When higher -altitudes are reached the interposition of haze decreases the already -deficient contrast, calling either for the development of more contrast -in the plate, or for the use of color filters to cut out the action of -the blue and violet light predominant in haze. Along the lines discussed -in the last section, it is not surprising to find that some plates are -better than others for bringing out gradations masked by haze, even -though no filters are used and though the plates are similar in color -sensitiveness. But the limitations to securing contrast by manipulating -the characteristic curve of the plate are soon reached, and it becomes -necessary to resort to haze-piercing color filters, used with _color -sensitive_ plates. - -Roughly, two general types of color sensitive emulsions may be -distinguished: first, those in which sensitiveness to green and yellow -is added to the natural blue sensitiveness, and second, those sensitive -in a useful degree to all colors of the spectrum. The former are called -_iso-_ or _ortho-chromatic_, the latter _panchromatic_ emulsions. -Spectrograms exhibiting the distribution of sensitiveness throughout the -spectrum for several representative plates are shown in Fig. 105. -Orthochromatic plates are adequate for use with light yellow filters and -have the slight practical working advantage that they can be handled by -red light. Panchromatic plates are necessary for use with dark orange or -red filters. They must be handled in total darkness or in an exceedingly -faint blue-green light, taking advantage of the common drop in -sensibility in that region of the spectrum. Plates can, indeed, be -sensitized for the red alone, leaving a gap of almost complete -insensibility in the green, as shown in the fourth spectrogram of Fig. -105. When used with a yellow filter these plates behave as do -panchromatic plates with a red filter. - -[Illustration: - - FIG. 105.—Spectrograms of representative photographic plates: _a_, - ordinary plate; _b_, orthochromatic plate; _c_, specially - green-sensitive plate; _d_, red sensitive plate, insensitive to - green; _e_, panchromatic plate; _f_, specially red-sensitive - panchromatic plate.] - -A rougher idea of color sensitiveness than is given by spectrograms is -furnished by the _tri-color ratio_, which is the ratio of exposure times -necessary with white light to give equal photographic action through a -certain set of red, green and blue filters, expressed in terms of the -blue exposure as unity. In an excellent panchromatic plate the three -exposures would be equal. In an orthochromatic plate the red exposure -will be too large to be figured. In interpreting either spectrograms or -tri-color ratios care must be taken that the _absolute_ exposures -necessary are known. Thus a relatively high red sensitiveness may mean -merely low absolute blue sensitiveness. - -Two methods are used in imparting color sensitiveness. Either the -sensitizing dye is incorporated in the plate emulsion before it is -flowed; or the plate is bathed in a dye solution not long before using. -The latter method gives higher color sensitiveness but poorer keeping -quality, and is not a practical method for field operations. Greatly -enhanced sensibility may be given by treatment with ammonia, but this -again is a method for laboratory rather than field use. - -=Resolving Power.=—A question which arises in connection with all -photography of detail is the size of the grain of the photographic -emulsion. Dependent on the size of the grain is the _resolving power_, -or ability to separate images of closely adjacent objects. This varies -with the speed, fast plates being of coarser grain than slow ones; with -the exposure; and with the method and time of development. In general, -it may be said that the resolving power of the plate does not enter -practically into aerial work, because the resolving power of all plates -so far found usable corresponds to a smaller distance than the size of a -point image as limited by the performance of the camera lens and the -speed of the plane. Remembering that ⅒ mm. is a fair value for the size -of a point image as rendered by the lens, the rôle of plate-resolving -power is shown by consideration of the following table. Resolving powers -are given in terms of lines to the millimeter just separable. - - Emulsion. Resolving Power. - Seed Graflex 25 - Eastman Aerial Film 37 - Hammer Ortho 44 - Cramer Isonon 48 - Cramer Spectrum Process 57 - Eastman Portrait Film 61 - -=Tabulation of Requirements for Aerial Emulsions.=—In terms of the -sensitometric quantities just discussed the general requirements for -aerial plates may be listed as follows: - -1. _Speed._ The speed usually connected with the contrast and density -required for the exposure times available is about 150 H & D. Faster -plates in general have too low contrast, but the highest speed that will -give the necessary contrast is desired. - -2. _Contrast._ The contrast capable of development without fog should be -from 1.5 to 2. This contrast should be produced by light of daylight -quality, and, in orthochromatic and panchromatic plates, with the yellow -or orange filters intended to be used with them. This contrast means a -gamma infinity approaching 2.5. - -3. _Speed of development._ A gamma of nearly 2 should be developed in 2½ -minutes at 20 degrees C. in the developers recommended below. - -4. _Fog._ Not over .25 for this degree of development, and not over .40 -for six minutes development. - -5. _Color sensitiveness._ This should in general be as high as possible. -In terms of certain representative filters (described in a subsequent -chapter) color sensitiveness should be such that with the white light -speed above specified the relative exposures through the filters shall -not be greater than as follows: - - No filter Aero 1 Aero 2 #21 #23a #25 - Panchromatic plate 1 3 4.5 7 9 12 - Ortho plate 1 2.5 3.5 6 - -=Relative Behavior of Plates and Films.=—The advantages of film from the -standpoint of weight and bulk have been discussed in connection with -aerial cameras. Were there no other considerations film would -unquestionably be the most appropriate medium for aerial photography. -There is, however, the question of ease of handling, to be treated in a -subsequent chapter, and the question whether the purely photographic -characteristics of film are satisfactory. Can the same speed, contrast, -and color sensitiveness be obtained on film as on glass? Is the picture -so obtained as permanent or reliable as the plate image? - -It must be confessed that up to the present emulsions on film have not -proved the equal of those on glass. It has been found by emulsion -manufacturers that the same emulsion flowed on film and on glass gives -better quality on the glass. Emulsions specially prepared for film fall -somewhat short of the best plate emulsions. It has also been found -harder to color-sensitize film, and to insure good keeping quality in -the color sensitized product. - -In addition to the question of photographic quality there arises the -matter of shrinkage and distortion. These are negligible with plates, -but are a more or less unknown quantity in film. Irregular shrinkages of -as much as two per cent. are found on experiment. This defect, of -course, would be an obstacle only in exact mapping work. - -=Positype Paper.=—The need sometimes arises in military operations to -secure prints ready for examination within a few minutes after the -receipt of the negatives. Even the 15 or 20 minutes within which a -negative can be developed and a wet print taken may be considered too -long. While such occasions are probably more apt to occur in popular -magazine stories than in actual warfare, it is important to have -available methods of producing prints with an absolute minimum of delay. -This need is met to some degree by a direct print process, commercially -exploited under the name of “Positype.” - -In this process the exposure is made directly on a sensitized paper or -card, which is developed, the image dissolved out, the residue exposed, -and again developed; thus furnishing a positive picture (reversed right -and left). The time necessary to develop a print ready for examination -need not be more than three minutes. Only a single print is available, -but this is all that would be called for under the extreme conditions -suggested. If later, copies are desired they may be made by the same -process. - -=Plates and Films Found Satisfactory for Aerial Work.=—The following -plates and films have been found particularly good for aerial -photography. The list is not intended to be complete. Furthermore, it -may be expected to be soon superseded, as the efforts of various -manufacturers are directed toward developing special aerial photographic -plates. - -Among orthochromatic plates: The Cramer Commercial Isonon, the Jougla -Ortho. - -Among panchromatic plates: The Ilford Special Panchromatic, the Cramer -Spectrum Process. - -Film: Ansco Speedex, Eastman Aero. - - - - - CHAPTER XIX - FILTERS - - -=The Function of Filters in Aerial Photography.=—The use of color -screens or filters has been very common in ordinary landscape -photography, for the purpose of securing approximately correct -renderings of the brightnesses of colored objects. Plates of the -non-color-sensitive type have their maximum of sensitiveness in the blue -of the spectrum (Fig. 105) and in consequence blue skies photograph as -white, while other colors are likewise reproduced on a totally wrong -scale. Filters for correct brightness rendering are calculated for a -given color sensitive plate so that the resultant reaction to the light -of the spectrum copies the sensitiveness of the eye, which is greatest -in the yellow-green. Such filters for use with the common orthochromatic -plates are of a general yellow color. - -Filters for aerial work are meant to serve quite a different purpose. -Correct tone or color rendering is of quite secondary importance to -another use of filters, namely, to cut or pierce aerial haze. It is -quite a matter of accident that the same general color of filter is -called for both to give correct color rendering and to pierce aerial -haze, namely, _yellow_. Yet on closer analysis it is found that quite -different types of yellow filter are demanded, spectroscopically -considered. - -Figure 106 (K_{1} and K_{2}) shows the spectral transmission curves of -the Wratten K_{1} and K_{2} filters, intended for correct color -rendering with orthochromatic plates. The absorption increases -_gradually_ toward the blue. In the same figure is shown on an arbitrary -scale the spectroscopic character of typical haze illumination, -increasing in brightness inversely as the fourth power of the -wave-length, that is, with great rapidity in the blue and violet. It is -evident from this that a much more abrupt absorption than that of the -K_{1} or K_{2} filter is desirable, because in the green of the spectrum -the haze light is comparatively weak, and more will be lost by any -absorption in this region through decreasing useful photographic action -than will be gained by cutting out the haze. This latter consideration -is important. The use of any filter means an increase of exposure; the -use of yellow filters multiplies it several times. Careful experiment -has shown that no filter of depth less than K 1½, to use the Wratten -filters as a basis for discussion, are of real value in haze piercing. -The _filter ratio_, or ratio of exposures with and without filter, is -4.7 for the K 1½ with the Cramer Isonon plate—a figure which shows the -importance of securing the necessary haze-piercing character with the -minimum absorption of useful photographic light. - -[Illustration: - - FIG. 106.—Characteristics of various filters.] - -=Practical Filters.=—Since the character of the absorption of the “K” -filters is not all that could be desired, new filters, both of dyed -gelatin and of glass, have been produced. The glass, a Corning product -having a very sharp-cut absorption, has not yet been produced on a -commercial scale with the high transparency in green, yellow and red -that selected samples have shown. The United States Air Service has -adopted filters of a new dye, called the EK, from the name of the -company in whose laboratory it was produced. These filters are -standardized in two depths of staining, called the “Aero No. 1” and -“Aero No. 2.” Their spectral transmission curves appear in Fig. 106, -along with those of certain darker filters useful only with panchromatic -plates for exceptionally heavy haze. The characteristic of these Aero -filters is their great transparency through all the spectrum except the -blue, whereby the greatest haze-cutting action is attained together with -a low filter factor. The filter factors of the Aero No. 1 and No. 2 with -Cramer Isonon plates are 3 and 5, respectively. - -=Effects Secured by the Use of Filters.=—The efficiency of yellow -filters for haze-cutting is best shown by photographs taken at high -altitudes with filters and without. Such illustrations are given in -Figs. 107 and 108, where the first photograph is one taken at 10,000 -feet without a filter, the second taken at the same altitude under the -same conditions, but with an orange filter. Both are on panchromatic -plates, and it will be seen that even with these plates the filter makes -all the difference between a useless and a useful picture. But it must -be clearly understood that the difference here lies between a plate -sensitive chiefly in the blue and violet, and a plate affected only by -the yellow, orange and red. The difference is not between what the eye -sees and what a plate with a filter sees, as is sometimes supposed. As -shown in Fig. 108, a filter enables the plate to photograph through the -haze between clouds, but not through the clouds themselves. In general, -no filter and plate combination which is feasible for aerial exposures -is capable of showing more than the eye can see if yellow or orange -goggles are worn. To do this it would be necessary for the photographic -action to take place by deep red or infra-red light, which would demand -exposures now out of the question. - -[Illustration: - - FIG. 107.—A photograph taken at 10,000 feet, without a filter.] - -Filters are almost always necessary in photographing from high altitudes -or in making distant obliques. At times, particularly after a heavy -rain, the air is clear enough so that filters may be dispensed with. -Clearing weather was therefore chosen whenever possible for making -obliques of the battle front. - -[Illustration: - - FIG. 108.—Photograph taken at same time and over same neighborhood as - Fig. 107, but with an orange filter.] - -=Filters for the Photographic Detection of Camouflage.=—In the -photographic as in the visual detection of camouflage, the problem is to -differentiate colors which ordinarily look alike, but which are actually -of different color composition. Particularly important are the -differences between natural foliage greens and the paints used to -simulate them. If these differ in their reflection spectra, a proper -choice of filter will show up the two greens as markedly different. Two -kinds of difference may be produced; either the two colors may be -changed in relative brightness, or they may be altered in hue. Thus -foliage green, due to its possessing a reflection band in the red of the -spectrum, which is absent in most pigments, may be made to appear _red_ -while the camouflage remains green or turns black. Filters which cause -changes of color are of course of no use for photographic detection of -camouflage, since the photographic image is colorless. Brightness -differences are alone available. - -Those same filters which have been worked out primarily for producing -brightness differences in visual detection of camouflage could be used -photographically, provided the plates employed were color sensitive, and -were as well screened to imitate the sensibility of the eye. But the -most useful visual filters are those causing color differences to -appear; more than this, the visual camouflage detection filters as a -class have low light transmissions, so that their usefulness in -photography is doubtful. Little work has actually been done with -camouflage detection filters for photography. Yet in spite of this -photography has been of real service in this form of detective work. Its -utility for the purpose comes from the fact that the natural -sensitiveness of the plate to blue, violet and invisible ultra-violet -acts to extend the range of the spectrum in which differences between -identical and merely visually matched colors may be picked up. -Consequently the plain unscreened plate has proved a very efficient -camouflage detector—so efficient in fact that all camouflage materials -have had to be subjected to a photographic test before acceptance. Fig. -171 shows how an ordinary photograph reveals the unnatural character of -the camouflage over a battery. - -=Methods of Mounting and Using Filters.=—The most primitive way of -mounting a gelatin filter is to cut a disc from a sheet of dyed gelatin -and insert it between the components of the lens. For this purpose the -gelatin must be perfectly flat, which is insured by its method of -preparation and test. One disadvantage of this method is that the filter -can be inserted and removed only upon the ground. It is less -satisfactory the larger the diameter of the lens, and the wastage of -filters due to insertion and removal is apt to be high. The camera -should be refocussed after filters of this kind are inserted. - -Glass filters, ground optically true, or gelatin filters, mounted -between optically flat glass plates, are the most convenient and -satisfactory. They may be mounted in circular cells to screw or attach -by bayonet catches to the front of the lens. Or they may be mounted in -rectangular frames to slide into transverse grooves in the camera body. -Fig. 44 shows the mount of this latter form adopted in the larger United -States Air Service cameras. This is particularly convenient if it is -desired to insert or change the filter while in the air—a practice not -generally considered feasible in war work with the photographically -inexperienced observer, but likely to be common with the employment of -skilled photographers for peace-time aerial photography. - -German cameras are reported in which the glass filter is carried behind -the lens, on a lever which also carries a clear glass plate of the same -thickness, to be thrown in when no filter is needed, thus maintaining -the focus. The performance of the lens will be impaired by this scheme, -unless it is specially calculated to offset the effect of the glass -introduced in the path of the rays behind the lens—optically true glass -has no effect on definition if placed in front of the lens. Glass -filters may also be placed in close contact with the plate or film, in -which case they must be much larger, but do not need to be of as good -optical quality. - -=Self-screening Plates.=—Mention must be made of a quite different mode -of realizing the filter idea, a method available where the sensitive -plate is always to be used with a filter. This is to incorporate a -yellow dye in the gelatin of the plate itself. The dye must be one which -has no direct chemical effect on the plate, but which acts simply as a -coloring agent for the gelatin. “Self-screening” plates, as they are -called, have been produced by the use of the dye called “filter yellow” -and have found some use in orthochromatic photography. They effect a -useful saving of light through the elimination of the reflection losses -at the surfaces of glass and gelatin filters. The filtering action of -the dye in the plate is somewhat different from its ordinary one, since -the deeper portions of the sensitive film are subject to greater action -than the surface, and this tends to diminish contrast. - - - - - CHAPTER XX - EXPOSURE OF AERIAL NEGATIVES - - -The principal factors governing the length of exposure in the airplane -camera have already been discussed under various headings. These are -briefly, the nature of the aerial landscape, the practically attainable -lens apertures, the form of the camera support, the speed of the plane, -and the characteristics of plates, films and filters. It is convenient -however, to re-assemble this information in one place, in such form as -to apply to the practical problem of determining the exposure to be -given in any specific case. - -=Limitations to Exposure.=—In the ordinary photography of stationary -objects, exposure is a variable entirely at the operator's command. -Plates of any speed may be selected, so that attention may be focussed -on latitude, color sensitiveness, and other tone rendering -characteristics. The exposure may be made of a length sufficient to -insure all the useful photographic action lying in the “correct -exposure” portion of the sensitometric curve. The exposure ratio of any -filter it is desired to use is a matter of indifference—its effect on -color rendering need alone be considered. - -Airplane photography is sharply distinguished from ground “still” -photography by its severe limitations as to the amount of the exposure. -The actual duration is definitely restricted by the high speed of the -plane. In peace work this can be offset in part by using slower planes -or by flying against the wind. The practical limitation to 1/100 second, -set by war-time requirements as to definition of fine detail, may be -increased to 1/50 of a second, or even more, where mapping of grosser -features is the object. A common, but entirely avoidable limitation, is -that due to vibration of the camera. By proper mounting this may be -entirely overcome, leaving the ground speed of the plane the only source -of exposure-limiting movement. The amount of light reaching the plate -constitutes a primary factor in exposure, and this is a matter of lens -aperture. Generally, lens aperture is smaller the larger the plate -required to be covered, and the greater the focal length. Because of -their larger aperture, the short-focus lenses which will be favored for -peace-time large-area mapping will permit more and longer working days -than have been the rule in long-focus war photography. The necessary use -of filters, particularly at the high altitudes which would be chosen in -mapping, in order to economize in the number of flights needed to cover -a given area, introduces an inevitable decrease in the amount of light -available at the plate, as compared with surface photography under the -same illuminations. - -Broadly speaking, it may be said that all the demands made in reference -to aerial photographic exposure work are to _decrease_ the amount of -light reaching the plate. Any surplus offered, as by the midsummer -noon-day sun, must be immediately snapped up, either by decreasing the -exposure to get greater sharpness, or by introducing filters to get -greater photographic contrast. The absolute exposure of the plate tends -to be kept at the irreducible minimum. As already stated, it lies, with -present photographic materials, on the “toe” of the “H & D” curve, just -reaching up into the straight line portion. - -=Estimation of Exposure.=—According to the foregoing argument the -problem of estimating an aerial exposure resolves itself largely into -one of deciding how short this may be made. Or, if the light is strong, -whether it is sufficient so that a filter may be introduced without -demanding more than the 1/100 second or thereabouts which is dictated by -the motion of the plane. - -Deciding upon exposures in the field has been largely a matter of -experience and judgment. A majority of the cameras in use during the war -were not furnished with shutters calibrated in definite speeds. -Consequently, the sergeant upon whom the decision usually devolved -became a storehouse of knowledge as to the slit widths and tensions -appropriate to each individual camera. This knowledge had to be acquired -from the results of actual photographic reconnaissances, or from special -test flights, both of them wasteful methods. But the chief objection to -this state of affairs lies in the fact that the knowledge thus acquired -is of no use to anyone else, nor is it applicable to other types of -camera. - -The first essential to placing exposure estimation upon a sound basis is -therefore an accurate knowledge of shutter performances. Either the -shutter speeds should be placed upon the camera by the manufacturer and -periodically checked, or a regular practice should be followed of -calibrating shutters, either at a base laboratory or even in the field. - -Assuming that the speeds of all shutters are accurately known, the -process of estimating the requisite exposure becomes less a matter of -mere guesswork and more nearly a matter of precision. For this purpose -data on the variation of light intensity during the day and during the -year (Figs. 101 and 102) should be taken as a guide. These data refer of -course to visual and not to photographic light, but since it is always -necessary to use color filters, which make the active light of -approximately visual quality, this is no valid objection. The effects of -clouds and mist must of course be learned largely by experience, but -with the above daylight data at hand, anyone in possession of definite -information on the correct exposure with a given plate for a known day -and hour need not go far wrong in estimating exposures at any other time -in definite fractions of a second. - -[Illustration: - - FIG. 109.—Chart showing aerial exposures for all times of the day and - year. Data on basis of F/5.6 lens, Jougla orthochromatic plate, and - clear sunlight, no filter. Exposures to be doubled and tripled for - overcast and cloudy weather.] - -_Exposure data charts._ Fig. 109 shows a chart, prepared in the French -service, indicating aerial exposures for all hours of the day throughout -the year. These are for clear sunlight, for a lens of aperture F/5.6 and -for “ortho” plates without a filter. They are based on what is probably -an over-estimate of the actual speeds given by the French shutters. For -“light” clouds the exposures are to be doubled, for “heavy” clouds -quadrupled, and for forests and dark ground “lengthened.” Charts of this -form should be extremely useful, but they were actually not of great -service because of the prevalent lack of knowledge of true shutter -speeds. - -_Exposure meters._ Aerial photography offers an excellent opportunity -for the use of exposure meters, particularly those of the type in which -a sensitive surface is exposed to the light for a measured time -sufficient to darken a predetermined amount. The sensitive paper of the -meter may either be exposed from the ground to the direct light of sun -and sky, or from the plane to the light reflected from the ground. The -first method will give figures subject to some correction for the -character of the ground to be photographed—whether fields, forests, or -snow. The second method is to be preferred where the shutter speed can -be adjusted in the air, according to the indications of the meter, or -where the filter can be selected and put in place during flight. Trials -with a commercial Wynne exposure meter, used in the latter manner, give -as a working figure an exposure of .001 second for each 4½ seconds taken -to darken the sensitometer strip to match the darker comparison patch. -This relation applies to a lens of aperture F/4.5, on Cramer Commercial -Isonon plates without filter. - - - - - CHAPTER XXI - PRINTING MEDIA - - -Skilled photographers can examine a negative and can interpret its -renderings with practically as much satisfaction as they get from a -print, whereby a considerable amount of time can be saved in an -emergency. The original glass negative should always be used when -accurate measurements are to be made. These and a few other cases -constitute the only use of a negative apart from its normal one, namely, -for producing positive prints, usually in large numbers. The commonest -form of print is on paper, although the most satisfactory print from the -photographic standpoint is the transparency on glass or celluloid film. - -=Transparencies.=—Transparencies are made by the regular photographic -processes of exposure and development, on glass plates or films placed -in contact with the negative, or in the appropriate position in an -enlarging camera. The sensitometry and the terms used to describe the -qualities of plate or film for this purpose are those already given in -connection with the general discussion of plates and films. But the kind -of emulsion to be selected is quite different from the aerial negative -emulsion. There is here no practical limitation to the speed, contrast -or latitude. Consequently, we can choose a positive emulsion on which -the exposure through the aerial negative falls entirely on the straight -line portion of the characteristic curve, thus reproducing all of its -tones, and the contrast of the negative may be increased to any desired -extent. The possibilities of positive emulsion are indeed rather greater -than the usual aerial negative can utilize. A range of clearly graduated -opacities of two or three hundred to one is possible, so that not only -can detail be well rendered in the high-lights, but also equally well in -dark shadows where, indeed, an increase of illumination is necessary for -it to be made easy to examine. This range is to be contrasted with the -1-to-7 range in the aerial landscape, which may be doubled by a -contrasty plate. In resolving power, the positive emulsion, which is -slow, exceeds the negative emulsion. It easily bears examination through -a magnifying glass, thus making any enlargement unnecessary in the -printing process. - -Glass transparencies are of course impractical for general distribution, -on account of their fragility. Heavy film transparencies are not open to -this objection, and, especially in the form of stereos, constitute the -most beautiful form of aerial photographic print. - -=Paper Prints.=—Prints on paper suffer by comparison with -transparencies, in the range of tones which they exhibit. This lies -between the white of the paper, which never has more than 80 per cent. -reflecting power, and its darkest black, which differs with the kind of -paper. In dull or mat papers the blacks will reflect as much as 5 per -cent.; in glossy papers, ordinarily used for aerial negatives, the -reflection from the black may be as low as one per cent., but in order -to get the benefit of this the paper must be so held as not to reflect -any bright object to the eyes. This deficiency in the range of paper -gradations is not so serious with aerial negatives as with ordinary -properly exposed negatives because of the small range of brightness in -the aerial view. - -The sensitometry of papers is similar to that of plates, with the -difference that reflecting powers take the place of transparency. As in -the case of transparency emulsions there is in papers no dominating -requirement for extreme speed, to which other characteristics must be -subordinated. Yet speed is of sufficient importance in handling large -quantities of prints so that aerial negative printing for military -purposes has been done almost entirely on the rapid enlarging papers, -rather than on the true contact printing papers, which are slower. - -[Illustration: - - FIG. 110.—Characteristic curves of bromide paper.] - -The two principal types of rapid enlarging papers, the bromide and the -“gas light,” exhibit certain characteristic differences which are -important to bear in mind in seeking to obtain any particular quality of -print. Bromide papers, of which “Nikko” is a good example, show -sensitometric curves rather like those of plates. That is, they increase -in contrast with continued development. At the same time, as is shown in -Fig. 110, they increase somewhat in speed with development; that is, -under exposure can be compensated for to a small degree by protracted -development. These characteristics of bromide paper can be utilized to -secure prints of a quality quite different from that of the negative. -Thus, if the negative has a long range of tones, a flat print can be -secured by full exposure and short development. If, as is apt to be the -case with aerial negatives, a print of greater contrast than the -negative is desired, a short exposure with long development is called -for. - -[Illustration: - - FIG. 111.—Characteristic curves of gas light paper.] - -The sensitometric curves of a typical gas light paper “Contrast -Enlarging Cyco,” are shown in Fig. 111. Here the contrast is a fixed -characteristic of the paper, and the only effect of changing development -is on the speed; that is, exposure and development are, within limits, -interchangeable. - -Choosing a printing paper is a matter of deciding on the contrast -required for the class of negative, and selecting a paper which will -give this contrast with a good range of tones from a clear white to a -deep black. The ideal paper would be one which was all straight line in -the H & D plot. In such a paper there would occur no loss of contrast in -the lighter tones when the high-lights were rendered by the clear white -of the paper. Too great contrast with a short straight line portion, -results in loss of detail at the ends of the scale. A negative -possessing a very great range of tones cannot be correctly represented -on one paper print—two printings are required, one for high-lights and -one for shadows, but this difficulty is rarely to be faced in aerial -views. The greatest demand for aerial printing papers has been for those -of considerable contrast, because of the flat character of the -negatives. - - - - - CHAPTER XXII - PHOTOGRAPHIC CHEMICALS - - -=General Considerations.=—Developing, fixing and other chemicals for -aerial work differ in no essential respect from those used in ordinary -photography. Full discussions of these are to be found in numerous texts -and articles. The aerial photographic problem is to select those most -suited for the under-exposed flat negatives characteristic of -photographs from the air. At the same time selection from among the -chemicals of appropriate quality must be governed by considerations of -the conditions surrounding work in aerial photographic laboratories. -These laboratories, especially in war-time, are apt to be most primitive -in their facilities. - -=Characteristics of Developers for Plates and Films.=—From the -standpoint of practicability, aerial negative developers should have -good keeping power, be slow to exhaust, and work well over a -considerable range of temperatures. From the standpoint of the -photographic quality desired in the negative, the developer should bring -up the maximum amount of under-exposed detail. This means that it should -impart the highest possible speed to the plate, with good contrast, and -low fog or general reduction of unexposed silver bromide. - -There are many characteristics to study in a developer: its effect on -inertia or speed, gamma infinity, fog, time of appearance, “Watkins -factor,” speed of development, temperature coefficient, dilution -coefficient, keeping power, exhaustion, length of rinsing, stain, color -coefficient and resolving power. These are defined and described as -follows: - -_Effect on inertia._ The meaning of inertia has already been given under -the discussion of plate speed. While this is a constant, independent of -time of development, for any one developer, it is altered appreciably by -change of the latter. - -_Time-gamma relation._ Contrast, symbolized by γ, has likewise been -discussed under plate sensitometry. Viewed from the standpoint of the -developer, the point of interest is the rate at which γ varies with -development, and the maximum contrast which can be reached or γ -infinity. Speed of development is commonly defined by the _velocity -constant_, symbolized by κ, which is arrived at mathematically from a -consideration of the time of development to produce two different -contrast values. High γ infinity is desired for aerial negatives, and -for rapid work κ must also be high. - -_Fog._ The opacity due to chemical fog is to be kept at a minimum in -aerial negatives, as it is chiefly prejudicial to under exposures. - -_Time of appearance and Watkins factor._ The time of appearance is -measured in seconds. The Watkins factor is a practical measure of the -speed of development, and is determined by the ratio of the time of -development required for a definite contrast, to the time of appearance. -It is useful also as a guide to development time. - -_Temperature coefficient._ This is the factor by which the time of -development at normal temperature (20 Cent.) must be increased or -decreased in order to obtain the same quality negative, for a change of -seven degrees either side of normal. - -_Temperature limits_ are the temperatures between which development can -be carried out with any degree of control or without serious damage to -the negative. These factors are of great importance where climatic or -seasonal changes have to be endured. - -_Dilution coefficient._ This is the factor by which the development time -is increased in order to maintain a given quality negative in different -dilutions of the developer. It is useful in tank development. - -_Keeping power._ The keeping power of a developer, mixed ready for use, -is determined by its ability to resist aerial oxidation. A developer of -poor keeping power, which must be made up immediately before use, causes -delay and waste of time whenever emergency work has to be done, whereas -a developer of good keeping power may be left in its tank ready for -instant use. - -_Exhaustion_ of a developer is the rate at which it becomes useless for -developing, due both to aerial oxidation and to the using up of its -reducing power by the work done in developing plates. It is conveniently -measured by the area of plate surface developable before the solution -must be renewed. - -_Length of rinsing._ The time required for rinsing between development -and fixing bath plays a not unimportant part in total development time. -Dichroic fog is caused with some developers if, due to insufficient -rinsing, any of the caustic alkali is carried over to the fixing bath. -Stains develop also if the fixing bath is old, or if light falls on the -unfixed plate while any developer remains in the film. - -_Color coefficient._ The function of the sulphite, which forms a -constituent of all developing solutions, is two-fold. It acts partly as -a preservative, and partly to prevent the occurrence of a yellow color -in the deposit. The yellow color, if present, increases the photographic -contrast. This phenomenon has been purposely utilized, particularly in -the British service, to give “stain” to negatives which otherwise would -show insufficient printing density. The color index or coefficient of a -negative (with a given printing medium) is the ratio of photographic to -visual density. If we take a pyro developer containing five parts of -pyro per thousand and ten parts of sodium carbonate, and then vary the -amount of sulphite from none to fifty parts per thousand, the color -index varies as follows: - - Sulphite Color Index - Parts per Thousand - 50 1.16 - 25 1.24 - 15 1.30 - 10 1.45 - 5 1.80 - 0 2.75 - -The color index is somewhat different with various kinds of printing -media. - -This staining effect is a variable one, depending upon length of -development, dilution of the developer, length of rinsing, temperature, -the fixing bath used (plain hypo being necessary for a maximum effect), -the length of washing after fixation and the properties of the water -used. Standardization of these conditions in the field is difficult; -hence any developer which will give the same effective contrast without -resorting to stain is to be preferred. - -_Resolving power._ Some developing processes and conditions will -introduce bad grain into the negative. Hence the resolving power which a -developer brings up must be investigated among its other -characteristics. - -=Practical Developers for Aerial Negatives.=—In the English service a -pyro metol developer was generally used, producing stained negatives. -The French, American and Italian practice was to use metol-hydrochinon, -without staining. A special chlor-hydrochinon developer, worked out by -the Eastman Research Laboratory for the United States Air Service, has -probably the greatest merit of any yet tried. A comparison, given below, -between it and a pyro metol formula used on a representative plate, -illustrates the use of the various bases of study given above. - - PYRO FORMULA - - Solution A Solution B - Pyro, 3.75 grams Sodium carbonate, 53 g - Potassium metabisulphite, 3.75 g - Metol, 3.05 g - Potassium bromide, 1.5 g - Water, 500 c.c. Water, 500 c.c. - - Use 1 part of A to 1 of B - - CHLORHYDROCHINON FORMULA - - Solution A Solution B - Chlorhydrochinon, 25 g Sodium carbonate, 30 g - Metol, 6 g Sodium hydrate, 10 g - Sodium bisulphite, 2.5 g Potassium bromide, 3 g - Sodium sulphite, 25 g - Water to 670 c.c. Water to 330 c.c. - - Use 2 parts of A to 1 of B - - Pyro Chlorhydrochinon - H & D speed 150 180 - Gamma infinity 1.45 2.12 - Fog (at maximum gamma) .32 .60 - Time of appearance 5 seconds 5 seconds - Watkins factor 25 10 - Velocity factor “κ” .320 .400 - Temperature coefficient 1.40 2.0 - Temperature limits 4° to 32° C 4° to 32° C - Keeping power 45 minutes 8 days - Exhaustion (100 c.c.) 30 sq. in. 300 sq. inches - Dilution coefficient 2 2 - Color coefficient 1.50 1.00 - Resolving power 47 53 - -Owing to the difficulty of securing pure chlor-hydrochinon a metol -hydrochinon of very similar properties has been worked out. Its -composition is - - Metol 16 grams - Hydrochinon 16 grams - Sodium sulphite 60 grams - Sodium hydroxide 10 grams - Potassium bromide 10 grams - Water to 1 litre - -To keep the ingredients in solution in cold weather, 50 c.c. of alcohol -should be included in every litre of solution. All things considered -this is probably the most practical and satisfactory developer for -aerial negatives. - -=Developers for Papers.=—The following formula has been found very -satisfactory for papers: - - Metol .9 gram - Hydrochinon 3.6 gram - Sodium carbonate 20.0 gram - Sodium sulphite 14.0 gram - Potassium bromide .5 to 1.0 gram - Water to 1 litre - -=Fixing Baths.=—For plates the following fixing and hardening bath is -recommended: - - Sodium thiosulphate (hypo) 350 grams - Potassium chrome alum 6 grams - Sodium bisulphite 10 grams - Water to 1000 c.c. - -During hot weather, the above quantities of chrome alum and bisulphite -are doubled. - -For papers the following: - - Hypo, 35 per cent. 100 volumes - Acid hardener 5 volumes - -The acid hardener is constituted as follows: - - Alum 50 grams - Acid acetic 28° 400 c.c. - Sodium sulphite 100 grams - Water to 1 litre - -=Intensification and Reduction.=—These processes have been little -employed in air work. Reduction is rarely necessary, for obvious -reasons. Intensification would often be of value, but the common -practice, which saves some time, is to use printing paper of strong -contrast for those negatives which are deficient in density and -contrast. When intensification is desirable or permissible, either the -ordinary mercury or uranium intensifier may be used. - -=Water.=—In the field it is found necessary in many cases to purify the -water that is to be used in mixing up chemicals. Water may contain -suspended matter or dirt, dissolved salts, and slime. It is important to -remove the suspended matter, as it may cause spots on the plates and -papers, while any slime would coagulate, forming a sludge in the -developer which would also tend to settle on the plates and cause marks -during development. The dissolved salts may or may not cause trouble. -Two methods of purification are possible: - -(_a_) Filter the water through a cloth into a barrel, add about one gram -of alum for every four litres of water, and allow to settle over night. -Draw off the clear liquid from a plug in the side as required. - -(_b_) Boil the water and allow it to cool over night. If the water -contains dissolved lime, boiling will often cause this to come out of -solution. - - - - - V - METHODS OF HANDLING PLATES, FILMS AND PAPERS - - - - - CHAPTER XXIII - THE DEVELOPING AND DRYING OF PLATES AND FILMS - - -=Field Requirements.=—Developing, fixing, drying and printing in the -field demand simple and convenient apparatus that may be carried about -and installed with the least amount of labor. On top of these -requirements military needs impose others that are more difficult. -_Speed_ is, on occasion, imperative. A print may be required within a -few minutes after landing, and many thousands within a few hours. -Quantity production must be achieved under the most primitive -conditions. Nothing, in fact, shows the calibre of the photographic -officer better than his choice of workplaces as the army moves forward. -Ingenuity and practical judgment are at a premium. Cellars, stables, dog -kennels, or huts hastily built from packing cases, must be equipped and -in working order over night. All the facilities offered by a great city -are urgently needed—water, electric light, power for driving fans—but -must be dispensed with if the photographic section is to be convenient -to the airdrome, whose portable hangars are most apt to be pitched in -the open country. Water must be carried, electricity generated, and to -the photographic problem is added the military one of concealment and -protection. Dugouts and bomb proofs must be built for supplies, and -“funk holes” for the men. Entire underground emergency extensions have -sometimes been built in stations occupied for extended periods, for -airdromes are a favorite bombing target. - -For getting the exposed plates to the photo section, messengers, on -motorcycles if possible, are employed. In some cases, where hangars and -photographic hut are forced to be widely separated, recourse has been -had to parachutes (Fig. 112), a device also employed to distribute -prints to infantry during an advance. - -[Illustration: - - FIG. 112.—Receiving pictures from plane by parachute.] - -For warfare of movement, especially in sparsely settled or devastated -country, where cellars are unavailable, the dark room must be taken -along. _Motor trucks and trailers_ (Figs. 113, 114, 115), the former for -hauling supplies and electric light generating plant, the latter fitted -as a complete developing and printing laboratory, form the headquarters -of each photographic section in the field. Usually altogether too small -for the amount of work required, they were extended by tents and -lean-to's, or ingeniously used as a nucleus for the organization of the -favored stable or cellar. - -[Illustration: - - FIG. 113.—Mobile photographic laboratory.] - -=Methods of Plate Development.=—Where speed is not required the simplest -and commonest mode of developing plates is in the tray, one plate at a -time. Common practice is to examine the plate at intervals during -development, and discontinue the operation on the basis of its -appearance. This is only possible if the plates used are insensitive to -some light by which the eye can see. Deep red light is suitable for -ordinary and most orthochromatic plates. A faint blue-green may be used -with some panchromatic plates. The best practice, however, is to develop -by time in total darkness, whereby all chance of dark room fog is -avoided. Development time for plates of the average exposure of the one -to be developed is either known from previous experience, or is found by -trial on the first one. Development by time results in negatives of -densities varying with the exposures, but, as was brought out in the -discussion of sensitometry, this difference can be compensated for by -the choice of the paper used for printing, and by its treatment. - -[Illustration: - - FIG. 114.—Interior of photographic trailer, developing room.] - -Where larger quantities of plates are to be handled _tank development_ -is adopted. In ordinary tank development the plates are placed in -grooved tanks, into which is poured first the developer, next the -rinsing water, and then the hypo. It has been customary in tank -development as practiced for peace-time work to use dilute developer, -requiring from ten to thirty minutes, but speed requirements in war-time -aerial photography dictate the use of full-strength quick-acting -developer. An improvement on the simple grooved tank is provided by -metal cages or racks, each holding a dozen or more plates, which may be -introduced or removed from the tank as a unit (Fig. 116). - -[Illustration: - - FIG. 115.—Interior of photographic trailer. Enlarging camera and - printer.] - -The _core rack_ system combines certain of the features of both tray and -tank development. Each plate is inserted in a separate metal frame with -projecting lugs to rest on the top of the tank and so suspend the plate -in the solution. The process of development is the same as in the tank -system, but any individual plate may be examined and removed. - -[Illustration: - - FIG. 116.—Tank and rack for tank development.] - -=Film Developing and Fixing.=—The problem of quickly handling roll film -of large size is one upon whose solution depends in large degree the -feasibility of film cameras for aerial work. It presents many -difficulties: a long film is unwieldy, is inherently subject to curling, -and takes up much space if it is handled entire. For small scale -operations roll film can be cut into short strips and developed either -by drawing through a tray or, if cost of developer is no object, in a -deep tank. In order to make the cutting apart of exposures easy in the -dark, film cameras should make some form of punch mark in the film -between the exposed parts, or the space between exposures should be -uniform, so that a print trimmer set to a definite mark may be used. -Racks for holding two or three feet of film, folded back on itself and -clasped by spring clothes-pins, are fairly practical. One object of the -use of film, however, is to greatly increase the number of possible -exposures; and where hundreds instead of dozens of exposures are to be -developed, this method takes up entirely too much time. - -Following the practice in moving picture development, _film developing -machines_ of various designs have been devised. Among these may be -described the G. E. M. machine; the Ansco machine; the Eastman apron -machine; the Brock frame and tank apparatus; the Eastman reel machine; -and a modification of the latter by the United States Air Service. - -The G. E. M. film developing apparatus, similar in idea to the Eastman -“apron” method of film developing, as exemplified in the familiar -amateur film developing machines, has the film wound in a spiral on a -long linked metal frame or chain. After being wound it is placed in a -tub of developer, from that to a tub of water, thence to a tub of hypo, -and finally to a tub of water, where it is washed in several changes. -The objections to the method are that it takes up much floor space for -the various tubs, and that it requires such large quantities of -solution. To develop a thirty-five foot length of 18 × 24 centimeter -exposures requires approximately 28 gallons of developer; for the -rinsing, 28 gallons of water, and the same for hypo, and at least three -times that for washing. In all 168 gallons of water must be brought to -the developing hut or lorry. - -The Ansco machine makes use of an idea frequently applied in the moving -picture industry. The film is carried spirally, upon two cross-arms -which bisect each other at right angles, and which contain vertical pins -around which the film is looped, beginning at the center and working -out. After it is wound it is placed in a tub of developer, as in the G. -E. M. machine. It has an advantage over this apparatus in that the shape -of the tubs or tanks is square instead of round. But it is equally -extravagant of space and water. - -This same criticism may be made of the Eastman apron apparatus for film -developing. This is similar to the G. E. M. machine, but differs from it -in using a perforated celluloid apron to support the film during the -various operations, instead of a metal chain. - -The Brock developing outfit consists of a rectangular wooden frame and a -three-compartment tank. The frame, which is approximately 3 by 4 feet in -size, is used as a support for the 4 inch wide film, which is wound -spirally around it, between guiding pins. A special support is provided, -on which the frame may be rotated as the film is fed off the camera -spool. The frame, with the film on it, is lowered successively into the -three narrow but deep compartments of the developing tank. The first -compartment holds developer, the next water, the next hypo. The amount -of developing solution required is rather large (96 gallons of water in -all for a strip of 100 4 × 5 inch exposures), but because of the small -surface exposed to the air, it keeps for a considerable period. The -chief demand for floor space with this apparatus is for feeding the film -on to the frame. - -In the Eastman twin reel machine the film is wound on a wooden drum or -reel of large diameter, to form a helix. The drum is suspended so that -the bottom edge touches the developing solution, and, upon revolving the -drum, every portion of the helix of film is brought into contact with -the developer. By shaping the developing tank so that it closely -conforms to the shape of the reels, a high economy in quantity of -developing agent can be achieved. When developing action is finished, -the developer is emptied out, rinse water put in; hypo follows, and then -comes the final washing with water. With this apparatus the whole cycle -is completed, for the 35 feet length of film above considered, with -seven gallons of water. - -The Air Service apparatus differs from the above only in the drying -method, which will be described below. - -Heavy cut film, such as is marketed under the name of Portrait Film, has -not thus far been used in aerial work, except for printing -transparencies. It is conceivable, however, that film in the cut form -may be used in some future design of camera. This may be developed -expeditiously in a tray, six or eight films being handled at a time, in -a pile, pulling out the lower one frequently and placing it on top. The -core rack system is also available for film in this form, special racks -with clips to hold the film being necessary. - -=Plate Drying.=—The drying of negatives on glass is a comparatively -simple matter, owing to the rigid nature of the emulsion support. A -large number of plates may be placed in a compact mass in the ordinary -plate racks of commerce with the wet sides accessible to a draft of air. -Two dozen plates separated from each other by a quarter of an inch and -left to dry spontaneously in a room of ordinary humidity and living -temperature will dry in two hours and a half. If the surface be wiped -with soft cheese-cloth or chamois, so as to absorb all the surface -moisture before the plates are placed on the rack, this time may be -appreciably reduced. By placing the plates in a forced draft of air, -from an electric fan, this time may be reduced to an hour. - -Extra rapid drying of plates may be accomplished by placing them in a -bath of alcohol before putting them in the racks. The alcohol displaces -all the water in the film, and is itself very quickly dissipated into -the atmosphere when the plate is taken from the tray. The plate must be -left in the alcohol tray long enough for the substitution of the alcohol -for the water in the film to take place. Five minutes is long enough. -The alcohol before use must be as nearly free from water as possible. -The best way to make sure of this is to place in the bottle of alcohol -some lumps of calcium oxide, which will take up the water and form -calcium hydroxide, which settles at the bottom of the bottle. - -Another method of quick plate drying takes advantage of the -extraordinary greediness of potassium carbonate for water. The wet -plates are placed in a saturated solution of potassium carbonate and -left for a minute. If a plate be now taken from the solution and its -surface wiped with a soft cloth, it will be found that the film has a -greasy, slippery feeling, but that it contains no water and can be -printed from at once. Plates so treated should be washed, however, at -some time in the succeeding four months, or the traces of potassium -carbonate left in the film cause deterioration. - -=Film Drying.=—Unlike the drying of plates, drying of film negatives is -a very puzzling problem, and may be considered as the crux of the -successful use of film in aerial cameras. - -Apron and similar machines have very poor drying efficiency if the film -is left in place, for not only the film but the apron or chain must be -freed of water. This may be hastened, as in the G. E. M. machine, by -blowing air through with fans, but even with their help drying a 35 foot -film is a matter of two hours or more. Passing the film through wringers -or a squeegee to remove excess water is a considerable aid; the film may -either be re-wound on a dry reel, to be put in a forced draft of air, or -may be hung up in short lengths or festooned, either method taking up a -great deal of space. The use of alcohol is not advisable as it may -abstract camphor from the celluloid and cause the film to become -distorted. - -The Eastman twin reel machine had an upper reel joined to the lower or -developing reel, with a chain and sprockets, so that the upper reel -revolved at the same time and rate of revolution as the lower, when the -lower was being revolved at the gentle speed appropriate to the -developing process. Fans blew a draft of air over the upper reel. This -method necessitated over an hour for drying. - -[Illustration: - - FIG. 117.—U. S. Air Service film developing machine for film 24 - centimeters wide.] - -The Air Service model of film developing and drying machine (Fig. 117) -introduces an essential modification in the drying scheme of the Eastman -apparatus. The upper reel is quite independent of the lower reel and is -revolved at a high rate of speed, so that a whirling action is -introduced into the drying. Large rotating fans at the same time drive a -considerable volume of air across the film surface, and the combination -of the two agencies makes it possible to dry 35 feet of 18 × 24 -centimeter film in 20 to 30 minutes. This for large numbers of pictures -makes the use of film even quicker than that of plates. The only -practical drawback to the apparatus is its bulk, which calls for a -separate room or trailer. This, however, seems to be inevitable in the -use of large roll film. - -Cut film can be dried with speed only if placed in a draft of warm air. -Drying boxes, with a chute or chimney and with fans to drive the air -through from an alcohol stove, will dry several dozen films in an hour. -The films must not be closer together than about one inch, which makes -the drying boxes rather cumbersome. - -=Marking Negatives.=—After development and drying, and before filing or -printing, each plate should be marked with data for purposes of future -identification. This is most easily done with pen and ink on the film -side (in reversed lettering) either along an edge in the unexposed -portion covered by the sheath or in a corner, so as to lose as little of -the photograph as possible. Just what data shall be inscribed is -dictated by the purpose for which the negative was made. The date, -altitude, time of day, true north (from known permanent features or from -shadow direction and time of day), number of the camera used, the focal -length of the lens. Other records, such as the plane and squadron -numbers, or even the pilot's and observer's initials, may be called for -(Fig. 75). For mapping work the scale of each of a set of negatives, -once found, may be marked, either in figures or by means of a line of -length corresponding to a fixed distance on the ground. Rectifying data -can similarly be inscribed, so that the negative can be printed in the -enlarging and rectifying camera with the minimum of delay. - - - - - CHAPTER XXIV - PRINTING AND ENLARGING - - -=Contact Printing.=—Single prints are made most simply in a printing -frame held at a short distance from a light source. When any quantity -must be made, as in turning out prints at high speed for distribution to -an army before an attack, _printing machines_ are employed. These -consist essentially of a light box, a printing frame of plate glass, and -a pressure pad. In the commercial models, such as the Crown and the -Ansco, which are equipped with electric light, merely bringing the -pressure pad down and clamping it automatically turns on the light, -while release of pressure terminates the exposure. - -The question of regulating the distribution of light is of considerable -importance with negatives taken by focal-plane shutters of non-uniform -rate of travel. In the McIntire printer (Fig. 119), the separate -electric bulbs are on long necks in ball and socket joints, so that they -can be brought individually closer to the printing surface or farther -away from it, thus permitting a wide range of “dodging.” This printer -also has an automatic time control for the light, a valuable device -where many prints from the same negative are desired. - -These machines are well suited for printing aerial negatives, either -plate or cut film, if used where a source of electric current is -available. The chief defect, which may be caused by faulty construction, -is imperfect contact between paper and negative, a cause of serious -unsharpness on prints destined for minute study in interpretation. - -[Illustration: - - FIG. 118.—Printing machine.] - -The printing of aerial negatives may be done either on roll or cut -paper, and if films are used, a further alternative is offered of -handling it either in the roll or in cut form. Where many prints are to -be made from one negative roll paper has some advantages, particularly -if a developing and drying machine is available. But for moderate -numbers the advantage is small, since cut prints can be developed quite -conveniently in goodly numbers in the ordinary trays. But the advantages -of keeping film in the roll form are very great, both in respect to -storage and in respect to handling during printing, as the rollers -provide the necessary tension and prevent the film “getting away.” - -[Illustration: - - FIG. 119.—Interior of McIntire printer, showing lamps adjustable in - position for “dodging.”] - -[Illustration: - - FIG. 120.—Film printing machine.] - -For the American Air Service, cut paper has been used exclusively. For -film printing, the Ansco machine has been equipped with roll pivots to -take film 24 centimeters wide which may be advanced in either direction -by turning large milled heads (Fig. 120). If we put rollers on the two -remaining sides of the box to handle paper we transform the printer into -the same form as a French machine, in which paper and film are moved at -right angles to each other. A disadvantage of this modification, -however, is the difficulty of examining the negative to be printed. - -=Stereo Printing.=—To make separate prints from the two elements of a -stereoscopic pair and mount them side by side after proper orientation -is too slow a process if quantities of prints are needed. One method of -multiple production is to make a master stereogram, and then produce -photographic copies of it, but there is inevitable loss of quality in -this copying process. An intermediate method is to print from both -negatives on the same sheet of paper. In order to do this the negatives -must be placed in rather large frames, with mats properly located to -guide the placing of the paper. The Richard double printing frame is a -practical device which simplifies the necessary manipulations. It -consists essentially of a platform pierced with three illuminated -openings. The two negatives are compared, superposed, and orientated -over the central opening and then shifted laterally, one to each of the -two side openings, which serve both as printing frames and masks. The -printing back slides on a rod, permitting the paper to be lifted up and -moved between exposures. Once the negatives are properly placed, stereo -prints can be turned out quickly and easily. - -=Enlarging.=—In the French service contact printing was the rule during -the war. The English practice, on the other hand, was to take small -negatives—4 × 5 inches, with 8 to 12 inch lenses—and enlarge them, -usually to 6½ × 8½ inches. For this purpose a regular part of the -English photo section equipment was the _enlarging camera_ (Fig. 115). -This may be briefly described as a short focus camera in which the -subject to be photographed is a negative, illuminated by transmitted -light, whose image is thrown by the camera lens on the paper or other -sensitive surface. By making the distance between negative and lens less -than that between lens and paper, the resulting print is an enlargement, -and _vice versa_. The scale of enlargement or of reduction is varied -over limits set only by the length of the camera and the amount of light -available. - -The lens employed must of course possess sufficiently high quality to -preserve all the sharpness of the negative, and focussing must be done -with accuracy. Next to the lens the most important element is the light -source. This may be of the point form, such as a concentrated filament -electric lamp, an oxy-acetylene lime light, or an acetylene flame. The -latter was extensively used in the English service, while acetylene -generators for emergency purposes formed part of each American photo -truck equipment. With point light sources we must use _condensers_ to -focus the light into the projecting lens. Much less efficient, but the -only recourse where large condensers are not available, is a diffusing -glass behind the negative, illuminated either by a bank of electric -lamps with mirrors or by a U tube mercury vapor lamp, where proper -current can be got. - -The device for holding the printing paper must permit quick changing, -but insure good contact. We may use either a spring plate to hold the -paper against plate glass from behind, or else a weight acting on a -lever arm of sufficient length. - -The need for some automatic means of focussing an enlarging camera has -been very generally felt. An illustration of such an enlarging camera is -that put out by Williams, Brown & Earle, of Philadelphia, known as the -“Semperfocal” (Fig. 121). In this camera the movements of the lens, -paper easel and negative are so inter-related and actuated with respect -to each other that the correct focus of the instrument is maintained for -any degree of enlargement or reduction. This feature is a great help in -making up mosaic maps, where prints of continuously varying scale -ordinarily occasion serious delay for individual focussing. - -[Illustration: - - FIG. 121.—“Semperfocal” enlarging camera, with mechanism for holding - image in focus at any enlargement.] - -Determining the correct enlargement for each negative of a mosaic is -perhaps the most important problem in the use of the enlarging camera -for aerial work. The correct setting of the camera may be found by -either of two methods: the negative may be previously scaled and marked -with a line on its edge, which must be projected to a definite size; or -the true location of several points in the picture as obtained from an -accurate map may be marked on the enlarging camera easel according to -the desired scale, and the negative image projected to coincide with -these. In either case, if an exact scale is desired, allowance must be -made for paper shrinkage, a matter which must be determined by previous -experiment. - -=Rectifying.=—Negatives taken when the plane is not flying level will be -distorted (Figs. 134 and 135). Contact prints from these will not fit -into a mosaic, and no mere enlargement or reduction will make them -available. It is necessary with these negatives to resort to a -_rectifying camera_. This is an enlarging camera built so that the -negative and print easel may be inclined about vertical and horizontal -axes, thereby purposely introducing a distortion sufficient to offset -the distortion of the negative. Thus, if the bottom of the printing -surface is moved away from the lens, that part of the picture will be -enlarged; if moved toward the lens, reduced. - -For small rectifications the common practice is to tilt the printing -surface alone, a method that is practical as long as this tilting does -not affect the focus so much as to require prohibitive stopping down of -the lens. For great distortions, such as that inherent in the principle -of the Bagley camera, it is necessary to tilt both negative and print in -order to preserve an approximate focus, a given portion of the negative -moving toward the lens as the corresponding portion of the print is -moved away. Both schemes for rectification are shown diagrammatically in -Fig. 122. - -=Developing and Drying Prints.=—The developing of prints follows closely -that of cut or roll film, and so need not be treated separately. - -The drying of emulsions on paper is more easily accomplished than the -drying of emulsions on glass, for two reasons: the emulsions on paper -are much more thinly coated, and there is diffusion of moisture into the -atmosphere from front and back of the printing medium. In the field a -common method has been to soak the prints in water-free alcohol and then -burn off the alcohol, thus securing a dry print within two or three -minutes after the conclusion of washing. A later method very generally -employed is to cover wooden frames three or four feet above the ground -with chicken wire or muslin, and on these lay the prints after soaking -them in alcohol. Below the frames currents of warm air rise from pans of -burning alcohol, previously used to soak the prints and now useless as -alcohol because of their high water content. - -[Illustration: - - FIG. 122.—Diagram showing enlarging with and without distortion: _A_, - enlarging without distortion; _B_, distortion for rectification of - print, by inclining printing surface; _C_, distortion, for - rectification of print, by inclining both negative and printing - surface.] - -Before putting them in alcohol it is advisable to squeegee all the -surface water from the prints. This may be expeditiously done by -removing them in mass from the final wash water upon a large ferrotype -plate, and either running the plate and prints together through a wash -wringer with light pressure, or covering the whole with a sheet of -blotting paper and pressing out the water underneath by means of a -rubber squeegee vigorously applied. - -For base work one of the modern automatic print-drying machines used in -commercial photography would be desirable. Glossy surfaces are given -prints by the usual ferrotype plate method. But this is too -time-consuming for war practice, and besides has but doubtful advantage -where papers of the glossy type are chosen. - - - - - VI - PRACTICAL PROBLEMS AND DATA - - - - - CHAPTER XXV - SPOTTING - - -“_Spotting_,” as distinct from mapping or from the photography of -continuous strips, is the photography of a definite individual -objective. In military work spotting or “pin pointing” includes the -photography of particular trenches or pivotal points in a trench system -before an attack (Fig. 123), of roads or bridges along which an advance -must pass (Fig. 124), of batteries or big guns which are the subject of -artillery fire (Fig. 125), both before and after their bombardment (Fig. -126), of gun puffs or exploding bombs (Fig. 131). - -The technique of spotting consists largely in getting properly over the -target and then securing the exposure at just the right moment. This is -chiefly a question of proper piloting; but the aid which can be offered -to the pilot by camera auxiliaries designed particularly for spotting -needs is very large. - -Discussion of the task of the pilot who must steer a photographic plane -accurately over a previously selected point of interest cannot be -undertaken without raising the question of who should take the picture, -pilot or observer? In the English service the most general practice was -for the pilot to be charged with the responsibility both of covering the -objective and of exposing. If a propeller drive was used on the camera, -this left to the observer only the task of changing magazines. If the -camera was hand operated the plates were changed either by the observer, -or else, as was frequently the case, distance operating devices were -attached, so that the pilot even then did everything except change the -magazines, and the observer was kept free to watch the sky for enemy -aircraft. A very desirable adjunct to the camera when plates are shifted -automatically or by the observer is a distance indicator, to show the -pilot when the shutter is set. Electrical indicators for this purpose -have been devised. - -[Illustration: - - FIG. 123.—Low view of trenches on the Yser, showing concrete - structures undamaged by bombardment.] - -[Illustration: - - FIG. 124.—The Piave River. To left, destroyed bridge, to right, - pontoon bridge under construction. - - Italian aerial photographic service.] - -[Illustration: - - FIG. 125.—Showing big gun hidden in forest. (Upper left-hand corner).] - -[Illustration: - - FIG. 126.—Example of spotting. Battery before and after bombardment.] - -If the camera is completely hand operated, as were most of those in the -French and German services, there is little choice but for the observer -to perform the entire operation. The exposing operation could have been -delegated to the pilot, but such was not the custom with the French or -with the American squadrons using French apparatus. In this method of -operation the observer depends on the pilot to get the plane over the -target, while the pilot depends on the observer to get the picture when -the target is covered. Ample opportunity is thus offered for -misunderstanding and disagreement. This can be avoided only by excellent -sights properly aligned, for both pilot and observer, and by some means -of communication between the two men concerned. - -[Illustration: - - FIG. 127.—Photograph, made with long focus lens to determine the - results of aerial bombing. The “Tirpitz” battery of long range naval - guns directed on Dunkirk.] - -The simplest _means of communication_ is of course direct conversation. -But this is only possible in those planes, such as the DH-9, in which -pilot's and observer's cockpits are immediately together, so that, by -shouting, any desired information can be conveyed with fair ease. When -the distance is increased to four or five feet, as in the DH 4, the -loudest shouts are totally lost in the roar of the engine and the blast -of the wind. Speaking tubes and telephones are now fairly good, but are -none too comfortable or convenient to have strapped on one's head and -face. A primitive device used to some extent in the war was merely a -pair of reins attached to the pilot's arms, by which he could be -directed which way to steer. There is much to be said for a simple -semaphore system, where an indicator in the observer's cockpit actuates -a similar dial in front of the pilot, indicating “right” or “left,” -“picture obtained,” “try again,” etc. If the observer has a sight by -which he can see far enough ahead to correct the pilot's error of -pointing, the need for an accurate sight for the pilot is diminished. - -[Illustration: - - FIG. 128.—Diagram showing relationship between focal length and area - covered by plate.] - -[Illustration: - - FIG. 129.—Diagram giving data on area covered at various altitudes by - representative lens.] - -[Illustration: - - FIG. 130.—Burchall Slide Rule, for calculating intervals between - exposures, and for other aerial photographic data.] - -In considering the question of sights, attention may again be called to -the poor “visibility” from the pilot's seat in the present prevailing -type of two-seater tractor plane. Blind directly in front, beneath, and -to either side (Figs. 7, 8 and 9), it is no unusual thing for a pilot to -entirely miss an objective, such as a railway line, which he can only -estimate to be beneath him by judging its distance from those objects to -either side which he can actually see. The English practice of leaving a -clear space of six inches to a foot between the fuselage and the -beginning of the wing fabric, allows the pilot to look down over the -side, a decided advantage. But for photographic purposes nothing can -compare with a good negative lens carrying fore and aft lines or wires, -so that the pilot can see his objective in ample time to head directly -for it. The lens should either be large enough so that its rear edge -gives the view directly downward, or supplemented by an additional lens -pointing directly down, so that the covering of the target is assured. -To locate such a lens in the front cockpit, free of all controls, is a -very hard task; even so its view is likely to be badly interrupted by -the landing gear. Nevertheless, so important is it, both in photography -and in bombing, to have a sight by which the plane can be accurately -directed that designers of planes should recognize this need and make -every effort to provide a suitable location. - -[Illustration: - - FIG. 131.—Aerial bombardment of Trieste. Note falling bombs in center - of picture; and exploding anti-aircraft shells over the water. - - Italian official photograph.] - -[Illustration: - - FIG. 132.—Example of spotting requiring exposure at exact instant. - Explosion following burst of bomb in ammunition dump. - - British official photograph.] - -Sights for the observer have been discussed already. Here again the -negative lens is to be preferred, but while the pilot's lens needs only -directing lines in the axis of the plane (unless he takes the picture), -the observer's lens needs both an accurate center mark and an additional -upper or lower sighting point. Accurate alignment of these marks with -the camera axis must be arranged for in precise spotting. - -Accurate spotting work requiring the delineation of fine detail calls -for cameras of considerable focal length. The camera of longest focal -length used in the war was the French 120 centimeter (Fig. 41). This was -employed with great success in such work as regulating the fire of heavy -railway guns brought into range only at night, to fire a few shots at -chosen angles. Photographs taken the next day would then show the exact -spot where each shell fell, and the damage it did, to serve as a guide -for the next night's operations (Fig. 127). The field of these cameras -is quite small—8 to 12 degrees—and so not only must sighting be exact -but the area covered on the ground must be accurately known. This is to -be calculated from the altitude, focal length, and plate size, by the -relation— - - distance on ground altitude - —————————————————— = ———————————— - plate length focal length - -Data derived from such calculations may be incorporated in tables, or -graphically in diagrams such as Figs. 128 and 129. - -[Illustration: - - FIG. 133.—The same subject a few minutes later. Height of smoke shown - by shadow. - - British official photograph.] - -These calculations and others required in mapping and stereo-work are -simply and quickly made by slide-rule devices. One of these, the -Burchell Photographic Slide Rule, developed in the English service, is -shown in Fig. 130. This consists of two dials, the center one of which -is mounted—usually by a pin pushed into a cork behind—so as to turn -freely, to permit its being set for altitude, focal length, ground -speed, plate size, etc., whereupon the area covered, or the appropriate -interval between exposures may be read off. - -Cameras for spotting work should be capable of exposure at the exact -moment desired. For if the camera is ever to catch the gun as it -discharges, the bomb as it falls (Fig. 131), or the shell as it explodes -(Fig. 132), the photograph must be taken within the instant. Automatic -cameras, exposing at regular intervals, while adequate for mapping, are -not fitted for many kinds of spotting. - - - - - CHAPTER XXVI - MAP MAKING - - -=Technique of Negative Making.=—Stated in its simplest terms, the whole -problem of making a photographic map from the air consists in taking a -large number of slightly overlapping negatives, all from the same -altitude, with the plane flying uniformly level. When trimmed and -mounted in juxtaposition, or pasted together so as to overlap in their -common portions, the prints from these negatives constitute a complete -pictorial map. There is thus furnished by a few hours' labor topographic -information which would be the work of months to obtain by other means. - -The making of map photographs involves all the special technique of -spotting, with much in addition. The pilot's task is not merely to go -over one object; he must navigate a narrow path, at a constant altitude, -on an even keel. If he is to make not merely a ribbon, but a map of -considerable width, he must take successive trips parallel to the first, -each displaced just far enough from the previous course to insure that -no portion is missed—a difficult task indeed. - -It is the observer's duty to so time the intervals between exposures -that they overlap enough, but not so much as to be wasteful of plates or -film. He must also change magazines or films so quickly as to miss no -territory, or if some be missed, his is the task of directing the pilot -back to the point of the last exposure, where they begin a new series. - -Level flying is entirely a pilot's problem. Its importance will be -realized when we consider the accompanying diagrams (Figs. 134 and 135), -where the effect on the resultant picture is shown of climbing, gliding, -or banking to either side. Prints from negatives distorted in this way -neither will be true representations of the territory photographed, nor -will they match when juxtaposed. In fact, they can be utilized only if -special rectifying apparatus is available for printing. Flying at a -constant altitude is similarly necessary if the prints are to be -utilized without enlargement or reduction in order to make them fit. - -[Illustration: - - FIG. 134.—Diagram showing effect of banking on aerial photograph.] - -[Illustration: - - FIG. 135.—Diagram showing effect of climbing and diving on aerial - photograph.] - -Assuming a skilled pilot who will do his part, the next step is to -calculate the exposure intervals in order to insure an adequate overlap. -If a negative lens is installed which has been marked with a rectangle -the size of the camera field, the simplest method is to estimate the -proper instant for exposure by watching the progress of objects across -the lens face. This of course requires constant attention, and it is -easier to do this only occasionally, in order to determine the ground -speed in terms of camera fields traversed per minute. Thereafter -exposures are to be made by time, as determined by a watch or clock. Any -desired degree of overlap can be chosen, and either estimated, or more -or less accurately fixed by lines marked on the negative lens at a -shorter distance apart than the edges of the field. The most usual -overlap is 20 per cent., except for stereos, which call for 50 to 75 per -cent. - -In the absence of a negative lens or some other sight to show the whole -camera field, it is necessary to resort to calculation from the speed -and altitude of the plane, the focus of the lens and the dimensions of -the plate. If _A_ is the altitude, _a_ the focal length of the lens, _d_ -the diameter of the plate in the direction of travel (usually the short -length is chosen for economy of flights to cover a given width), _f_ the -fractional part by which one negative is desired to overlap the next, -and _V_ the ground speed of the plane, then we have, by simple -proportion, that the interval between exposures, _t_, must be— - - _Ad_(1 - _f_) - _t_ = ————————————— - _aV_ - -If _A_ = 2000 meters, _d_ = 18 centimeters, _f_ = ⅕, _a_ = 50 -centimeters, and _V_ = 200 kilometers per hour, this relation gives— - - 2000 × .18 × .8 × 3600 - _t_ = —————————————————————— = 10.3 seconds - .5 × 200,000 - -The principle of overlapping map exposures is shown in the accompanying -diagram (Fig. 129), together with data calculated as above for a 4 × 5 -inch plate. - -It is particularly to be noted that it is the _ground speed_ of the -plane that is used. This may be calculated by knowing the air speed and -the wind velocity and direction. Fig. 136 shows the method of doing this -graphically. First an arrow is drawn representing the direction it is -desired to fly. Next a second arrow is drawn of length to represent the -wind velocity. This must be inclined toward the first arrow in the -direction of the wind, and its head is to touch the head of the first -arrow. Then with the farther end of this second arrow as a center, -describe a circle of such a length as to represent the air speed of the -plane, in the same units as the wind velocity. Connect the point where -this circle cuts the arrow of flight direction to the center of the -circle by a straight line. This line constitutes the air speed arrow, -giving the direction it is necessary to fly, at the given air speed, to -make the course desired. The length of the flight direction arrow -between its head and its point of intersection with the air speed arrow -gives the ground speed. - -[Illustration: - - FIG. 136.—Diagram showing method of calculating ground speed from air - speed and wind velocity.] - -When the wind is ahead or astern this calculation reduces to the simple -subtraction or addition of the wind velocity to the air speed of the -plane. Whenever possible, mapping should be done up and down the wind -(Fig. 137). If the plane is “crabbing,” the above calculations for -overlap are only valid if the camera can be turned normal to the -direction of travel over the ground. If the camera cannot be so turned -the corners of the successive pictures overlap instead of their sides, -with quite unsatisfactory results (Fig. 138). - -Calculation of the distance apart of the parallel flights necessary to -make a map of any width is done by the use of a formula similar to the -longitudinal overlap formula above, distance figuring instead of time. -Using the same symbols, and denoting the distance by _D_, we have— - - _Ad_(1 - _f_) - _D_ = ————————————— - _a_ - -With the same figures as before, but substituting 24 centimeters for the -plate dimension, this relation gives— - - 2000 × .24 × .8 - _D_ = ——————————————— = 768 meters - .5 - -[Illustration: - - FIG. 137.—Overlaps made when flying with or against the wind.] - -[Illustration: - - FIG. 138.—Unsatisfactory overlaps made when plane is “crabbing.”] - -It is of course largely a pilot's problem to steer the plane over -parallel courses at a given distance apart, although the observer, -noting conspicuous objects through a properly marked negative lens, may -direct the pilot by any of the means of communication already mentioned. - -An alternative method of securing parallel strips, which is to be highly -recommended where enough photographically equipped airplanes are -available, is for several planes to fly side by side, maintaining their -proper separation (Fig. 139). - -=Cameras and Auxiliaries for Map Making.=—Mapping can be done quite -satisfactorily by hand operated or semi-automatic cameras, provided the -observer has not too many other duties. On the other hand, the operation -of exposing at more or less definite intervals of time, irrespective of -the object immediately presented to the camera, is a largely mechanical -one. It naturally suggests the employment of an automatic mechanism, -whose speed of operation only is it necessary to watch. - -If a non-automatic camera is used the timing of exposures may be done by -watching a negative lens, as described above, or by reference to a -clock, assuming that the ground speed is known through calculation. A -very practical advance over the ordinary use of a clock is to attach a -stop-watch to the shutter release, so that it is turned back to zero and -re-started at each exposure (Fig. 70). In passing, it may be noted that -if the stop-watch hand makes an electric contact which throws the -shutter release, then the device constitutes an attachment for turning -any semi-automatic camera into an automatic. The most suitable cameras -for mapping are unquestionably those of the entirely automatic type. The -use of such cameras always demands a knowledge of the ground speed. This -demand has led to many suggestions for _ground speed indicators_. The -common idea of these is to provide a moving part on the plane—either a -disc of large diameter, or a chain, or a revolving screw—whose speed may -be varied until any point upon it appears to keep in coincidence with a -point on the moving landscape below. The ground speed is then to be read -off a properly calibrated dial. Or, as a further step, the frequency of -the exposures may be directly controlled by the ground speed indicator -mechanism. The entire control of the camera would then consist merely in -occasional adjustment of the ground speed indicator. - -[Illustration: - - FIG. 139.—Planes starting out to make a map by flying in parallel.] - -While entirely possible in theory, these devices are not easy to work -with in practice, because the plane is always subject to some pitching -and rolling, which make it difficult to hold any object constantly on -the moving point. This is especially true at high altitudes, where the -apparent motion of the earth is quite slow compared to the swervings of -the plane. This objection is in part removed if the ground speed -indicator is carried by a gyro stabilizer. - -Ordinary mapping does not demand such exquisite rendering of detail as -does trench mapping. Nor is it necessary to fly in peace-time at such -high altitudes as in war. In consequence, mapping cameras are preferably -of the short focus, wide angle type, say, 25 centimeter focus for an 18 -× 24 centimeter plate. Film is to be preferred over plates because of -the greater number of exposures it is possible to make on a flight. The -shutter of the mapping camera must be extremely uniform in its rate of -travel so that the elements of the map may match in tone (Fig. 140). A -mount which permits the camera to be turned normal to the direction of -flight, such as the British turret mount (Fig. 87), is particularly -desirable if flying across the wind is necessary, as will often be the -case in mapping strips between towns or between flying fields. Devices -to indicate compass direction and altitude are called for in new and -poorly mapped territory, and may be expected to receive intensive study -in the future. The question of their utility is, however, bound up with -the whole question of the sphere of aerial photographic mapping. Up to -the present this has been almost entirely a matter of filling in details -on maps obtained by the regular surveying methods, or of making -pictorial maps for aviators. To what extent primary mapping can be done -by the airplane is yet to be determined. - -[Illustration: - - FIG. 140.—A strip map, showing effect of uneven focal plane shutter - action.] - -At this point mention must be made of special cameras for securing -extremely wide angle views, thereby minimizing the number of flights. -The _Bagley camera_, devised by Major Bagley of the U. S. Engineers, is -an example. It has three lenses, a middle one pointing directly -downward, and one to either side at an angle of 35 degrees. The pictures -obtained with the side cameras are of course greatly distorted, and must -be rectified in a special rectifying camera. The resultant definition is -not good, but as the maps are made on a much smaller scale than the -original pictures, this is not a serious objection. It is a matter for -the future to decide whether the additional labor on the ground -necessary for the rectifying process is to be more expensive than the -extra flights which must be made with the ordinary types of cameras -covering a smaller angle. - -=Printing and Mounting Mosaics.=—With an ordinary set of overlapping -negatives the first step toward producing a map is to _scale_ the -negatives. For this purpose one should be selected which by comparison -with a map shows no distortion, and which is on the desired scale, or is -known to have been made at the average altitude of flight. A sketch map -of the territory should then be drawn, on this scale, based on available -maps. This sketch is preferably made on a large ground glass illuminated -from behind (Fig. 141). On this all the negatives should be laid, and -their proper relative positions sought. When this is done it is evident -at once whether all the territory has been covered, and whether there -are any superfluous negatives. Each negative should then be examined as -to its scale and distortion. If it can be made to fit the scale by -simple enlargement or reduction, a line can be drawn on one edge of a -length indicating its scale. This line will later be used as a guide in -the enlarging camera. If the picture is badly distorted it must either -be replaced by another negative, or if rectifying apparatus is -available, it must be set aside for the making of a rectified print. - -[Illustration: - - FIG. 141.—Scaling negatives for mosaic map-making.] - -The next step is to make prints from the negatives, which may be done -either by contact, or, necessarily if differences of scale must be -compensated, in the enlarging camera. If prints to an exact scale are -required the shrinkage of the paper must be determined and allowed for. -The prints must all show the same tone, and must be uniform from edge to -edge. If the focal-plane shutter is not uniform in its travel, as is -frequently the case, this means that the print must be “dodged,” or -exposed more at one edge than the other, by locally shielding the plate -and paper during exposure. A case of the step-like effect caused by -uneven shutter action is shown in Fig. 140. The effect due to uneven -shutter action is of course absent with a between-the-lens shutter, -which constitutes a strong argument in favor of that type for use in -mapping cameras. - -[Illustration: - - FIG. 142.—Arranging prints for a mosaic map.] - -When the prints are made they must be mounted together on a large card -or cloth background. For a very small mosaic they may be juxtaposed by -simple examination, matching corresponding details in successive prints. -For a mosaic of any size an accurate outline map must be drawn on the -surface to which the prints are to be attached. The prints are then laid -out on this outline, moved to their correct positions, and held down by -pins (Fig. 142). When they are all arranged the final mounting may be -begun. The excess paper, beyond what is necessary for safe overlaps, may -be trimmed off, exercising judgment as to which print of each adjacent -pair is of the better quality, and utilizing it for the top one at the -overlapping junction. If one print shows serious distortion it may be -placed under its fellows on all four edges, thus minimizing its weight. -The edges are best made irregular by _tearing_. Straight edges are apt -to force themselves on one's attention in the final mosaic and give an -erroneous impression of the existence of straight roads or other -features. Both forms of edging are shown in Figs. 124 and 143. - -An alternative method of securing the final print mosaic, where film -negatives are used, is to trim successive film negatives so that the -trimmed sections will exactly juxtapose, instead of overlap. The -sections are then mounted, by stickers at their edges, on a large sheet -of glass, and printed together. Captured German prints show that this -was the method commonly used with the German film camera (Fig. 62). - -It will be noted that the procedure which has been described and -illustrated by Figs. 142 and 143 assumes the previous existence of a map -accurately placing at least the chief features of the country covered. -This draws attention at once to the limitations and true sphere of -aerial photographic mapping at the present time. With the cameras thus -far it is not possible, nor is it attempted, to do primary mapping of -unknown regions. Distortions due to lens, shutter, film warping and -paper shrinkage considerably exceed the figures permitted in precision -mapping. From the standpoint of geodetic accuracy the cumulative errors -of deviations in direction, altitude and level, peculiar to flying, -would soon become prohibitive. - -[Illustration: - - FIG. 143.—A partly completed map. Prints mounted over an outline - sketch map to proper scale.] - -The great field for aerial photographic mapping in the near future lies -in filling in detail on maps heretofore completed as to general -outlines, or, as in the war, on maps far out of date. The war-time -procedure in country largely unknown, such as Mesopotamia, was probably -closely that which will be necessary in peace. Conspicuous points in the -landscape were first triangulated from friendly territory, and from -these the outline map was drawn, whose details were to be supplied by -aerial photographs. Much of the “mapping” of cross country aerial routes -so far done is frankly of a pictorial nature, showing conspicuous -landmarks and good landing fields—extremely valuable and useful, but not -to be confused with precision mapping. In assembling mosaics of this -kind the elaborate procedure described above is not followed. The -process is the simple one of juxtaposing adjacent prints as accurately -as possible by visual examination. Errors are of course cumulative, but -as long as exact distances are not in question this is no matter. - - - - - CHAPTER XXVII - OBLIQUE AERIAL PHOTOGRAPHY - - -Oblique views from the airplane are of very great value. While vertical -views are more searching in many respects, they do nevertheless present -an aspect of the earth with which ordinary human experience is -unfamiliar. Consequently they are difficult to interpret without special -training. They suffer, too, from the military standpoint, from the -limitation that it is with vertical extension just as much as with -horizontal that an army has to contend in its progress. Elevations and -depressions of land show on an oblique view where they would be entirely -missed in a vertical one. For illustration, study the picture of part of -the outskirts of Arras (Fig. 144), presenting moat, walls and -embankments, all of which would be serious obstacles, but would hardly -be noticed on a vertical view. Pictures taken from directly overhead are -eminently suited to artillery use, but oblique views of the territory to -be attacked, taken from low altitudes, formed an essential part of the -equipment of the infantry in the later stages of the war. - -Pictorially, oblique views are undoubtedly the most satisfactory. The -most revealing aspect of any object is not one side or face alone, but -the view taken at an angle, showing portions of two or three sides. Best -of all is that taken to show portions of front, side and top—the -well-known but heretofore fictitious “bird's-eye view” (Fig. 145). This -possibility is ordinarily denied the surface-of-the-earth photographer, -but the proper vantage point is attained in the airplane. - -Aerial obliques may be taken at any angle, although a distinction is -sometimes made between obliques of high angle and panoramic or low angle -views (Fig. 146). In addition to ordinary obliques, a very beautiful -development is the stereo oblique. Both kinds of oblique photography -call for special instrumental equipment and technique. - -[Illustration: - - FIG. 144.—The outskirts of Arras. Low oblique showing contours.] - -=Methods and Apparatus for Oblique Photography.=—The simplest method of -taking oblique pictures from a plane is to use a hand camera pointed at -the desired angle. Its limitations are in the size and scale of the -picture obtainable, and in the inherent limitations to the method of -camera support. A step in advance of this is to mount the camera above -the fuselage, on the machine gun ring or turret, in place of the gun. -Considerably greater rigidity is thus obtained, and heavier cameras can -be utilized, although the wind resistance is a serious factor. Excellent -obliques have been made in this way, even with 50-centimeter cameras, -but the scheme is impractical in military planes, because of the removal -of machine gun protection. - -[Illustration: - - FIG. 145.—Oblique view of Capitol and Congressional Library, - Washington.] - -[Illustration: - - FIG. 146.—Fort Alvenslegen, near Metz. - - Photo by Photographic Section A. E. F.] - -If the camera is fixed in the fuselage in its normal vertical position, -obliques may be and have been taken by the simple expedient of _banking_ -the plane steeply. This is not to be recommended as a standard -procedure, especially for taking a consecutive series of exposures. - -The most satisfactory arrangements for taking obliques are two; first, -to mount _the camera obliquely in the plane_, and second, to use a -_mirror or prism_, in front or behind the lens of the vertically mounted -camera. The first method has been employed chiefly by the French, the -latter by the English, whose gravity fed cameras could not be mounted -obliquely. - -Taking up first the oblique mounting of cameras, we find two ways of -doing this: longitudinal mounting and lateral mounting. In longitudinal -mounting the camera projects forward and downward, usually from the nose -of a pusher or bi-motored plane. With this form of mounting (Fig. 147) -it is necessary of course to fly directly toward the objective. If this -is a portion of enemy trench, which must be photographed from a height -of 400 or 500 meters, the plane will be directly on top of its objective -a few seconds after the exposure is made, and be a conspicuous target, -in imminent danger of destruction. Moreover, only a single short section -of the trench would be obtained for each crossing of the line. The one -case where resort to this method is practically forced is with the -120-centimeter cameras which simply cannot be slung athwart the plane. -There is a slight advantage in this method of carrying in that the -motion of the image is less if the objective is approached, instead of -being passed at the side, and so longer exposures can be made. The -longitudinal mounting has, however, been very generally superseded by -the lateral. - -[Illustration: - - FIG. 147.—120-centimeter camera mounted obliquely in the fore-and-aft - position.] - -Methods for mounting cameras obliquely for taking pictures through the -side of the plane have been discussed in detail in connection with -camera mountings and installations (Fig. 93). The chief difficulties are -want of space, obstacles at the side such as control wires and -longerons, and failure of the camera to function properly at an angle. -Even in the broad circular sectioned fuselage of the Salmson plane, -quarters are so cramped that the French 50-centimeter camera when -obliquely mounted cannot be used with the 12-plate magazine, and -recourse is made to thin flat double plate-holders. Holes in the side of -the fuselage should clear all wires and should command a view -unobstructed by the wings—which often means that the camera must be -carried behind the observer's cockpit, irrespective of the suitability -of that space from other standpoints. Cameras dependent for their action -on gravity, such as the deRam and English L type, are unsuited for -oblique suspension. - -[Illustration: - - FIG. 148.—Mirror on camera cone for taking oblique views.] - -For cameras which, because of their method of operation or shape cannot -be slung obliquely, the only way to take obliques is to employ mirrors -(Fig. 148) or prisms. These must be of the same optical quality as the -photographic lens. They are both necessarily of considerable weight -because they must be of large area of face to fill the entire aperture -of an aerial lens. Mirrors are lighter than prisms, but must be quite -thick to prevent distortion of the surface due to any possible strains -to their mount. Right angle glass prisms have been used by the English -with the 8 and 10 inch L cameras. The prisms were uniformly tilted to an -angle of 12½ degrees from the horizontal. - -Glass mirrors can be silvered either on the rear or front surface. If on -the rear, both surfaces must be accurately parallel, which means much -greater labor and expense than if the front surface can be utilized. The -difficulty with front surface mirrors is that the metallic coating is -easily tarnished or scratched, especially if silver is used, which is -almost imperative, since all the other metals have considerably lower -reflecting powers. (Gold might serve both as mirror and color filter, -because of its yellow color.) Placing the mirror inside the camera body -in part obviates this trouble, but means the use of a special elbow lens -cone. In any case the mirror or prism occasions at least a 10 per cent. -loss of light. Pictures taken by reflectors of any kind are reversed, -and must either be printed in a camera, or on transparent film which may -be viewed from the back. - -The most usual condition for making obliques is to fly very low (300 to -600 meters), with the line of sight of the camera from 12 to 45 degrees -from the horizontal. This low altitude necessitates very short -exposures, to avoid movement of the image. The picture may be taken -either the long or the short way of the plate, depending on the -character of the object and the information desired. It is to be noted -that successive oblique pictures cannot be mounted to form a continuous -panorama—this being possible with obliques only if they are taken from -one point, as from a captive balloon. If successive views are made on a -straight flight at intervals so as to exactly juxtapose in the -foreground, they overlap by a large margin the middle, and a point on -the horizon, if that shows, will be in the same position in every -picture. Mosaics of obliques could be made only by some system of -conical mounting. - -=Sights for Oblique Photography.=—Any of the sights previously discussed -for vertical work, such as the tube sights, are applicable to obliques. -They must, however, be suited for mounting at an angle, in a position -convenient for the observer. In addition, provision must be made for -adjusting the angle so that the lines of sight of camera and finder are -parallel. Mounting outside the fuselage is practically the only feasible -way, and is less objectionable with oblique than with vertical sights, -as oblique sighting does not require the observer to stand up and lean -over the edge of the cockpit. Windows in the side of the fuselage, -either of celluloid or non-breakable glass, are a great aid to oblique -observation. Marks upon the transparent surface can be utilized for the -rear points of a sight of which the front point is a single fixed bead -or rectangle. - - - - - CHAPTER XXVIII - STEREOSCOPIC AERIAL PHOTOGRAPHY - - -One of the most striking and valuable developments in aerial photography -has been the use of stereoscopic views. Pairs of pictures, taken with a -considerable separation in their points of view and studied later by the -aid of the stereoscope, show an elevation and a solidity which are -entirely wanting in the ordinary flat aerial vista. Often, indeed, these -attributes are essential for detecting and recognizing the nature of -objects seen from above. Stereoscopic aerial photography has been justly -termed “the worst foe of camouflage.” - -=Principles of Stereoscopic Vision.=—The ability to see objects in -relief is confined solely to man and to a few of the higher animals in -whom the eyes are placed side by side. When the eyes are so placed they -both see, to a large extent, the same objects in their fields of view. -Owing to the separation of the eyes the actual appearance of all objects -not too far away is different, and it is by the interpretation of these -differences that the brain gets the sensation of relief. Thus in Fig. -149 the two eyes are shown diagrammatically as looking at a cube. The -right eye sees around on the right-hand face of the cube, the left eye -on the left-hand face of the cube. The two aspects which are fused and -interpreted by the brain are shown in the lower diagram. - -Stereoscopic views or stereograms, made either by photography or, in the -early days, by careful drawing, consist of pairs of pictures made of the -same object from two different points. For ordinary stereoscopic work -these points are separated by the distance between the eyes, -approximately 65 millimeters or 2¾ inches. These two pictures are then -so viewed that each eye receives its appropriate image from the proper -direction, whereupon the object delineated stands out in relief. - -[Illustration: - - FIG. 149.—The principle of stereoscopic vision.] - -Fusion of the two elements of the stereoscopic picture can take place -without the assistance of any instrument, if the eyes are properly -directed and focussed, but this comes only with practice. Holding the -stereogram well away from the face the eyes are directed to a distant -object above and beyond, in order to diverge the axes. Then without -converging, the eyes are dropped to the picture, which should spring -into relief. It is necessary in moving the eyes from the distant object -to the near stereogram to alter their focus somewhat, depending on how -near the stereogram is held; and the success of the attempt to fuse the -images depends on the observer's ability to maintain the eyes diverged -for a distant object while focussing for a near one. Near-sighted people -(on taking off their glasses) fuse the stereoscopic images quite easily, -since their eyes do not focus on distant objects even when diverged for -them. Transparencies are easier to fuse than paper prints, but in any -case where a stereoscope is not used the separation of image centers -should not be more than that of the eyes. - -[Illustration: - - FIG. 150.—Common form of prism stereoscope.] - -=Stereoscopes.=—The easier and more usual method of fusing the -stereoscopic images is by a _stereoscope_. The simplest form consists -merely of two convex lenses, one for each eye, their centers separated -by a distance somewhat greater than that between the eyes. Their -function is to bring the stereogram to focus, and, by the prismatic -action of the edges of the lenses, to converge the lines of sight which -pass through the centers of the two pictures to a point in space in -front of the observer. The two lenses should be mounted so as to provide -for the adjustment of their separation to fit different eyes and print -spacings. The most common form of stereoscope is that designed by -Holmes, for viewing paper print stereograms (Fig. 150). It has prismatic -lenses of an appropriate angle to converge pictures whose centers are -three inches apart, instead of the lesser distance appropriate to -stereograms intended for fusing without an instrument. No adjustment is -provided for varying the lens separation, but the print can be moved to -and fro for focussing. - -[Illustration: - - FIG. 151.—Box stereoscope.] - -[Illustration: - - FIG. 152.—Diagram of mirror stereoscope.] - -Another form of stereoscope, one of the first produced, is the mirror -stereoscope (Fig. 152), now used extensively for viewing stereo X-ray -pictures. It consists of two vertical mirrors at right angles to each -other, with their edge of contact between the eyes. The two prints to be -studied are placed to right and left, an arrangement that permits the -use of prints of any size. The convergence point is controlled by the -angle between the mirrors. The Pellin stereoscope (Fig. 153) utilizes -two pairs of mirrors in a way to permit the use of large prints. The -prints are, however, placed side by side on a horizontal viewing table, -which avoids certain difficulties of illumination met with in the -simpler mirror form. The box form of stereoscope (Fig. 151) using either -prisms or simple convex lenses, is particularly adapted for viewing -transparencies, although the insertion of a door at the top provides -illumination for paper prints. The Schweissguth design (Fig. 154) is -intended primarily as an aid to selecting the portions of the prints to -be cut out for mounting. The platform on which the pictures rest is -composed of two long rectangular blocks, on which are plates of glass -raised sufficiently to permit the prints to be slid underneath. The -space between the blocks allows the unused portion of the photograph to -be turned down out of the way. Prints of any size can thus be moved -about until the proper portions for stereo mounting are found. Either -block can be moved in its own plane and also to and from the eye, -whereby two prints of somewhat different scales can be fused. - -[Illustration: - - FIG. 153.—Pellin double mirror stereoscope.] - -=The Taking of Aerial Stereograms.=—The normal separation of the eyes is -altogether too small to give an appearance of relief to objects as far -away as is the ground from a plane at ordinary flying heights. In order -to secure stereoscopic pairs it is therefore necessary to resort to a -method originally employed for photographing distant mountains and -clouds. This is to take the two pictures from points separated by -distances much greater than the interocular separation—by meters instead -of millimeters—corresponding to the positions of the eyes on a veritable -giant. In the airplane this is accomplished by making successive -exposures as the plane flies over the objective, at intervals to be -determined by the speed, the altitude and the amount of relief desired -(Fig. 155). - -An all important question which arises immediately is: What separation -of points of view shall we select? If the exposures are too close -together there will be little relief; if too distant the relief will be -so great as to be unnatural, even offensive. Obviously we cannot here -establish a criterion of _natural_ appearance, since the natural -appearance to ordinary human eyes is devoid of relief. We may, however, -define _correct relief_ as that obtained when _the apparent height of -elevated objects is right as compared with their extension or plan_. - -[Illustration: - - FIG. 154.—Schweissguth stereoscope, used for selecting portions of - prints to be mounted.] - -[Illustration: - - FIG. 155.—Method of taking stereoscopic pictures.] - -In order to secure this condition it is necessary, first, that each -element of the stereoscopic pair be correct in its perspective. This is -fortunately an old photographic problem, already well understood. Its -solution is to view the photograph from a distance exactly equal to the -focal length of the camera lens. Since the normal viewing distance is -not less than 25 centimeters, lenses of this focal length at least are -requisite for correct perspective. Secondly, it is necessary for correct -relief that the two views be taken with a separation equal, on the plane -of the plate, to the separation of the eyes, or 65 millimeters. If _d_ -is the interocular distance, a the viewing distance, identical with the -focal length of the lens used, and _A_ the altitude, then _D_, the -distance between exposures, is given by the relation— - - _d_ _D_ - ——— = ——— - _a_ _A_ - -For _a_ = 25 centimeters, _D/A_ = 6.5/25, approximately ¼, or the -interval between exposures must be a quarter the altitude. With a 50 -centimeter lens this becomes ⅛, and so on. These figures show the -fallacy of the suggestion sometimes made that we take stereoscopic -pictures by two cameras placed one at the extremity of each wing. - -When lenses of more than 25 centimeters focal length are employed, the -stereoscope should be one capable of throwing the convergence point -farther away than the customary 25 centimeters. In the simple lens type -of instrument we can do this by bringing the centers of the lenses -closer together, and by making the focus agree with the convergence -point by adjustment of the distance between lenses and stereogram. If -enlargements are used they should be treated in all respects as -originals made by lenses of the greater foci corresponding to the scale -of the enlargement. - -When all the conditions are covered, the appearance presented in the -stereoscope is that of a _model_ of the original object at a distance -_a_, and _a/A_ times natural size. If pictures are made at exposure -intervals less than those indicated for correct relief, they show -insufficient relief. This does not, however, give an unnatural effect, -because anything between no relief and “correct” relief appears natural -with large objects which are not ordinarily seen in relief by eyes not -Brobdignagian. Conversely, stereograms made with too large exposure -intervals show exaggerated relief. Yet this is often no objection. It is -indeed rather an advantage if we wish to bring objects of interest to -notice. Consequently, so long as the exaggeration of relief is not -offensive, the permissible limits of exposure interval are pretty large. -Actually, the eye tolerates such great deviations from strictly normal -conditions that satisfactory stereoscopic effects are obtained for -pictures viewed at very different distances from the focal length of the -taking lens, and with the axes of the eyes parallel or even diverging, -although there is some strain whenever focus and convergence points -differ. On the whole, therefore, it may be said that the conditions -above laid down for correct relief are only a normal, to be approximated -as nearly as is practicable. - -Having established the correct relation of taking points for stereos the -next problem is how to determine these when in the plane. The simplest -way is by means of a _stereoscopic sight_. This consists essentially of -two lines of sight (fixed by beads, crosses, or other objects), inclined -toward each other at the angle determined by the ratio of the ocular -separation to the focal length of the lens. If the back sight is made a -single bead or cross, the rest of the stereo sight will consist of two -beads or crosses, separated from each other by the ocular distance of 65 -millimeters, and distant from the back sight by the focal length of the -lens (Fig. 157). The first picture is taken when the object is in line -with the forward pointing line of sight, the second when it lies along -the backward pointing one. Like other sights, the stereoscopic sight may -be attached either to the camera, or if this is fixed in position, to -any convenient part of the plane. A very simple sight for vertical -stereoscopic photography consists of an inverted V painted on the side -of the fuselage, so that the eye can be placed at the vertex and sighted -along either leg. - -The common method of determining the space between exposures is by the -_time_ interval. If _V_ is the speed of the plane, and _t_ the desired -time interval, we have, from the last equation— - - _D_ _dA_ - _t_ = ——— = ———— - _V_ _aV_ - -If _A_ = 2000 meters, _d_ = 65 millimeters, and _a_ = 25 centimeters, -and if the plane is traveling 200 kilometers per hour, the time interval -must be— - - .065 × 2000 × 3600 - —————————————————— = 9.4 seconds - .25 × 200,000 - -At 1000 meters altitude the interval will be half this, and so on in -proportion. If the pictures are taken with a 50 centimeter focus camera, -and are hence to be viewed at 50 centimeters convergence distance -instead of at 25, the time will again be halved. These relations are -clearly shown in the diagram (Fig. 156). Here the left-hand portion -shows how to find the stereoscopic base line at each altitude for each -focal length; while the right-hand portion shows how to translate this -into time interval for any plane velocity. The Burchall slide rule (Fig. -130) shows another way to arrange these data in form for rapid -calculation. - -[Illustration: - - FIG. 156.—Chart for calculating intervals between exposures for - stereoscopic pictures.] - -Plates used for stereoscopic negatives should be at least twice as long -as the ocular separation, if correct relief is desired, and the full -size of the stereoscope field is to be utilized. This relation follows -at once if we consider that we wish to cut from each negative a -rectangle 65 millimeters wide, and that the image of the target has -shifted 65 millimeters between exposures. If the plate is larger than -this there is opportunity to select the view, or to pick several. If the -plate is smaller the elements of the stereogram must be narrow strips. -This, however, holds only for contact prints. - -The ordinary English practice in making stereo negatives is to take -successive pictures with an overlap of 60 to 75 per cent. This practice -is probably dictated by the 4 × 5 inch plate, since 60 per cent. overlap -on 4 inches means a separation of just over an inch and a half instead -of 2¾, but it leaves 2½ inches of picture common to the two negatives. -With ¾ overlap the common portion is 3 inches, which permits of cutting -2¾ inch prints, and allows some latitude for irregular motion of the -plane or for chance error in calculation of intervals. Data on the basis -of ¾ overlaps for a 4-inch plate are shown in connection with Fig. 155 -which shows in diagrammatic form the variation of exposure interval with -height, together with other points of interest. - -=Elevation Possible to Detect in Stereoscopic Views.=—Can the actual -difference in elevation be discovered by the use of stereoscopic views? -An approximate idea may be obtained from the following considerations: -Suppose we have two small point-like objects, one above the other, such -as a street lamp globe and the base of the lamp pillar. In a view taken -from directly overhead these will be superposed, and so will not be -capable of separation. But, as the point of view is shifted sideways, -the two objects separate, until a point is reached where they can just -be distinguished as double. When this condition holds for either picture -of the stereoscopic pair it will be possible to obtain stereoscopic -relief. - -Now the separation which can just be distinguished is commonly assumed -to be one minute of arc. This angle corresponds to about 1/3400 the -distance from the eye to the object. If the object is assumed at a -distance _a_ from the face, and on a line with one of the eyes, which -are separated by the distance _d_, then (all angles being small) the -object must be of height _a/d_ times the horizontal distance which -corresponds to one minute. For 25 centimeters' viewing-distance this -quantity is about 4, so that the least perceptible elevation is 4/3400 -or about 1/900. The stereogram having been made under conditions -giving correct relief, this fraction is also the fraction of the -altitude of the plane when the photograph was taken which may be -detected. An object as high as a man (6 feet) should be visible as a -projection in a stereoscopic view taken at 6 × 900 = 5400 feet. This -relation—1/900—holds (irrespective of the focal length of the lens), -as long as the conditions for correct relief are maintained. - -=Stereoscopic Aerial Cameras.=—Cameras for aerial stereoscopic -photography need in no way differ in construction from those made for -mapping or spotting, provided only they permit exposures to be made at -short enough intervals. The addition of special sights, as already -discussed, constitutes the only real difference between single view and -stereoscopic aerial cameras. But even without such sights ordinary -aerial cameras are applicable to stereo work by the usual procedure of -determining the exposure spacing by time. - -One scheme employed for taking low stereos, where the interval is only -two or three seconds, is to mount two cameras in the plane, exposing -them one after the other at the correct interval. Another method which -has been tried with success is the use of a double focal-plane shutter -in a single lens camera (Fig. 157). The two shutters are side by side, -with their slots parallel to the line of flight. To take a stereo -negative we expose first the shutter nearer the tail of the plane, and -then the other, after an interval which can be calculated from the speed -and altitude, or, better, determined by a stereoscopic sight. The two -views are thus obtained on a single plate. Prints from these negatives -are transposed right and left, and, if the prints are viewed in an -ordinary stereoscope, have to be cut apart and transposed for mounting, -or else this may be done to the negatives. - -[Illustration: - - FIG. 157.—Aerial hand camera fitted with two complimentary shutter - slits and double sight, for stereoscopic photography.] - -In this connection attention may be drawn to an alternative method of -viewing stereograms, which may be used on transposed prints—a method -which needs no instrument, and so has sufficient advantage to even -warrant mounting ordinary stereoscopic pairs in the transposed position -for observation. This method consists in crossing the optic axes, in the -fashion illustrated in Fig. 158. A finger is held in front of the face -in such a position that the left stereogram element and the finger are -seen in line by the right eye; the right element and the finger by the -left eye. The proper position is found by alternately closing each eye, -and advancing or retracting the finger. Then both eyes are opened and -converged on the finger tip, which is thereupon dropped, leaving the -picture standing out in relief. An opportunity to try this method is -afforded by Fig. 159. - -[Illustration: - - FIG. 158.—Method of fusing transposed stereoscopic images by crossing - the optic axes.] - -=Stereo Obliques.=—The theory of making oblique stereo pictures is -identical with that of other stereos. The only problem peculiar to -obliques is that of making the exposures at short enough intervals -apart. This problem is due largely to the fact that oblique views are -ordinarily taken from low altitudes, for the purpose of “spotting” -particular objects, rather than for mapping the gross features of an -extended area. The same problem of how to secure a short exposure -interval is met with when we attempt to take vertical stereos from a low -altitude, but as already discussed, it is much preferable from the -pictorial standpoint that pictures of definite small objectives be made -obliquely. - -Another reason for taking stereo obliques from points but little -separated is of some interest in connection with the discussion above -given of “correct” and “natural” relief. When the relief is “correct” -the object appears, as already stated, to be a small model in its true -proportions, standing at the convergence distance. When the eyes are -converged to a small object 25 to 50 centimeters away all objects beyond -are hopelessly transposed and confused. This does not happen when we -look at large distant objects, since their background is at a distance -effectively but little beyond them. As a result, when a stereo oblique -is made in “correct” relief of such an object as the Washington monument -with buildings beyond, the confusion of the background presents an -appearance entirely contrary to our visual experience with objects as -large as the neighboring buildings are known to be. This effect may be -avoided by choosing a uniform background such as grass, or by taking the -pictures very much closer together, at the expense of “correct” but at a -gain in “natural” relief. - -Stereo obliques can of course only be made with any facility by -laterally pointing cameras. From the calculations already given it -appears that a “correct” stereo oblique of an object 500 meters away -will mean exposures only two or three seconds apart, too short an -interval for any of the ordinary plate-changing and shutter-setting -mechanisms; and the case is even worse should less relief be desired. -One solution of this problem has been the use, already mentioned, of two -cameras mounted together, either side by side or one over the other, -with separate shutter releases. Both releases may be controlled by the -observer, using a sight, or else pilot and observer may work in harmony -as has been recommended in the English service, where the pilot releases -one shutter and the observer counts time from the instant he sees the -first shutter unwind and releases the second. - -[Illustration: - - FIG. 159.—Oblique stereogram made with stereoscopic aerial camera - (Fig. 157). To be viewed by crossing the optic axes (Fig. 158).] - -A very satisfactory apparatus for the taking of stereo obliques consists -of a 10-inch focus hand-held camera (Fig. 157), provided with a -two-aperture focal-plane shutter. The right-hand half of one curtain -aperture is blocked out, the left-hand half of the other. The first -pressure on the exposing lever exposes one-half of the plate, the second -the other. A stereoscopic sight of the type already described is placed -on the bottom. To make an oblique stereo negative the camera is held -rigidly by resting the elbows on the top of the fuselage and the first -exposure is made when the object comes in line with the rear sight and -the leading front sight. The eye is then moved so as to look along the -line of the rear sight and the following front sight, and when the -object is again in alinement the second pressure is given the exposing -lever. Fig. 159 shows a stereo oblique made by this camera. The elements -are transposed right and left, and the stereogram may be viewed by -crossing the optic axes as shown in Fig. 158, or the two pictures may be -cut apart and remounted. - -=The Mounting of Aerial Stereograms.=—The first step in making the -printed stereogram is to select two pictures taken on the same scale, -but from slightly different positions. These may be two chosen from a -collection made for other purposes, or else a pair taken at distances -calculated to fit them for stereoscopic use. The next step is to mark -the center of each picture, either with easily removed chalk or with a -pin point. They are then superposed, and afterward carefully moved apart -by a motion parallel to the line joining their centers when superposed. -The final step before mounting is to mark out and cut the two elements, -their bases being parallel to the line of centers, their horizontal -length the distance between the optic axes of the stereoscope (or as -near this as the size of the prints will permit). They are then mounted -on a card, with their centers separated by approximately 65 millimeters. -The right-hand view is the one showing more of the right-hand side of -objects, and vice versa. This process of arranging, cutting, and -mounting is shown clearly in Fig. 160. In this case the stereoscopic -elements lie symmetrically about the line joining the centers of the -original prints. This is not necessary, as they may be selected from -above or below this line so long as their bases are parallel to it. A -simplification of this method consists in superposing the two prints, -laying over them a square of glass of the size to which they are to be -cut, then turning it so that a side is parallel to the line of centers, -and cutting around it through both prints with a sharp knife. The -principle and results are of course the same with both methods. - -[Illustration: - - FIG. 160.—Method of arranging stereoscopic prints for cutting.] - -If large numbers of stereoscopic prints are required it is necessary, -for economy of time, either to photograph a finished stereogram and make -prints from this copy negative, or to set up special printing machines. -Under the general discussion of printing devices a stereoscopic printer -is described (the Richard) in which the two negatives are placed so that -stereo prints can be got by two successive printings on one sheet of -paper. - -=Uses of Stereoscopic Aerial Views.=—Attention has already been called -to the characteristic flatness of the aerial view. Neither the picture -on the retina nor that on the photographic plate affords any adequate -idea of hills and hollows. Unless shadows are well defined, small local -elevations and depressions cannot be distinguished from mere difference -in color or marking. Even in the presence of shadows it is often only by -close study that differences of contour are noticeable. But with -stereoscopic views these features stand out in a striking manner. Taking -our illustrations from military sources, we may note the use of -stereoscopic pictures to detect undulations of ground in front of -trenches (Fig. 161). They reveal the hillocks, pits, small quarries, -streams flowing behind high banks, and other features which make the -attack hard or easy. Commanding positions are shown, the boundaries of -areas exposed to machine-gun fire, and the defilades where the attackers -may pause to reform. Concrete “pill boxes” are located in the midst of -shell holes of the same size and outline, and can be differentiated from -them. - -Railway or road embankments and cuts can be detected and studied to -extraordinary advantage in stereoscopic pictures. Thus what appears to -be a mine crater on a level road, easily driven around, may be a gap -blown in an embankment, a serious obstacle indeed. Bridges, observation -towers and other elevated structures jump into view in the stereoscope -when often they have entirely eluded notice in the ordinary flat -picture. Once presented in relief, camouflaged buildings or gun -emplacements, however carefully painted, are ridiculously easy to pick -out. - -[Illustration: - - FIG. 161.—Typical stereogram of military detail. Fuse by looking at a - distant object over the top of the page, and quickly dropping the - eyes to the print.] - -Practical peace-time applications of stereoscopic views can easily be -foreseen following the lines of war experience. Such, for instance, -would be the study of proposed railway or canal routes. A series of -stereograms would obviate the necessity of contour surveys, at least -until the exact route was picked and construction work ready to start. - -Apart from their utilitarian side, however, stereoscopic views have very -great pictorial merit. Stereoscopic pictures of cathedrals, public and -other large buildings, have often great beauty, and afford opportunities -for the study of form given by no other kind of representation, short of -expensive scale models. They may very well lead in the near future to a -revival of the popularity of the stereoscope. - -=Impression of Relief Produced by Motion.=—An appearance of solidity can -be obtained in _moving pictures_ by the simple expedient of slowly -moving the camera laterally as the pictures are taken. As an -illustration, if the moving picture camera is carried on a boat while -structures on the shore are photographed, when these are projected on -the screen they appear in relief, due to the relative motion of -foreground and background. As relief of this sort is not dependent on -the use of the two eyes, it demands no special viewing apparatus. This -idea has been utilized to a limited extent in ordinary moving picture -photography by introducing a slow to-and-fro motion of the camera, but -this can hardly be considered satisfactory, since this motion is so -obviously unnatural. - -In moving pictures made from the airplane the normal rapid motion of the -point of view is ideal for the production of the impression of relief in -the manner just described. For instance, in moving pictures of a city -made from a low flying plane, the skyscrapers and spires as they sweep -past stand forth from their more slowly moving background in bold and -satisfying solidity. In fact, such pictures probably constitute the most -satisfactory solution yet found of the vexing problem of “stereoscopic” -projection. No better medium can be imagined for the travel lecturer to -introduce his audience to a foreign city than to throw upon his screen a -film made in a plane approaching from afar and then circling the -architectural landmarks at low altitudes. - - - - - CHAPTER XXIX - THE INTERPRETATION OF AERIAL PHOTOGRAPHS - - -Oblique aerial photographs if on a large enough scale are even easier to -interpret than are ordinary photographs taken from the ground, since -they practically preserve the usual view, and add to it the essentials -of a plan. With verticals, however, this is far from the case. In them -all natural objects present an appearance quite foreign to the ordinary -mortal's previous experience of them. This may be easily demonstrated by -taking any aerial view containing a fair amount of detail and trying -systematically to identify each object. A necessary preliminary to doing -this accurately is acquaintance with and study of the ground -photographed, or of similar regions, and of objects of the same -character as those likely to be included. - -The interpretation of military aerial photographs is of such importance, -and has become such an art, that it is the function of special -departments of the intelligence service. Extended courses in the subject -are now given in military schools. This instruction must cover more than -the interpretation of aerial photographs as such. General military -knowledge is essential, so that not only may photographed objects be -recognized, but the significance of their appearance be realized. -Whether attack or retreat is indicated; whether a long range bombardment -is in preparation, or a mere strengthening of local defences. - -The natural difficulties of interpreting aerial views are enormously -increased by the unfamiliar nature and frequently changed character of -the military structures, and particularly by the attempts made to -conceal these from aerial observation by selection of surroundings and -by camouflage. The small scale of the photographs, in which a machine -gun shows as a mere pin point, adds to the uncertainty, with the net -result of making interpretation a task of minute study and deduction -worthy of a Sherlock Holmes. - -Little detailed information on interpretation can be profitably written -in a general treatise, partly because the illustrations available are of -a highly technical military character, partly because original -photographs instead of halftone reproductions are practically imperative -for purposes of study. Nevertheless some general instructions, -applicable to any problem of interpretation, may be given, as well as a -few illustrations, drawn from military sources, which will serve to show -the detective skill necessary. - -First of all it is important that the print or transparency be held in -the right position. The shadows must always fall toward the observer; -otherwise, reliefs will appear as hollows and hollows will show as -hills. The reason for this is that the body ordinarily acts as a shield, -preventing the formation of shadows except by light falling toward the -beholder. Thus in Fig. 162 the slag heap looks like a quarry when the -shadows fall away from one. The necessity for proper direction of -shadows is, it may be noted, in conflict with the ordinary convention -for the orientation of maps—at least in the northern hemisphere. A city -map, made by sunlight falling from the south, presents its shadows as -falling away from the observer, when it is mounted with its north point -at the top, as is customary. As a consequence buildings in aerial -photographic mosaics of cities occasionally look sunken instead of -standing out. - -[Illustration] - - Wrong way. Shadows falling away Right way. Shadows falling toward - from observer. observer. - - FIG. 162.—The wrong way and the right way to hold a photograph for - interpretation. - -[Illustration: - - FIG. 163.—Guide to interpretation of trench details.] - -The relation between the shape of the shadow and the object casting it -must be well learned. This is a part of the training of every -architectural draftsman, but the appearance of shadows from above has -not heretofore been a matter of importance. The difference between high -and low trenches, between cuttings and embankments, between shell holes, -occupied or unoccupied, and “pill boxes,” must be detected largely from -the character of the shadows. Which elevations and depressions are of -military and which of merely accessory nature, whether this black dot is -a machine gun or a signaling device, whether that dark spot is an active -gun port or an abandoned one—these are all matters of shadow and of -light and shade study. Several illustrations of these points appear in -Figs. 163, 164 and 165. - -[Illustration: - - FIG. 164.—Guide to interpretation of shell holes and other pits.] - -[Illustration: - - FIG. 165.—Illustrating the importance of distinguishing between - objects of similar appearance but different military importance.] - -Shadows may be used to get exact information as to directions and -magnitudes. If we know the time of day at which a picture is taken, the -direction of the shadows will give the points of the compass. A chart -for doing this is shown in Fig. 166. The length of a shadow is a measure -of the height of the object casting it, and the exact relation between -the two dimensions is determined by the day and hour. Fig. 167 embodies -in chart form the values of this relationship for all times of the year -and day, while Fig. 168 shows the kind of picture in which shadow data -could be utilized to great profit. - -[Illustration: - - FIG. 166.—Location of true north from direction of shadows. Place the - dial on the photograph, the hour line corresponding to the time it - was taken being pointed in the direction of the shadows. North lies - between the two arrows, the exact direction being obtained by - joining the center of the dial to the point on the figure of eight - corresponding to the date on which the picture was taken. (Number on - figure of eight represent the 1st of the month.)] - -Minute changes, both in light and shade and in position, must be watched -for with great care. Naturally growing foliage and the cut branches used -for camouflage differ in color progressively with the drying up of the -leaves. Hence a mere spot of lighter tone in a picture of a forest, -especially if the picture is taken through a deep filter, becomes -instant object for suspicion. The complete study of any position calls -for photographs of all kinds—verticals, obliques, and stereos. -Stereoscopic views are the worst foe to camouflage. A bridge painted to -look like the river beneath is labor thrown away if the stereo shows it -to be a good ten feet above the real river! - -[Illustration: - - FIG. 167.—Length of shadow of object one meter high, at different - times of the day and year, for latitude of Paris.] - -[Illustration: - - FIG. 168.—Bethune, August, 1918, illustrative of interpretation by - shadows.] - -[Illustration: - - FIG. 169.—Typical trench photograph showing first and second lines, - communicating trenches, listening posts, machine gun emplacements, - and barbed wire.] - -A few illustrations of the more ordinary and obvious objects whose -detection is the subject of aerial photography are shown in accompanying -figures. Fig. 169 pictures a typical trench system, with barbed wire. -The trenches show as narrow castellated lines, from which run the zigzag -lines of communicating trenches, saps, and listening posts. The minute -pockmarks behind the main trench lines are shell holes and machine gun -pits. The barbed wire shows as double and triple gray bands, intricately -criss-crossed at strategic points. Another form of defence, intended for -the same purpose as the barbed wire of the western front, is that -furnished by overthrown trees in forest regions. Fig. 170 reveals a -mountain fortress surrounded by a zone of felled trees, and indicates in -striking manner the value of the information a single aerial photograph -may furnish to an attacking force. Fig. 123 shows on a comparatively -large scale opposing trench systems in which a natural obstacle—a -river—separates the adversaries. Nicks and dots indicate machine guns to -the skilled eye, and several rectangular structures are revealed as -concrete buildings which have survived unscathed the shell fire which -has obliterated, and caused to be rebuilt, nearly every other element of -the trench system. - -[Illustration: - - FIG. 170.—A mountain fort surrounded by felled timber.] - -Isolated battery emplacements (Fig. 171) must be carefully studied to -learn if they are in use. The chief indication is given by the paths the -men make in going and coming; these show as fine light lines, -obliterated by growing vegetation if long disused. Another indication is -the blast marks in front of the gun muzzles; occasionally the sensitive -plate will catch the actual puff of smoke as the gun is discharged. - -[Illustration: - - FIG. 171.—Three stages in the life of a battery.] - -Railways of various gauges show as thin lines, crossed by ties, and -exhibiting the characteristic curves and switches. They are particularly -important to detect because they naturally lead to guns or supplies of -importance. Abandoned railways from which the rails and ties have been -removed leave their marks on the ground and must be carefully -distinguished from lines in actual operation. - -Aviation fields are easily recognized by the hangars, often with “funk -hole” trenches alongside for the men to take shelter in during air raids -(Fig. 172). Other characteristic features are the “T” which shows the -direction of the wind to the returning pilot, and of course the planes -themselves, standing on the ground. But the field may be inactive, and -the planes merely canvas dummies, so that to pierce the disguise, all -paths, ruts, and other indications of activity must be minutely studied. - -Overhead telegraph and telephone lines are revealed when new by a series -of light points (Fig. 174), where the posts have been erected in the -fresh turned earth. Later, when the fields through which they pass are -cultivated, the post bases show as islands left unturned by the plow. In -winter the wires reveal their position by black lines in the snow caused -by drippings. Buried cables are indicated while building by their -trenches, and for some time afterward by the comparatively straight line -of disturbed earth. - -[Illustration: - - FIG. 172.—Aviation field, showing hangars, planes, landing “T” and - refuge trench.] - -Just as the detective of classic story makes full use of freshly fallen -snow to identify the footprints of the criminal, so does the aerial -photographer utilize a snowfall to pierce the enemy's attempts at -deception. Tracks in the snow show which trenches or batteries are in -actual use. Melting of the snow in certain places may mean fires in -dugouts beneath. Black smudges in front of trench walls show where guns -are active. Guns, wire and other objects, however carefully painted to -match the gray-green earth, stand out in violent contrast to this new -white background (Fig. 173). - -[Illustration: - - FIG. 173.—Trenches and barbed wire in the snow of an Alpine ridge. - Italian Air Service photograph.] - -[Illustration: - - FIG. 174.—A fully interpreted aerial photograph.] - -After the aerial photograph has been interpreted the results of the -interpretation must be made available to the artilleryman or the -attacking infantryman. This may be done by legends marked directly on -the photograph. Another method is to mount over the photograph a thin -tissue paper or oilskin leaf, with the interpretation marked on it. A -yet more elegant method consists in outlining all the chief features of -the photograph in ink, writing in the points of importance in -interpretation, and then bleaching out the photograph with potassium -permanganate solution. Photographic copies of the resultant line drawing -are then mounted side by side with the original photograph. Fig. 174, -which shows a fully interpreted photograph, is an example of this kind -of mounting. - - - - - CHAPTER XXX - NAVAL AERIAL PHOTOGRAPHY - - -The problems of naval aerial photography are quite different from those -of military aerial work, and on the whole they are more simple. At the -same time, photography has played a considerably less important part in -naval aerial warfare than in land operations. Photography as a necessary -preparation for attack has not figured in naval practice nearly so much -as have the record and instruction aspects. To some extent this is due -to the nature of the naval operations in the Great War, to some extent -to the limitations of ceiling and cruising radius of the naval aircraft. - -A photographic reconnaissance, preceding and following a bombardment of -shore batteries; a photographic record of the ships at anchor, as at -Santiago; a photograph of the forts defending a channel, as at Manila; -photographs, quickly developed and printed, of an approaching fleet—all -these are possibilities of great usefulness in naval warfare between -contestants both of whom “come out” and carry the struggle to the -enemy's gates. But in the recent war the use of the submarine, -operations under cover of fog, the striving for “low visibility,” and -the considerable distances to be traversed to reach the enemy lairs, -have conspired to limit the development of photography as a major aid to -naval combat. Probably when the whole history of the conflict is told we -shall learn that the Zeppelins which cruised over the North Sea, keeping -the Allied fleet under observation, had a regular routine of -photographic work. In the Italian zone, where much of the enemy -territory and several important naval centers lay at only short -distances over the Adriatic, the naval photographic service more nearly -rivalled that of the army than in the English, French and American zone -of activity in the Channel and North Sea. - -[Illustration: - - FIG. 175.—A lighthouse, as the naval flier sees it.] - -[Illustration: - - FIG. 176.—A threatened submarine attack. Throwing out a smoke screen - to protect a convoy. British official photograph.] - -The majority of the photographs made in the British service were -obliques, taken by short focus (6 to 10 inch) hand-held cameras. This -type was employed partly because of difficulties to be noted presently, -in using other forms of cameras, but more especially because such -pictures sufficed for the kind of information desired. A hand-held -camera formed part of the outfit of each flying boat and dirigible, but, -unlike land reconnaissance, planes ascending primarily for picture -taking were unknown in their naval service. Usually no photographic -objective was predetermined—photographs were made only if objects of -interest were come upon. Mapping also formed no part of the seaplane's -work. Four plates would be carried, instead of as many dozens in the -land machine, and often these would come back unexposed. There were of -course some photographic flights planned out beforehand, for the purpose -of photographing lighthouses and other landmarks whose appearance from -the air should be known to the naval aviator (Fig. 175). Among the -accidental and record types of photograph come convoys (Fig. 176), whose -composition and arrangement were made a matter of record, particularly -if any ship was out of its assigned position. Photographs of oil spots -on the sea surface, or other results of bomb dropping, were necessary -evidence to establish the sinking of a submarine (Fig. 179). Pictures of -all types of ships friendly, neutral, and where possible, enemy—were a -much needed part of naval equipment, in particular pictures of friendly -destroyers and submarines, which should not be bombed by mistake. For -safe navigation it was essential to have photographs of uncharted wrecks -(Fig. 181), of buoys out of place and of ships failing to return signals -or otherwise to comply with rules. The great majority of the pictures -were taken from altitudes of not more than 300 meters. - -[Illustration: - - FIG. 177.—Submarine coming to the surface. - - U. S. Naval Air Service photograph.] - -[Illustration: - - FIG. 178.—Dropping depth bombs.] - -[Illustration: - - FIG. 179.—The submarine destroyed. Destroyer on tell-tale oil patch. - British official photographs.] - -[Illustration: - - FIG. 180.—A convoy at anchor in port.] - -Hand-held cameras for naval work have practically the same design as -those for land work. In view of the smaller number of pictures taken on -naval trips, and the consequent absence of any need for great speed in -changing plates, the ordinary two-plate dark slide has been found -satisfactory in the English service. But these are much less convenient -than the bag magazines used in the U. S. Naval hand camera (Fig. 31). -The sights on the naval hand camera are preferably of the rectangular, -field indicating type, especially useful in photographing extended -objects such as convoys. As the flying boat travels comparatively slow, -it is easy for the observer to stand up to take pictures, and the sight -is conveniently placed on top. But if held out over the side for -verticals the sight must be on the bottom. Rectangular sights in both -positions are provided in the English camera (Fig. 186). Naval cameras -should be immune from moisture, which means doing away with all wooden -slides or grooves. A praiseworthy practice is to carry the camera in a -waterproof bag. - -[Illustration: - - FIG. 181.—Airplane photography as an aid to salvaging. Position of - wrecked merchantman twelve fathoms down revealed by photograph from - the air. - - Photograph by British Air Service.] - -[Illustration: - - FIG. 182.—A sea plane.] - -[Illustration: - - FIG. 183.—A flying boat.] - -[Illustration: - - FIG. 184.—A dirigible or “blimp”—possibly the photographic aircraft of - the future.] - -[Illustration: - - FIG. 185.—English “Type 18” hand camera on bracket for exposing - through side window of flying boat. - - British official photograph.] - -[Illustration: - - FIG. 186.—Camera mounted in bracket from forward cockpit of flying - boat. - - British official photograph.] - -Cameras other than of the hand-held form have been little used in sea -planes, owing to the difficulties of installation. The hydro-airplane, -consisting of an ordinary airplane fuselage mounted on two pontoons -(Fig. 182), can carry the same kind of photographic equipment as the -land machine. But if it has a single central pontoon this is not -feasible. The hydro-airplane is, however, largely superseded by the -flying boat (Fig. 183), whose fuselage, of boat form, rests directly on -the water. In this type of sea plane, views taken vertically downward -are not easy to make. In the larger flying boats the hull projects out -horizontally a matter of several feet beyond the side of the cockpit. An -ordinary outboard mounting is therefore out of the question. The camera -must either be held out at arm's length or else mounted on a long -bracket (Fig. 186). The usual place for carrying the camera is in the -front cockpit with its magnificent all-round view. Obliques can, too, be -taken in great comfort from the side windows behind the wings, as shown -in Fig. 185. The possibility of cutting a hole in the bottom of a flying -boat to take care of a vertical camera is not entertained in British and -American naval circles. Nevertheless it is the regular practice in the -Italian service, with their small high ceilinged flying boats. In them a -round hole is cut in the floor, stopped with a plug and rubber gasket. -After the boat rises into the air the hole is opened, and the regulation -Italian camera is set securely in a frame on the floor over the hole -(Fig. 187). Photographs are taken to the capacity of the camera, and if -it is desired another camera is put in its place, till all its plates -have been exposed, and then even a third. Before coming down the hole -must of course be closed again. Sliding doors have been designed to -close this aperture, but have not proved sufficiently water-tight, -although such a device could undoubtedly be worked out. - -[Illustration: - - FIG. 187.—Italian flying boat with camera mounted on the floor.] - -With its space for five or more passengers, and with its low speed, the -modern flying boat affords an excellent craft for photographic work. -There is ample room for any size of camera, and for any style of -mounting, if we assume that there is no objection to an opening in the -bottom. The low ceiling of these ships, however, prevents their use for -certain forms of aerial photography which should be of the greatest -importance. Operations against shore stations—harbors, docks, shipyards, -ships at anchor, and fortifications—cannot be undertaken for fear of -anti-aircraft guns and hostile land planes. The solution of the problem -of carrying and launching fast high flying planes from ships will -immediately extend the usefulness of aerial photography to coastal work. -In the recent war, such of this as was done, along the Belgian coast—the -shore batteries, and the results of naval operations at Zeebrugge and -Ostend—was done by land planes from territory held by the Allies. The -photographic equipment of sea planes of the type suggested will of -course present special problems, but the apparatus used will be apt to -approximate closely to that of the land planes. - - - - - VII - THE FUTURE OF AERIAL PHOTOGRAPHY - - - - - CHAPTER XXXI - FUTURE DEVELOPMENTS IN APPARATUS AND METHODS - - -Prophecy is an undertaking that always involves risk. The prophet's -guess of what the future will bring forth is based only on the -tendencies of the past, the most urgent needs of the present, and the -activity of his imagination. He may easily—and he usually does—entirely -overlook certain possibilities which may arise apparently from nowhere -and which profoundly affect the whole trend of development. Conditions -which dominate at the present time—such as military necessity—may -happily drop into the background and free the science from some of its -severest restrictions. With this caution, some future possibilities in -apparatus and methods may be presented along the lines already used in -discussing the present status of aerial photography. - -=Lenses.=—From the military standpoint the next steps in lens design -would be toward telephoto lenses on the one hand, and on the other -toward lenses of short focus and wide angle. The telephoto lenses used -for spotting would be of long equivalent focus—a meter and more—but of -handy size, that is, not more than 50 centimeters over all working -distance. The wide angle short focus lenses would be designed for low -flying reconnaissance or quick mapping work. They would also be demanded -for peace-time mapping projects, where the largest possible amount of -territory should be covered in a single flight. Both types of lens -should be pushed to the extreme in aperture, for short exposures and the -maximum of working days will always be demanded. - -=Cameras.=—Peace-times will give the necessary opportunity to develop -self-contained and therefore simply installed cameras. They will at the -same time be made very completely automatic but simple to operate in -spite of their complexity. Such cameras have, during the war, been the -ideal of all aerial photographers, but the time has been too short since -the necessary conditions have been understood for that lengthy -development work and those complete service tests which are so necessary -to develop all automatic apparatus. Several designs which are now being -perfected may be counted on to take us a long way toward this ideal. - -On the other hand, that military ideal which leaves the camera operator -the greatest possible freedom for other activities, is apt to be -entirely reversed in peace. The camera operator can now be required to -be an expert, who will be free to change plates or filters and to -estimate exposures, instead of giving his best efforts to the problem of -defence. For him a simple and reliable hand-operated or semi-automatic -camera is entirely satisfactory, and the great expense of complicated -automatic apparatus has no longer its former justification. - -=Camera Suspension.=—Perhaps the most pleasing prospect before the -aerial photographer as he turns from war to peace work is that of having -planes built for and dedicated primarily to photography. Instead of his -camera being relegated to an inaccessible position, picked after the -plane design has been officially “locked;” instead of yielding first -place to controls, machine gun and ammunition; instead of being -jealously criticised for the space and weight it takes up, the camera -can now claim space, weight, and location suitable for any likely aerial -photographic need. High speed no longer will be vital, and slower -planes, permitting longer exposures in inverse ratio to their speed, -will be chosen for photographic purposes. - -A development which is sure to intrigue many investigators is the -gyroscopically controlled camera. This has its chief _raison d'être_ in -precision mapping, whose possibilities from the air will undoubtedly be -intensively studied at once. With the automatically leveled camera will -come renewed attention to indicators of time, altitude, and direction, -with the ultimate goal of producing aerial negatives that show upon -their face the exact printing and arranging directions necessary to put -together an accurate map. - -=Sensitive Materials.=—Manufacturers of plates and films will direct -efforts toward producing emulsions of good contrast, high color -sensitiveness and high effective speed, especially when used in -conjunction with the filters necessary for haze penetration. Exposure -data will be accumulated and exposure meters appropriate for aerial work -will be developed. - -=Color Photography.=—Color photography from the air by any of the -screen-plate or film-pack methods is probably out of the question -because of the long exposures required. The screen-plates are unsuitable -also because of the relatively large size of their grain compared to the -detail of the aerial photograph. Ordinary three-color photography, using -three separate negatives, is always subject on the earth's surface to -the difficulty that the three negatives must be exposed from the same -point of view, either in succession or by means of some optical -arrangement which is costly from the standpoint of light. In -photographing from the air this difficulty of securing a single point of -view for the three photographs is absent. Three matched cameras, side by -side in the fuselage, have identical points of view as far as objects on -the earth below are in question. Consequently, three-color negatives are -entirely possible, and indeed will be simple to make as soon as plates -of adequate color sensitiveness and speed are available. Probably the -new Ilford panchromatic plate has the necessary qualities. - -=Night Photography.=—The searching eye of photography was so omnipresent -in the later stages of the Great War that extensive troop movements and -other preparations had to be carried out either in photographically -impossible weather or else at night. The natural reply to the -utilization of the cover of night is to “turn night into day” by proper -artificial illumination. At first thought it might well appear that the -task of illuminating a whole landscape adequately for airplane -photography is well-nigh hopeless by any artificial means. On one hand -we have the very short exposures alone permissible; on the other the -fact that the intensity of daylight illumination is overwhelmingly -greater than those common in the most extravagant forms of artificial -illumination. - -Toward the close of the war, however, actual experiments made with -instantaneous flashes of several million candlepower showed that if -proper means were provided to insure the flash going off near the -ground, and if its duration were made no longer than about 1/50 second, -interpretable photographs were obtainable on the fastest plates. It -appears, therefore, merely a matter of manufacturers perfecting the -technique of flash production, and of inventors providing the launching -and igniting devices to push this kind of photography to the practical -stage. The achievement of night photography cannot fail to have an -enormous effect on future tactics. - -The technique of night photography may take either of two directions. On -one hand we may develop flashes of the requisite intensity to give all -their light in 1/100 second; on the other hand, it may prove more -feasible to use flashes of longer duration and to arrange for the camera -shutter (of the between-the-lens type) to be exposed synchronously with -the middle of the flash. One way, frequently suggested, to use these -longer flashes would be to trail the charge on a long wire, through -which the ignition is effected electrically. This is not likely to be -satisfactory, however, for the resistance of a wire is so great that -when the plane flies at any practical height, the trailed flash, if it -reaches near the ground, will be forced to a very great distance behind. -Probably the best solution will involve accurate synchronizing of the -fuse in the freely dropped sack of flash powder with the exposing -mechanism in the camera. - - - - - CHAPTER XXXII - PICTORIAL AND TECHNICAL USES - - -Aside from their element of novelty, aerial photographs have undoubted -qualities of beauty and utility. The “bird's-eye view” has always been a -favorite for revealing to the best advantage the entire form and -location of buildings and of other large objects. Heretofore such views -have usually had to be drawn by an imaginative artist. - -[Illustration: - - FIG. 188.—Rheims Cathedral.] - -Aerial oblique views possess the virtues both of pictures and of plans. -They are destined to be extensively used in the study of architecture -(Fig. 188). Cathedrals, castles, town halls, particularly those still in -their cramped medieval surroundings where they can never be seen in -their entirety from the ground, come forth in all their beautiful or -quaint proportions from the airman's point of vantage. Stereoscopic -aerial views are destined to occupy a valuable position also. Stereo -prints of the famous buildings of Europe, taken from the air, will give -to the prospective traveler or the arm-chair tourist a many fold more -accurate idea of their construction than will any number of mere surface -views. - -[Illustration: - - FIG. 189.—A portion of Vienna seen from the air, during a “propaganda - raid.” - - Italian official photograph.] - -[Illustration: - - FIG. 190.—The Rialto bridge, Venice. - - Italian Air Service photograph.] - -[Illustration: - - FIG. 191.—A partly developed suburb.] - -A vertical aerial photograph is most closely akin to a map, but has -advantages over any ordinary surveyor's product. As a guide it is -infinitely superior to the best draftsman's diagram, for it provides a -wealth of detail whereby the traveller may definitely locate himself. At -a single glance he notes the objects of interest within his radius of -easy travel. The guide-book of the future will therefore be incomplete -without numerous aerial views, both vertical and oblique. As an -illustration of the peculiar merit of the view from the air, consider -the photograph of Vienna made during d'Annunzio's “propaganda” -bombardment. Or the picture of the Rialto bridge (Fig. 190). No ordinary -photograph from land or water suggests the central roadway and no map -shows the beauty of its elevation. Both are shown here, as well as an -intimate view of the arched and pillared courtyard of the Fondaco de' -Tedeschi to the right. - -[Illustration: - - FIG. 192.—A sea-side resort.] - -[Illustration: - - FIG. 193.—A bathing beach seen from the air.] - -Airplane photographs will undoubtedly be widely used in certain fields -of advertising. Architects and real estate agents may be expected to -display their wares by the aid of aerial views. A well-planned country -estate or golf course, or a suburban development (Fig. 191), can be -shown with a completeness, both as to environment and stage of progress -which no other form of representation can approach. A sea-side resort -can now show the extent and grouping of its natural and artificial -amusement features in a single picture (Fig. 192). Even the extent of -its bathing beach under water is revealed to the aerial photographer -(Fig. 193). Real estate agents can utilize aerial photographic maps of -cities to great advantage. On these their properties can be pointed out, -with the nature of their surroundings shown at a glance, together with -their relation to transportation, schools, churches, shopping districts, -parks, or factories. The future purchaser of lots in a distant boom town -will no longer be satisfied with a map outlining the streets with -high-sounding names. He will demand an authentic aerial photograph, -showing the actual number of houses under construction, the streets, -gutters and sidewalks already laid, the size and planting of trees. - -[Illustration: - - FIG. 194.—Mt. Vernon from the air.] - -[Illustration: - - FIG. 195.—A contrast in roofs. The Capitol retains its individuality, - while the White House loses all character when seen from above.] - -The study of landscape gardening is another field for which the aerial -photograph is peculiarly fitted. A collection of oblique pictures of the -châteaux and palaces of Europe showing their approaches and grounds, or -of the historic estates of our own South, (Fig. 194), will be worth more -to the prospective designer of a country estate than maps and ground -pictures can ever be. Closely allied to landscape gardening is city -planning, for which the aerial map will be quite indispensable. The -appearance of a city from the air may indeed become a matter of pride to -its inhabitants, and not only the arrangement of streets and parks, but -even the character of the roofs of the buildings, be the subject of -study (Fig. 195). - -[Illustration: - - FIG. 196.—An aviation field under construction; early stage.] - -Engineers and constructors will depend more and more on preliminary -photographic surveys as a basis for locating their operations. At the -later stages of their work they will use aerial photographs for -recording progress. Periodic photographs of buildings in process of -construction, such as are now made from the ground, are much more -illustrative when made from the air. Only from above is it possible to -obtain in a single picture the progress of the complete project, such as -the construction of an aviation field (Figs. 196 and 197) or of a -shipyard. The building of large structures—bridges, hotels, ships on the -stocks—particularly demands aerial views if the foreground is not to -eclipse the center of real interest. - -[Illustration: - - FIG. 197.—An aviation field under construction; later stage.] - -News events will soon call for an aerial photographic service. Already -we are seeing newspapers and magazines featuring aerial photographs of -the entry into conquered cities and the parades of returning fleets. -Accidents, fires, floods and wrecks, of either local or national -interest, can best be represented by this newest form of photography. - -[Illustration: - - FIG. 198.—The crater of Vesuvius. - - Photograph by Royal Italian Air Service.] - -The photographing of wrecks, fires and floods suggests the importance of -aerial views to insurance underwriters, who require the most minute -information on the characteristics of buildings in every neighborhood, -and on the extent and nature of damage done. Marine insurance companies -might with profit use the airplane camera to help estimate the chances -of salvage of a stranded ship or a vessel foundered in shallow waters -(Fig. 181). - -[Illustration: - - FIG. 199.—Waves set up by a ship—of interest to the naval architect.] - -Numerous scientific uses for aerial views seem likely. Prominent among -these is their use in geology, for the study of the various forms of -earth sculpture. Pictures from the air of extinct volcanoes will give -information as to their configuration that would otherwise require -months of painstaking survey to obtain. Aerial photographs of active -volcanoes (Fig. 198), showing the results of a succession of -outbursts—one obliterating the other—would prove of the greatest value, -especially when studied in conjunction with other scientific data, the -whole making a record unobtainable by any other means. - -In earthquake regions—notably Southern Italy and Japan—the changing -coast lines, shallows and safe harbors, could be promptly ascertained -after the subsidence of each fresh shock, with a consequent keeping open -of trade routes and often the saving of life. River courses, glacier -formations, cañons, and all the larger natural formations which man -usually sees only in minute sections, and which he must build up in his -mind's eye or by models, are today quickly and accurately recorded by -the camera in the air. Such formations as coral reefs, whose -configurations can now be accurately learned only by laborious surveys -of a limited number, could be studied in quantity and with heretofore -unknown satisfaction as the result of a single expedition with a -ship-carrying seaplane and aerial camera. - -Another scientific field—probably one of many similar ones—lies in the -study of the waves set up by ships (Fig. 199). These are of extreme -importance in the realm of naval architecture, but before the day of the -airplane could never be easily studied in full scale. - - - - - CHAPTER XXXIII - EXPLORATION AND MAPPING - - -Aerial photographic mapping in war-time has been almost entirely -confined to inserting new details in old maps. For such work some -distortion or a lack of complete information on altitude and directions -is not a serious matter, because the known permanent outlines serve as a -basis. Furthermore, in so far as outline maps are concerned, as -distinguished from pictorial maps, these have been drawn on the ordinary -scales, and with the ordinary conventions of engineering map practice. - -Aerial photography may be used in the future in practically the same -way, as an aid to the quick recording of those minute details which -would ordinarily consume an enormous amount of labor to survey directly. -The region shown in Fig. 200 affords a good illustration. A discouraging -amount of time and effort would be required to map this section of -Virginia by the usual methods, while the smallest curve of creek and -shore is instantly and completely recorded on a single photographic -plate. But there are other possibilities, diverging from this -application both toward greater and lesser requirements for precision. - -Pictorial maps, in which the actual photographs figure, promise to be an -essential part of the airman's equipment, whether he be pilot or -passenger, mail carrier or sportsman. Without any pretention to detailed -accuracy of location, these maps will show, in strip or mosaic form, the -general appearance of the country to be traversed, with particular -reference to good landing fields and other points of interest to the -aviator. Vertical pictorial maps may be supplemented by obliques giving -the view ahead, whereby the pilot may direct his ship. Thus the -Washington monument as seen by the pilot from Baltimore is a truer guide -than is the country beneath him. The crossing of mountain ranges is -another case where the oblique picture will be more useful than the -vertical (Fig. 201). - -[Illustration: - - FIG. 200.—An aerial photographic survey of ground difficult to cover - by ordinary surveying methods.] - -[Illustration: - - FIG. 201.—Seeking out mountain passes.] - -Contrasted with the merely pictorial maps will be precision surveys. -Whether it will prove practical to make these entirely from the air is -still an open question. It is to be assumed that cameras can be -constructed with lenses having negligible distortion of field, with -between-the-lens shutters to obviate the distortions due to the -focal-plane type, with auxiliary devices for indicating compass -direction, altitude, and inclination, or with gyroscopic mounting so -that an inclination indicator is unnecessary. The application of aerial -photography to precision mapping will depend upon the perfection which -such cameras attain, as estimated by the permissible errors in this form -of mapping. Entire dependence on photography, as in uncharted regions, -is likely to be worked up to slowly, beginning with a stage of rather -complete triangulation of natural or artificial points—say three in each -constituent picture—then through several stages each successively -employing fewer and fewer well determined points. The photographic -mapping of some of our Western States will be greatly facilitated by the -100-yard squares into which the land is divided and already marked in a -manner which shows clearly in aerial photographs. - -A theoretical possibility is the plotting of contours from stereo-aerial -pictures. Given two elements of a stereoscopic pair, taken from points -whose separation is known, the position of any point in space shown in -the stereoscopic view can be determined by the use of the -stereo-comparator. This is an instrument already employed in mountain -photo-surveying, which consists essentially of a compound stereoscope in -whose eye-pieces are two points movable at will so that the relief image -formed by their fusion can be made to coincide with any chosen part of -the landscape. The chief difficulty in the application of this idea to -aerial work is to fix the base line. This problem may be met in some -cases by using stereo obliques, and getting the base line by -simultaneously made vertical photographs of well surveyed territory -beneath. Possibly also methods can be developed by which photographs -from two or more known altitudes may furnish the requisite data. - -[Illustration: - - FIG. 202.—Business section of Hampton, Virginia. A survey made by a - single instantaneous exposure.] - -[Illustration: - - FIG. 203.—Mosaic map of the City of Washington. White rectangle shows - portion included in next figure.] - -[Illustration: - - FIG. 204.—Portion of Washington mosaic, full size.] - -City mapping is a field for which aerial photography is peculiarly -fitted (Fig. 202). A complete map of a large city is a labor of years. -In fact, a modern city is always dangerously near to growing faster than -its maps. An aerial map, on the contrary, can be produced in a few -hours. Paris was mapped with 800 plates in less than a day's actual -flying. Washington was completely mapped in 2½ hours, with less than 200 -exposures. The entire map is shown, on a greatly reduced scale, in Fig. -203, while Fig. 204 shows a small portion of it in full size, from which -can be obtained an idea of the dimensions of the original. These maps, -while not accurate enough for the recording of deeds and mortgages, yet -serve the majority of needs. There is indeed no reason why with long -focus cameras, given several accurately marked points, the photographic -map of a piece of real estate should not be made with all the accuracy -needed, still leaving the whole process of partial surveying helped out -by photography an enormously simpler one than the usual method. - -Rougher types of surveying, in open country, offer a most promising -opportunity. Railway surveys, showing the character of the country: -passes through mountain ranges: the available timber and other materials -of construction. Canal routes, with the available sources of water -supply, and the best choice of course to avoid deep cuttings and -aqueducts. Irrigation projects, with the natural lakes, river courses -and valleys, which may be dammed to form storage basins. Coast, river -and harbor surveys are possible by aerial means with a promptness and -frequency which should entirely revolutionize the making of maps of -waterways. Shifts in channels and shallows, even of considerable depth, -stand out prominently in the aerial photograph. The actual bottom, if -not more than three or four meters down—as in a bathing beach—shows in -the aerial photograph (Fig. 193), while the varying surface tints caused -by light reflected from the bottom at far greater depths are readily -differentiated by the camera from the air. An instantaneous photograph -will thus perform the work now done by a week's soundings. Fig. 205, -taken near Langley Field, shows how the aerial photograph may be used to -chart natural channels, while Fig. 206 shows the dredged channels of the -port of Venice. Navigation of such a river as the Mississippi with its -shifting bars may come to be guided by monthly or even weekly aerial -photo maps. - -[Illustration: - - FIG. 205.—Shallows and channels revealed by the aerial photograph.] - -[Illustration: - - FIG. 206.—Venice from the air, showing dredged channels. - - Italian official photograph.] - -[Illustration: - - FIG. 207.—Bengasi, a North African town, surveyed for the first time - from the air. - - Italian official photograph.] - -[Illustration: - - FIG. 208.—Thurnberg on the Rhine. - - Photograph by photographic section A. E. F.] - -Among other uses for aerial photography will be the location of timber. -As one illustration, may be taken the discovery of mahogany trees. Their -foliage at certain times of the year is of characteristic color. This -may be recorded on color sensitive plates with a scientifically chosen -filter, and the cutting expedition sent out with the photograph as a -guide. In this as in other cases where rough or unexplored country is to -be covered, it is a question whether the airplane will after all be the -most feasible craft, on account of its necessarily rapid rate of travel, -and its need for known landing fields. The dirigible of large cruising -radius, which can seek its landing field at leisure, is probably -indicated for this kind of work. It may indeed, as already hinted, prove -to be the chief photographic aircraft of the future. - -Archæological surveys offer a fascinating opportunity for airplane or -dirigible balloon photography to render scientific service. Buried in -desert sands or overgrown with tropical vegetation the ancient cities of -Asia Minor, of Burma, and of Yucatan evade discovery, and even when -found remain unmapped for decades. Discovery and mapping can now go -hand-in-hand. The topography of barbaric or colonial towns and villages, -whose importance could not warrant elaborate surveys, but which should -nevertheless be a matter of record, will be quickly and easily plotted -by photography (Fig. 207). To this day who knows how the streets run in -Timbuctu, and how, save from the air, can we ever map the teeming cities -of China? He who would follow in the footsteps of Haroun-al-Raschid can -even now explore the by-ways of Bagdad by the aid of the Royal Air Force -photographic map! - - - - - INDEX - - - Aberrations, lens, 47, 54 - - Aberration, spherical, 47, 54 - chromatic, 48, 54 - - Acetylene light, 284 - - Advertising, use of aerial photography in, 392 - - Aero 1 and Aero 2 filters, 241 - - Airplane, as camera platform, =20= - types of, 24 - - Air speed, 27, 308 - indicator, 33, 34 - - Alcohol, use of in plate drying, 275 - use of in print drying, 286 - - Altimeter, 33, 174 - reading recorded on film, 135, 171 - - Altitudes of flight for photography, 61, 224 - - Anastigmatic type of lens, 47 - - Aperture, lens, 39, 44, 56, 57 - - Archæological surveying, 413 - - Astigmatism, 49, 50, 54 - - Auxiliaries, camera, =163= - installation of, 214 - - - Bagley camera, 286, 314 - - Balance, of camera, 96 - of plane, 157, 208, 217 - - Balloons, 15, 16, 18 - - Banking, 27, 324 - - Batteries, storage, 150, 156 - - Bay, camera, 120, 210 - - Bellows, camera, 65, 95 - - Biplane, 24 - - Blast marks in front of guns, 363 - - Bleaching out print to leave interpretation marks, 367 - - Boat, flying, 25, 374 - - Bowden wire, 103, 106, 108, 111, 114, 116, 118, 126, 129, 136, =163= - - Brightness, range of, 221, 225 - - Burchell photographic slide rule, 303, 339 - - - Camera, airplane, =39=, 384 - automatic, 18, 43, 90, 116, =124=, 125, 311 - B. M., =120=, 203 - Bagley, 286, 314 - Brock automatic plate, =126= - C type, 43, 87, 103, =109= - classification of, 43 - deMaria, 86, 89, 103 - deRam, 82, 93, =121=, 129, 157, 205, 214, 326 - E type, 43, 87, 103, =109= - elements of, =42= - film, _see_ Film cameras - Folmer automatic plate, =126= - hand held, =95=, 321, 369 - English, =99= - German, =99= - U. S. Air Service, =100= - Ica, =103= - Lamperti (Italian), =112=, =211= - L. B., =120= - long focus, 103, 301, 324 - L type, 43, 82, 94, 102, =117=, 162, 210 - M., =106=, 203 - non-automatic, 43, =102= - Piserini and Mondini, =111= - semi-automatic, 43, =116=, 149 - stereoscopic, 341 - - Camouflage, filters for the detection of, 225, =243= - stereoscopic views and, 329, 358 - - Ceiling of plane, 27, 130 - - Channels, detection of by photography, 408 - - Chemicals, photographic, =257= - - Chlorhydrochinon developer, 261 - - Chromatism, lateral, 49, 54 - - City planning, use of aerial photography in, 396 - - Clock-work for driving cameras, 149, 155 - - Clouds, 224, 242 - - Collimator, 66 - - Color, coefficient or index of negative, 259 - filters, _see_ filters - photography, =385= - sensitive plates, 15, 174, 233, 237 - sensitiveness of film, 131 - sensitizing, methods of, 235 - - Coma, =47=, 48 - - Communication, means of on plane, 296 - - Compass, 33, 173 - reading recorded on film, 135, 171 - - Cone, camera lens, 42, 114 - interchangeable, 108, 120 - - Construction operations, aerial photographic records of, 396 - - Contact, imperfect in printing, 279, 284 prints, 45, =279= - - Contacts, electric, on plane, 163 - - Contours by stereo aerial photography, 404 - - Control, distance, of camera, 110, 163 - speed, of camera, 136, 144, 157 - - Controls, duplicate, 19, 25, 195, 209, 214 - of plane, 21, 26 - - Convergence point, 337 - - Cord for adjusting shutter aperture, 82 - - Core rack development, 271 - - Contrast, in brightness on earth's surface, 221 - in photographic emulsions, 15, 230, 236, 258 - - Counter, exposure, on magazine, 88 - on release, 164 - - Covering power of lens, 44, =49=, 50, 58 - - Crabbing, 27, 308 - - Cradles, camera, 195 - - Cross wires, 23 - insertion of camera through, 42, 210 - - Curtain, auxiliary shutter, 80, 84, 106 - speed of travel of shutter, 74 - uniformity of, 76, 82, 84, 86, 315 - - Cylinders, relation between vibration and number of, 185 - - - Dark slides, double, 87, 99 - - Daylight, intensity of, 222 - - Definition, lens, 44 - - Density, of air, effect of on propeller, 154 - of photographic image, 228 - - Developers, for plates and films, =257=, 260 - for papers, =262= - - Developing machines, film, 133, =273= - Ansco, 273 - Brock, 274 - Eastman, 274 - G.E.M., 273 - - Development, core rack method of, 271 - factor, 228 - film, =272= - methods of, =267=, 269 - of prints, =286= - speed of, 236, 267 - tank, 270 - time, 269 - - DH-4 plane, 210, 217, 296 - photographic, 213 - - DH-9 plane, 296 - - Diaframs, to equalize illumination of plate, 78 - lens, 48, 58 - - Dilution coefficient of developer, 258 - - Dirigibles, 15, 16, 413 - - Distortion, absent with between-the-lens shutter, 74 - barrel, 51 - due to camera tilting, 206, 286, 305, 315 - in aerial maps, 317 - lens, 39, 44, 51, 54, 56, 62 - pin-cushion, 51 - produced by film shrinkage, 237 - produced by focal plane shutter, =74= - produced by glass plate in front of film, 131 - with wide angle lenses, 63 - “Dodging” in printing, 279, 315 - Doors in plane for camera to work through, 214 - - Drying of films, =267=, 276 - of plates, 267, 275 - of prints, =286= - - - Earth, appearance of from plane, 30 - - Eastman apron film developing machine, 274 - twin reel film developing machine, 274 - - Efficiency, propeller, 155 - shutter, 70, =72=, 76 - - English aerial photographic practice, 45, 46, 283, 291, 340 - - EK filters, 241 - - Electric, drive for cameras, 116, 119, 123, 145, 149 - generator operated by motor, 146 - motor, characteristics of, 151, 156 - motor, service, 163 - - Elevation possible to detect in stereoscopic views, =340= - - Emulsions, photographic, characteristics of, =227= - - Enlarging, 45, =279=, 283 - camera, 283 - - Enlarging versus contact printing, 59 - - Exhaustion of developer, 259 - - Exploration, use of aerial photography in, 401 - - Exposure, data charts, 250 - distance between for mosaic maps, 307 - distance between for stereos, 336 - estimation of, 248 - limitations to, 247 - meters, 251 - meter, Wynne, 251 - of aerial negatives, =247= - relation between motion of plane and, 68, 185 - under, period, 230 - - - Field, angular, of lens, 57, 302 - flatness of lens, 56 - - Field laboratory, mobile, 268 - - Film cameras, 43, =130=, 134 - Brock, 138 - Duchatellier, 131, 136 - F type, 134, 171 - G.E.M., 138 - German, 64, 76, 139, 317 - K type, 142, 203, 214 - - Film, celluloid, 130 - backed, 133 - changing in the air, 137, 144 - color sensitiveness of, 237 - cut, 275, 278 - development of, 130, 272 - drying, 131, 276 - means for holding flat, 130, 131 - relative performance of compared to plates, 237 - satisfactory kinds for aerial work, 238 - shrinkage, 237 - - Filters, 15, 58, 106, 174, 224, 233, =239=, 241, 243 - effects secured by use of, 241 - - Filters, gelatin, 244 - glass, 245 - holders for, 67, 106, 224 - ratio, 240 - - Fixing bath, 262 - - Flaps, auxiliary to shutter, 80, 84, 85, 98, 99, 101, 119 - to protect lens, 214 - - Flash lights, 18, 386 - - Flying boat, 374 - - Focal length, relationship of lens characteristics to, 55, 57 - requirements for, 39, 42, 58, 61, 103 - - Focal plane, 50 - shutter, 70, 71 - distortion by, 74 - performance of, 86 - types and representative, 80 - - Focus, depth of, 44 - effect of temperature on, 41, 65 - fixed, in aerial cameras, 40 - ground and air, 65 - - Focussing, automatic, 284 - by parallax, 65, 66 - - Fog, atmospheric, 224 - photographic, 237, 258 - - French aerial photographic practice, 45, 46, 283 - - Friction disc speed control, 136, 159, 160 - - Fuselage, 21, 87 - shape and size depending on type of engine, 23 - - Future of aerial photography, =383= - - - German aerial photographic practice, 63, 103, 209 - - Glass, optical, used in lenses, 44 - - Gloves, handling apparatus through, 41, 89 - - Governor for camera speed, 159 - - Graphite, use of to prevent static discharge, 134 - - Gravity, action of in aerial cameras, 41, 112, 115, 119 - center of, should not change in cameras, 125, 207 - change of center of, in magazines, 92, 112 - depended on in magazines, 88 - handles best at center of, 96 - pseudo, in moving vehicle, 188 - support at center of, 182, 203 - - Ground speed, 27, 307 - indicator, 311 - - Guide-books, illustration of, 15, 392 - - Guides for inserting magazines, 41 - - Gyroscope, 189 - - Gyroscopically controlled instruments, 29, 174, 192, 312 - - Gyroscopic erector, 188 - mounting of camera, 187 - stabilizer, Gray, 190 - - - Hardener, acid, 262 - - Hand operation of deRam camera, 121, 125, 129 - - Haze, 15, 30, 223, 233, 239 - - Head resistance and weight, equivalent, 156 - - Heat, effect of on plate sensitiveness, 175, 232 - - Heater, electric, in camera, 142, 174, 232 - - Horizon, photography of to indicate - inclination, 174 - position of, 30 - - Hurter and Driffield sensitometric curve, 227 - - - Illumination of field by lens, 50, 54, 56, 57 - - Image of point source, size of, 49, 54, 56 - size of in relation to focal length, 59 - - Incidence, angle of assumed by plane at high altitudes, 206 - - Inclinometer, 33, 35, 171, 173 - - Indicators, distance, 163, 295 - - Inertia, 229, 257 - - Installation, camera, 24, 208 - - Instructions, operating, to be placed on apparatus, 42 - - Instruments, airplane, 30 - - Instrument board, 30 - photographing, 170 - - Intensification of aerial negatives, 262 - - Intensity of daylight, 222 - - Interpretation of aerial photographs, 17, 40, 351 - - Interval between exposures, 40, 158, 305, 307 - for stereoscopic pictures, 334, 338 - methods of regulating, 124 - - Isochromatic plates, 233 - - Italian photographic practice, 122, 377 - - - Jamming of cameras in operation, 119, 120 - - - K_{1} and K_{2} filters, 239 - - Keeping power of developer, 259 - - Kites, 15, 16, 18 - - - Laboratory, mobile photographic, 269 - - Landscape gardening, use of aerial photography in, 395 - - Latitude of plates, 229 - - Lens, 42, 39, =44=, 383 - aperture, 39 - characteristics, 46 - mounts, 65 - suitable for aerial photography, 62 - symmetrical, 52 - telephoto, 61 - testing and tolerances, 52 - unsymmetrical, 52 - wide angle, 62 - - Levels, spirit, on camera, 95 - - Light, distribution of in aerial view, 221 - trail method of testing camera mountings, 183 - - Loop, centrifugal force in, 29 - - - Machine gun ring as camera mount, 321 - - Magazines, =87= - bag, 88, 101 - Bellieni, 92 - Chassel, 92 - deMaria, 89, 98 - Ernemann, 89 - Folmer, 90 - Fournieux, 92 - Jacquelin, 92 - Piserini and Mondini, 90 - Ruttan, 92 - - Magazine racks, 94 - installation of, 217 - - Mapping, 64, 135, 185, 186, =304=, 401 - precision, 317, 404 - - Maps, mosaic, 17, 39, 64, 314 - sketch, 314 - - Marking of negatives, 278 - - Metol-hydrochinon developer, 261 - - Mirrors for oblique photography, 324, 326 - - Monoplane, 24 - - Motions of camera, 179 - - Motive power for aerial cameras, =145= - - Mounting, camera, 102, =103=, 179, 183 384 - bell-crank, 120, 198, 203 - Brock, 139, 207 - center of gravity, 205 - floor, 195 - G. E. M., 138, 207 - Italian, 207 - outboard, 194 - parallel motion, 198 - pendular, 185 - tennis ball, 196 - - Mounting, camera, turret, 312 - of prints, 316 - of stereograms, 346 - - Movement, of film during exposure, 75, 142 - of image, permissible, 68 - - Moving pictures from plane, 350 - - Mud splashing on camera, 214 - - - Naval aerial photography, =368= - - Negative lens sight, =168=, 299, 305 - - Night photography, =386= - - Numbering devices in cameras, 169 - - - Oblique views, 39, =320= - angles at which taken, 40, 321 - exposures for, 69 - filters for, 242 - use of hand cameras for, 95 - - Observer, function of in aerial photography, 291, 295, 304 - - Oil spray from motor, 214 - - Opacity, 228 - - Opening for camera, 211 - - Opposite directions, shutter to move alternately in, 76, 139 - - Orthochromatic plates, 233 - - Overlaps, for mapping, 307 - for stereoscopic views, 40 - on a turn, 64, 139 - - - Panchromatic emulsions, 233 - plates, 49, 238 - - Panoramic views, 321 - - Parallax method of focussing, 65, 66 - - Parallel, flying in, 308 - - Photographic planes, special, 213 - - Pilot, function of, in aerial photography, 291, 295 - - Pinpoints, 39, 291 - - Pistol grip for hand cameras, 95, 96 - - Plate holders, 87 - - Plates, bathed, 235 - behavior of compared to film, 237 - color sensitive, 233 - iso- and ortho-chromatic, 233 - panchromatic, 233 - satisfactory kinds for aerial work, 238 - self screening, 245 - shape of, 63 - size of, 43, 62 - - Plumb line, behavior in banking plane, 28 - - Ply-wood veneer construction, 23, 211 - - Positype paper, 238 - - Potassium carbonate for drying plates, 276 - - Power required to drive cameras, 125 - - Pressure, of shutter curtain, 131 - plate, for holding film flat, 131, 141 - - Printing, =279= - contact, 279 - machines, 279 - media, 252 - - Prints, paper, 253 - development of, =286= - - Prisms for oblique photography, 324, 326 - - Propeller, characteristics, 152 - constant speed, 129, 159 - drive for cameras, 102, 116, 119, 135, 136, 144, 157, 158 - position of, 120 - variable speed, 159 - - Pump, for producing suction on film, 132 - - Punch marks on film, 144, 272 - - Pyro developer, 261 - - - Racks, negative, 271 - - Real estate, aerial mapping of, 393 - - Rectifying, 286, 305 - camera, 314 - - Reduction of aerial negatives, 262 - - Release, shutter, 96, 99 - duplicate, for pilot, 164 - - Release, shutter, time controlled, 124 - - Relief, criterion for correct, 335, 344 - exaggerated, 337 - impression of, produced by motion, 349 - - Resolving power of plates, 59, =235=, 260 - - Richard stereo printing frame, 283, 348 - - Rinsing of plates, 259 - - Rubber, sponge, use of in camera mountings, 195, 203 - - - Safe lights, photographic, 269 - - Safety catch on camera mounting, 203 - device on camera driving mechanism, 162 - - Salvaging of ships, aerial photography and, 399 - - Seaplane, 25, 374 - - Self screening plates, 245 - - Semaphore signalling in plane, 297 - - Semperfocal enlarging camera, 284 - - Sensitized materials, requirements for, 225 - - Sensitometry, =227= - of papers, 253 - - Shadows, compass directions from, 356 - proper direction for, in examining prints, 352 - - Shaft, flexible, 119, 123, 142, 161 - - Sheaths, plate, 87, 88, 93, 126, 170 - - Shrinkage, film, 237, 317 - paper, 285, 315, 317 - - Shutter, 42, 68 - between-the-lens, 58, 70, 112, 115, 316, 387 - efficiency, 70, =72=, 76 - focal plane, 70, 71, 73 - focal plane, double for stereo work, 341, 345 - focal plane, moving alternately in - opposite directions, 76 - focal plane, types of, 80 - - Shutter, Folmer, 80 - Ica, 81 - Klopcic, 78, 84, 98 - release, 96, 99, 124 - speed, 39, 40, 58, 70, 249 - testing, 76 - testing apparatus, 77 - - Sights, =164=, 166, 296, 301, 327 - adjustable for angle of incidence of plane, 169 - attached to plane, 167 - negative lens, 168, 299, 305 - rectangular, 98, 167, 373 - stereoscopic, 338 - to indicate size of field, 166, 373 - tube, 101, 166 - - Single-seaters, carrying cameras in, 114, 211 - - Slide rule, photographic, 303, 339 - - Solenoid, 151, 163 - - Sound, not to be used for indication in plane, 164 - - Spacing of pictures in film camera, 144 - - Speaking tubes on plane, 296 - - Speed, of development, 236, 267 - of plates, 228, 236 - criteria of, 230 - effect of temperature on, 175, 232 - variable, control of camera, 144, 151 - - Spotting, 64, 125, =291= - - Spring motors, 149, 155 - - Springs, use of in aerial cameras, 41 - in magazines, 88 - - Stabilized camera, 95, 187 - - Static electric charges on film, 131, 133 - - Stereo-comparator, 404 - - Stereo-oblique views, 115, 321, 343 - - Stereoscopes, 331 - - Stereo printing, 283 - - Stereoscopic cameras, 341, 344 - effect, absence of at flying heights, 30, 334 - - Stereoscopic photography, 329 - mounting, 346 - pictures, fusion of without instruments, 330, 343 - sights, 338 - views, 39, 40, 64 - uses for, 348 - vision, principles of, 329 - - Stop-watch attached to shutter release, 310 - - Strap, on hand camera, to go around observer's neck, 99 - on plate magazine, 87 - to go over hand, on hand camera, 100 - - Stream lines, 26 - lined hood, 214 - - Suction for holding film flat, 131, 132, 142 - advantages of continuous and intermittent, 132 - - Surveying by aerial photography, 401 - - - Tank development, 270 - - Tearing edges of prints, 317 - - Telephones on planes, 296 - - Telephoto lens, 61 - - Temperature, coefficient of development, 258 - effect of on focus, 41 - effect of on mechanical functioning, 41, 102, 125 - effect of on plate speed, 232 - limits of development, 258 - - Test chart, lens, 52 - - Threshold value, 230 - - Thrust, propeller, 152 - - Timber, location of by aerial photography, 413 - - Tone rendering, correct, 226, 230, 239 - - Touch, sense of, not dependable in plane, 41, 125, 164 - - Trailer, photographic truck and, 268 - - Transmission of power to camera, 161 - - Transparencies, 252, 330 - - Transparency, 227 - - Trays, camera, 195 - - Tri-color ratio, 235 - - Triplane, 24 - - Tuning fork, used in shutter tester, 79 - - Turbine, wind, for driving camera, 116, 127, 144, 147, 158 - - - Uniformity of curtain speed in focal plane shutter, 76, 82, 84, 86, 315 - - Unit system of camera construction, 42 - - Uprights, camera, in plane, 209 - - Uses for aerial photography, =388= - - - Velocity constant, 258 - - Veneer construction, 23, 209, 211 - - Venturi tube, 132, 142, 144 - - Vibration, 16, 18, 26, 40, 41, 58, 102, 179 - - Volcanoes, photography of, 15, 399 - - - Water for mixing chemicals, 262 - - Watkins factor, 258 - - Weight and head resistance, equivalence, 156 - of film compared to plates, 101, 130 - of deRam camera, 123 - of hand cameras, 96 - of K type camera, 144 - of K film roll, 144 - of L type camera, 117 - of M type camera, 109 - of storage batteries, 157 - - Wind, flying against, 68, 308 - motor, 146 - - Windows in side of plane, 214, 328 - - Wire, barbed, appearance of in aerial photograph, 360 - - - - - * * * * * * - - - - -Transcriber's note: - - 1. 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