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diff --git a/old/53494-0.txt b/old/53494-0.txt deleted file mode 100644 index 4691cb2..0000000 --- a/old/53494-0.txt +++ /dev/null @@ -1,4333 +0,0 @@ -The Project Gutenberg EBook of New Edition of Hints on Silver-Glass -Reflecting Telescopes Manufactured b, by George Calver - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. 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: New Edition of Hints on Silver-Glass Reflecting Telescopes Manufactured by Mr. G. Calver, F.R.A.S. - with Directions for Silvering, Adjusting, &c. - -Author: George Calver - -Release Date: November 10, 2016 [EBook #53494] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK SILVER-GLASS REFLECTING TELESCOPES *** - - - - -Produced by Eric Hutton and the Online Distributed -Proofreading Team at http://www.pgdp.net - - - - - - - - - - NEW EDITION OF HINTS ON Silvered-Glass Reflecting TELESCOPES - - MANUFACTURED BY - - MR. G. CALVER, F.R.A.S. - - WITH DIRECTIONS FOR SILVERING, ADJUSTING, &c. - - 1877. - - - GEORGE CALVER, - - HILL HOUSE, WIDFORD, - - CHELMSFORD, ESSEX. - - -[ Illustration: decorative ] - - - - - HINTS ON Silvered-Glass Reflecting Telescopes. - - -Of the various forms of Telescopes now in use, each has its own -peculiar advantages; but the Silvered-Glass Reflector is undoubtedly -gaining favour among our practical observers. A well-figured -speculum, being perfectly free from chromatic aberration, gives, in -a proper condition of the atmosphere, the finest possible definition -of the Moon and planets, the markings and colours of these objects -being excellently seen; while coloured stars, such as Albireo (β -Cygni), or Almaach (γ Andromedæ), are exceedingly well shown, the -beautiful contrast of the stars in the former being especially -noticeable in a reflector. The advice of “F.R.A.S.” (in the “English -Mechanic,” March 21st, 1873) as to the choice of a Telescope, may -here appropriately be quoted. After expressing a preference for -refractors when measuring close double-stars, he says, “But should -the object of your correspondent be merely to regard the wonders and -beauties of the Heavens, or notably, should he purpose to devote -himself to the study of the physical structure of the Moon and -planets, then by all means let him obtain the largest reflector he -can afford; its absolute achromatism tells most astonishingly on -these last-named objects.” This is the opinion of one who has great -practical knowledge of the different forms of Telescopes. - -If Achromatic Telescopes of large aperture could be made as cheaply -as reflectors, and in as convenient a form, they would doubtless be -preferred for general star-work, although the aberrations, -especially that of colour, cannot be so perfectly corrected. A -silvered-glass reflector is, however, much cheaper than a refractor, -and, when the aperture exceeds five or six inches, is much handier -to work, and occupies less space, being only about half the length -of an achromatic of the same aperture. It is true that a reflector -will give less light than an achromatic of equal aperture—but this -is, in certain conditions of the atmosphere, a distinct advantage, -the extra aperture to give the same light adding to the definition -and penetrating power. An example of this is seen in the beautiful -definition given by an unsilvered mirror on brilliant objects, as -the Sun, Moon, and Venus. In large achromatics, the distressing -excess of light has often to be reduced by diminishing the aperture -or using a higher power than is convenient; and in such cases a -lower and more suitable power can be employed with a reflector. - -When the air is unsteady, the definition of Reflectors, owing to -their tubes being open, is more liable to fluctuate than that of -refractors, although when a reflector does not give good definition -on account of the atmosphere, a refractor of the same aperture will -certainly not perform satisfactorily. Sir John Herschel has shown -that when the air in the tube of even an achromatic is disturbed in -turning from one object to another, good definition does not return -until it is brought to rest. In order to reduce the vibration of the -air in the open tube, and also that of the stand, to a minimum, -reflectors require to be very firmly and steadily mounted, and to -have iron tubes. - -Many of the specula made by me are now in observatories, where they -have been compared with achromatics of first-rate quality with the -most satisfactory results. For instance, several 6-1/2 inch specula -were tried with two excellent achromatics of 4-1/2 and 6 inches -aperture, when the planetary definition was considered to be -superior with the reflectors; and the appearance of the star-discs, -with equal apertures, differed little from the beauty and hardness -of the images given by the achromatics. In viewing stars of great -altitude, the use of the refractor is extremely inconvenient to the -observer, whose position is necessarily very uncomfortable, while -with the Newtonian reflector any part of the heavens may be observed -with the same ease and comfort. In short, a good speculum, well -mounted and situated, is sure to be both pleasing and satisfactory. - -It is perhaps unnecessary to remind the reader that, when the -defining powers of a telescope are put to the test, as much depends -on the acuteness of the observer’s eye and the practice he has had, -as upon the perfection of the instrument and the fineness of air. It -is a mistake to suppose that when the stars appear to be the -brightest to the unaided eye, definition will be at its best, though -this may happen; when it does so, the astronomer should make the -most possible use of the opportunity, as such nights are very -scarce. As a rule, the best nights are those when there is the -slightest possible mistiness of the atmosphere, and for the faintest -stars absence of bright moonlight. The 5 inch mirror is guaranteed -to divide stars one second apart with ease, and closer ones in very -fine air. The 8-1/2 will split such difficult pairs as γ^2 -Andromedæ, and μ^2 Boötis. An acute eye will master these stars with -even a less aperture on very favourable occasions, γ^2 Andromedæ -having been seen with a 7-inch stop of a 10-inch mirror, and Mr. -Sadler, of Honiton Rectory, has split this star with his 6-1/2-inch -telescope. It sometimes happens on favourable nights that the most -difficult objects will be seen with the same telescope, which on -other occasions had failed to show them as well as a much smaller -aperture had done in very fine air. These remarks equally apply to -the observation of minute stars and planetary detail. The amateur -must follow the advice of the Rev. T. W. Webb, given in “Celestial -Objects” (pages 15-17), and must cultivate that virtue applicable to -all scientific investigation, namely, patience. The following -interesting and difficult objects may be looked for, the powers used -for their observation should vary from 150 to 300, and for the very -closest stars up to 500, or even still higher. - - - TESTS FOR SPECULA. - -LIGHT TESTS. - - 94 Ophiuchi. 5″ : 7, 13. - 58 Ceti. 3″·5 : 6·5, 14. - γ Crateris. 3″ : 4, 14. - 15 Monocerotis. 25″, 15″ : 6, 9·5, 15. - τ Orionis. 15″, 20″ : 4, 15, 12. - υ Ceti. 6″ : 4·5, 15. - ε Trianguli. 5″ : 5·5, 15. - 179 Piscium. 3″ : 8·5, 15. - 110 Herculis. 55″ : 5, 16. - μ Andromedæ. 45″ : 4, 16. - β Equulei. 35″, 3″, 50″ : 5, 13, 16, 14. - 85 Virginis. 30″ : 6, 16. - 55 Andromedæ. 25″ : 5·5, 16. - 178^a Delphini. 20″ : 7·5, 16. - 212 Libræ. 20″, 10″ : 6, 16, 8. - 14 Monocerotis. 10″ : 6, 16. - 94 Ceti. 5″ : 5·5, 16. - δ Aquilæ. 1″·5 : 3·5, 16. - - -SEPARATING TESTS. - - 33 Orionis. 2″ : 6, 8. - 52 Orionis. 1″·8 : 6, 8. - δ Cygni. 1″·8 : 3·5, 9. - 2 Camelopardi. 1″·7 : 5·5, 7·5. - π Aquilæ. 1″·7 : 6, 7. - σ^2 Cancri. 1″·4 : 5·5, 7. - 9 Leonis. 1″·2 : 7·5, 7·5. - η Orionis. 1″ : 4, 5. - ε Arietis. 1″ : 5, 6·5. - 4 Lyncis. 1″ : 6, 7·5. - 37 Pegasi. 0″·8 : 6, 7·5. - 749ξ Tauri. 0″·8 : 7·1, 7·2. - 46 Arietis. 0″·8 : 8, 9. - λ^a Cygni. 0″·7: 5, 6. - β Delphini. 0″·7 : 5, 5·5. - 20 Draconis. 0″·7 : 7, 7·5. - 287 Draconis. 0″·7 : 7, 8. - 178^b Delphini. 0″·6 : 4·5, 6. - φ Draconis. 0″·6 : 4·5, 6·5. - γ^2 Andromedæ. 0″·6 : 8·5, 9. - μ^2 Boötis. 0″·5 : 8, 8·5. - ι^a Boötis. 0″·5 : 4·5, 4·5. - 7 Tauri. 0″·5 : 6, 6·5. - 108 Draconis. 0″·5 : 9, 9. - η Herculis. 0″·3 : 3, 8. - 42 Comæ Berenices. 0″·3 : 4·5, 5. - ω Leonis. 0″·3 : 6·5, 7·5. - - -Since writing the first edition of my catalogue, the writer has -received many gratifying accounts of the success of the -“Silvered-Glass Reflector.” Many private letters have been received -by him from observers, expressing their delight and satisfaction. -This has been so encouraging that no pains have been spared, nor any -opportunities neglected, in turning to the best purpose every -valuable lesson that continued practice and experience may have -suggested in the manipulation of specula from time to time, in order -to secure the best means for obtaining the highest excellence of -defining power. - -Many modifications and improvements have been made in working and -testing mirrors, and special machinery and appliances constructed -for large sizes. But to complete my conditions suitable for truly -figuring and testing specula of considerable size, I found it -necessary to remove from the traffic and tremor of a town to a still -and tranquil situation in the country. - -The truth of the curve is so sensibly and seriously affected by -vibration constantly going on in and near a town, that it is liable -to a variety of defects, and the surface becomes wavy and “plucked.” - -As an instance, I may mention that my first and most convincing -proof of the advantage of the stiller situation was tested by an -18-inch speculum (on which much labour had been bestowed); it was -laid aside, but successfully finished after removal, and without -undulations or any perceptible defect, and the Observer wrote me, -that in good air, he “saw Sirius as a brilliant white dot, without a -ray or appendage of any kind, and celestial photographs obtained -with it are very fine.” Such results were exceedingly gratifying, as -they were obtained with much less labour and uncertainly, and the -tedious process of the final touches had not to be repeated so many -times. - -It is said that the celebrated Alvan Clarke, from the same effects -of tremor, never finished an object-glass to his satisfaction -above-ground; and Dr. Draper, testing his mirrors at the centre of -curvature, to avoid draughts, &c. in an ordinary apartment, resorts -to an underground one. - -In this little book of “Hints” it may be useful to remind those who -possess a speculum of fine quality that they are not produced by a -“rule of thumb”—so to speak—and that the difference between _a_ -speculum and a really _fine_ one, giving a maximum of defining and -illuminating power, is the result of considerable labour and -thought, and deserves careful usage. - -The Rev. Cooper Key, an amateur of much experience, writes in the -“English Mechanic,” that he was eight months (working sometimes -eight hours a day), giving his 18 inch speculum its final touches -and corrections. - -A well known correspondent of the “English Mechanic,” “Hyperion,” -tells us he found it impossible to test his 8-1/2 inch mirror in an -ordinary room, and had to resort to a tunnel under the clay of his -garden. Those who have the means and perseverance to make their own -mirror, should be careful not to proceed with the finishing touches -until an hour at night when their workshop, if in a town, is free -from tremor. - -First secure a well-ground and carefully-centred disc, let the -polish be as perfect as possible before any attempt is made to -figure. Care must be taken that every square or portion of the -polisher is of the same consistency and temperature, that the disc -may not be acted on irregularly. - -To give the pitch this quality it must be well boiled and “pulled,” -so that no air bubbles are in the squares, as these cause expansion -or contraction as the temperature of the apartment varies, or that -of the pitch and glass from friction. - -It is much the best plan to keep the workshop to the same uniform -temperature as the polisher was made for, allowing no draughts to -pass over the mirror while working, nor the gas or lamp to be near. -When the polisher is warmed—which it should be after laying aside -for any time—it should be warmed _equally_. Neglect of the above in -making the polisher, or any incautious handling of either the disc -or polisher, will be sure to cause defects, which cannot be cured -but by retracing the early steps in the fine grinding with an -accurately centered tool. - -The polishing successfully accomplished, the process of correcting -for parallel rays is next proceeded with. At every step all possible -care must be taken to prevent the mirror from running into an -irregular curve. The importance of this cannot be too strongly -urged, if a speculum capable of doing the _best_ work is desired, as -the curve _must_ be true, regular, or uniform, to give the _highest_ -defining and illuminating power. An under-corrected mirror, if of a -regular curve, will perform much better than a compound correction, -exhibiting at the focal point much less lateral aberration. - -A brief explanation of this may be of use as far as the limits of -these pages will admit. It would be impossible to teach, by a mere -description of methods, how to commence and finish off a speculum of -good quality, even if every working secret and minute detail were -unreservedly explained. The only way is to master sufficient theory, -and the rest will come by prolonged care and perseverance. I state -this because amateurs who have been desirous of enjoying the -gratification of observing with a mirror of their own making have -written for information which they have been quite willing even to -pay for liberally, but have felt disappointed, and perhaps thought -it somewhat discourteous, on being told that what they wanted was -impracticable and could not be satisfactorily attempted by the -Optician in writing. In some cases I have finished amateurs’ work on -agreeable terms, and which has led to a pleasant correspondence or -acquaintance. These remarks may prevent some future disappointment. - -Every one is familiar with the fact that the parabola is the only -concave surface that can reflect rays of light falling on its -surface from an object at an infinite distance—such as the stars and -planets practically are—to one common focus, or without aberration. -This series or column of rays (which is equal in diameter to the -opening of the speculum), reach the mirror without making an angle -to it or to each other, they travel side by side, they are all of -one length, and are reflected to a point, and are therefore all of -the same length at the proper focal distance, viz., half the radius -of the curvature of the concave. The properties of the parobola make -the nearest possible approach to it of the utmost importance. - -It has been said that a considerably under-corrected surface, if of -a true and uniform curve, is better than one with less aberration, -with zones or sections of various curves. To explain this, let us -suppose an artificial star at some short distance, say 50 yards, the -parabola would not form an image of this at the eye-piece without -aberration; it has for this distance too short a focus for the -central rays, and the best disc is inclined to the inner focus, -because the rays from the object _diverge_ towards it, instead of -travelling parallel; and they reach the surface (the central rays -compared with the marginal), at an angle equal to the semi-diameter -of the mirror. - -But, if instead of a parabolic mirror an elliptical one be used, -which has one of its foci at 50 yards’ distance, the image will be -perfect. Now place the artificial star at 500 yards, the image will -now be attended with perceptible aberration. The longer focus of the -ellipse must be worked further and further from the mirror by -shortening the focus of the central rays. Correct it for this -distance, and again remove the artificial star to a still greater -distance, repeating the corrections as before and carrying the outer -focus towards the object, and the inner towards the mirror, until -the rays from the object become more and more parallel, and the -curve is nearer and nearer to the parabola, or that eccentricity of -ellipse which acts better and better for distant objects or parallel -rays. - -It is evident from this that if the ellipse is corrected very -considerably towards the parabola, without irregularities, and -_every part_ of the surface corrected regularly from the centre to -the edge (no part hastening more than another), that such a surface, -though under-corrected, is much better than if one portion is fully -and another under-corrected, especially if the more imperfect -portion is towards the edge. Such a compound correction may show -little or no _outstanding_ aberration at the focal point, but the -rays do not find a focus at the same regular pace as they would from -a regular curve—one edge of a zone will be coming into focus when -another would be going out. With the focussing screw they are -focussed to the place where they appear to collapse, and are most -satisfactory, though, _in reality_, the rays at the best place bend -over each other, and the definition is imperfect. From a star, which -is only a point, this may be more tolerated, but on the planets, -where the image has a sensible diameter, and is perhaps magnified -many times, this imperfect curve is very inferior to the regular -one, whose error is all of _one kind_. The light is all there -_somewhere_, but not with any good effect. There is no _proper_ -illumination or definition, as rays are employed which are crossing -the optical axis at _varying_ angles, and the result is confusion. - -So the amateur who sets himself the pleasant task of making his own -speculum (for there are many who can better afford the labour than -the capital to purchase, and whose capabilities are thus superior to -their means), need not be discouraged and give up the pursuit -because he cannot obtain the best results by getting a perfectly -parabolic glass. But if he has been successful in removing a -considerable amount of the spherical error, and advanced to the -elongated ellipse by maintaining a truthful curve, “let well alone” -with this disc, and proceed no further, but commence _another_, -taking care not to alter the first until the second is _better_, and -_then_ an improvement of the first may be attempted. - -In the second attempt, should the amateur lose control over the -_regularity_ of the surface, let him try it as an experiment against -the first on the planets, and he will not fail to appreciate the -difference, and will be stimulated by fresh courage to get as near -to the parabola as possible with the same accurate curve. - -To produce a true and uniform curve is, however, the acme of -troubles, whether it is desired to obtain the spherical, or -parabola, or any other curve. It is generally supposed that the -spherical curve is a very easy matter, so easy indeed that it is -difficult to avoid. This is a very great mistake—a spherical curve -of undeviating truth is as difficult a problem as a true parabola. -The spherical curve is the _only uniform_ curve, it has but _one_ -focus, and the polisher must coincide with, and be of the same -radius as the glass, at every instant. This is why the optician -strives to obtain a semi-polish with the grinder to lessen the risk -of losing his curve on the polisher, for the curves of an object -glass are spherical. The curve most liable to be obtained by the -amateur is the spheroidal, a curve with its marginal rays _shorter_ -than the central, or half the radius of curvature. - -There are no means with the _telescope_ of telling the spheroidal, -approximating to the sphere, _from_ the sphere. There are _no_ means -of analyzing the exact character of a curve _equal_ to certain -methods at the centre of the curvature, but to accomplish this -requires much practice and observation, with “surroundings” -perfectly free from vibration. If the amateur can overcome this, and -lives in or very near a town, he should only work at the polishing -and figuring during the late hours of the night, when traffic, &c. -have subsided. Then, by carefully preventing any draughts in the -apartment, and with the mirror of the same temperature as the air in -the room, he will then be able to see how varied and numerous are -the chances of error in working a mirror, and the great care -necessary to avoid or cure them. He will find the surface -exceedingly prone to receive zones and irregularities during work, -and much more so than to “work true.” - -The necessity for avoiding incautious handling or heating may be -realized by the following little experiment when one can manage and -understand it:—Place the tip of one finger on the surface, as it -hangs in the dark room ready for testing, and with very gentle -pressure let it remain long enough to spell one’s name; it will then -be seen that the feeble heat of the finger has, by expansion, raised -a mound on the surface of the glass, and though this amount of -swelling must be _very_ small, yet it is enough to cast a shadow -across the surface, as if something were laid on it, and quite ten -minutes will elapse before the heat will leave this spot and the -surface again become level. Now if the polisher were placed on the -glass while this hillock was there, a permanent hollow would be the -result. For a full account of these methods (of which Foucault was -the discoverer) the reader is referred to Sir John Herschel’s and -Dr. Draper’s works on the telescope. - -Care must be taken not to leave too much aberration, as then the -central disc is formed too positively outside the focus, and the -rest of the light from the object appears as obtrusive rings and -false light. The over-correction is bad, and acting as a negative -lens the disc is formed too near the mirror. Such a correction, -besides being objectionable on almost all classes of objects, -prevents the use of the “Barlow” lens, and acts badly with all kinds -of positive or Ramsden eye-pieces, which improve _under_-corrected, -but “make bad worse” with over-corrected surfaces. - -If these few and brief hints should stimulate the industrious and -persevering student to make his own telescope, and thus enjoy the -fruits of his own labours (which may add a relish and a pleasure to -his astronomical work), they will have served the purpose for which -they are written. - -After a “Hint” on the choice of focus for the mirror, it only -remains to say a few words about the plane, as this, with the large -speculum, are the only parts that need be home-made as far as the -optical work is concerned. Never adopt a “dumpy” for general use -where high powers are sometimes wanted, for small and moderate -sizes, say 8 to 12 diameters, and for large sizes not less than -about 6 diameters of the mirror, as the larger ones practically -admit of a shorter focus. The short ones can be mounted somewhat -cheaper, but never choose them on _this_ account, they will not make -so satisfactory an instrument, and no adaptation of “Barlow’s” will -make it so. - -To make the plane, provide three well-turned metal discs of 7 inches -or 9 inches diameter. Iron is the best, as the work will go down -closer, and the plates or discs may be very thick—say an inch—and -not so liable to “spring.” These turned discs must be scraped and -ground perfectly flat on each other, until they are in good contact -all over, so that there is nothing between the faces when testing -them. In the earlier stage of “truing,” oil and colour can be used. -When these discs are proved true, a disc of plate glass, same size, -is cemented with pitch on one of them, or on a thick disc of glass, -and care must be taken that it is not strained while on the block -during working. Truly grind this plate on one of the tools to a fine -semi-polish. Polish on the same tool with a piece of thick silk or -alpaca, laid over and cemented down by a solution of resin dissolved -in turpentine (as much resin as the turpentine will dissolve), then, -with the tip of the finger, thinly smear over the tool and bind -round the edge with cord, and the silk will keep in place. Fill up -the texture of the silk or alpaca evenly with damp rouge, keep it -uniformly damp, and never let the rouge and water work about. It may -be polished on very hard strained pitch, but pitch for the amateur -is not so safe, as it is liable in his hands to alter its form and -destroy the truth of the plane, but it is the quickest and handiest -if it can be managed. - -After the polish is _perfect_ remove the plate from the support and -cut into squares a little larger than the size of the intended flat. -Test these in the telescope on a _star_. If one plane turns out bad, -the whole will most likely be so, and another plate must be worked -with renewed energy and care, for a bad plane will spoil the action -of the speculum however good, for there is no way of counteracting -the curved surface of the flat or second reflector. The edge of the -plate for about an inch should not be used. - -When a good plane is found it can be edged by turning a piece of -wood a little less in diameter than the minor axis of the plane -required, turn the end square, and mark a line around it distant -from the end equal to the diameter, and cut through to the opposite -edge, and it will give the oval and will appear round at 45°, this -will mount nicely in brass tubes of the proper size, and a cover -should be made to fit easy. - - - “THE EQUATORIAL.” - -Fig. 1 is a modification of the German principle, and it should be a -sufficient recommendation to remark that it is the principle used by -a maker of such experience as Mr. John Browning, and is, without -doubt, the very best style of mounting for a fixed equatorial, -especially when clockwork is employed, because clock power is -applied to the polar axis itself direct from the driving worm. - -The driving part, viz., the worm and wheel, which is out of the -observer’s way (being between the standards), is not liable to meet -with accident, and the driving wheel being near the lower end of the -axis, is at the most rigid part, viz., the foundation of the -instrument. As it has a long and very firm polar axis, and is -connected with the foundation plate, it secures the utmost -steadiness and freedom from strain. - -An equatorial mounting, with two long and stiff shafts for the axes, -has always the advantage of firmness; the _holding_ portions of the -instrument being in masses, are not liable to receive injury from -blows, and thus be put radically out of order. _No means should be -resorted to to make an equatorial of light weight_; an instrument -cannot be light and slim and at the same time _firm_ and _steady_—no -amount of steadying rods, splines or strings, will make a slender -tripod for a refractor so steady as a firmly made one, with proper -size and weight in the parts. - -Another important advantage of this kind of mount for a large size -is, that although it has considerable weight, it is very convenient -to move and set up, being built of convenient parts, which are easy -of separation and removal. The uprights or standards are separate, -and are bolted to the bed-plate, the upper and lower discs are -readily detached from the stand. The cradle and tube are in this -construction separable. The tube of the telescope being suspended -over the _side_ of the stand, is in the most convenient position for -observing objects at any altitude, as the stand is out of the way -and clear of the telescope. The various disadvantages and objections -to the driving clock being _carried_ by the telescope are here -avoided. The clock, which is large and very powerful, is bolted down -to the iron bed-plate, and the telescope, not having to carry the -clock or weight, the balance is never altered nor the rate of the -clock disturbed, and thus a strong clock, keeping regular time, and -working much longer can be used. They will drive 2-1/2 hours. It -will be readily understood that a driving clock to enable the -instrument to do the most exact and best work must be powerful and a -good time-keeper. Lord Lindsay has said that “the clock should have -twice as much power as is _used_,” otherwise the spindles, &c. are -pent up, and it is moreover sensible to any extra weight or work -being put upon the instrument. - -To give an idea of the efficiency and regularity of this clock, I -may mention that a Newtonian telescope of 18 inches aperture, -intended to carry a photographic camera of 112 lbs. weight, needed -no additional provisions to do the extra work, the rate and power -not being affected. There is a mechanism in the clock “for making up -of time” and in setting the hour circle can be used with a joint -handle or hooks as a fine screw-motion, and can be applied whether -the clock is going or not. There is also a provision at the -foundation for throwing the instrument out of the meridian, to -follow the motion of the moon or planets with the clock and “maker -up.” The above mounting is equally suitable for “Cassegrains” and -“Newtonians,” to both of which forms it has been successfully -applied up to 18 inches of aperture. It is well suited for large -telescopes. - -Fig. 2 is a mounting on the same principle, but with a shorter polar -axis, and the column is in one casting. It is well suited for -moderate sizes, and the circles, &c. are applied exactly as in Fig. -1, but when a clock is wanted there is no mounting equal to the -former. - -The “Educational” is a plain 6-1/2 inch, of 4 to 6 feet focus, and -is made to this pattern, with revolving body and screw-motion for -following. It is made as portable as it can be, and is a steady, -good-working instrument, and much approved of. - - - THE UNIVERSAL ALT-AZIMUTH. - - (FIG. 3.) - -In this mounting I have seen little or no alteration needful, except -that the tube is now made to balance so that the eye-tube can be -reached for objects in the zenith without the observer having to -elevate himself, and the elevating rod can be clamped without the -lever, the legs of the stand are more curved and have more spread in -them. - -This “Alt-Azimuth” stand has met with much approval; and where -portability is of consideration and the observer has to set up and -remove the entire instrument after every night’s work, this, or the -Angle-Block stand, will be found the most convenient, more so than -an equatorial in any form. There is much less weight to remove, and -being in three convenient parts is more readily put together and -separated in the dark. If the stand and trunnions can be left in the -open air a very small covering will serve to protect them, and then -the tube alone has to be removed. The equatorial cannot be too -highly recommended where it can be a fixture and undisturbed, as -when once got into proper adjustments its advantages can then be -realised, but not unless it is a permanent fixture. The circles of a -portable equatorial can only be used for very rough reading, and -consequently the adjustments are never in order, and the readings -are of very limited use indeed. - - - THE IMPROVED “ANGLE-BLOCK” STAND. - - (FIG. 4.) - -With respect to the tube, it is mounted like the Alt-Azimuth, but -with the plane of the horizontal movement corresponding to the -latitude of the place of observation, and therefore following -objects with one screw movement is in reality a telescope with -equatorial motions. It need not be of heavier construction than the -Alt-Azimuth, and there is not the double weight of counterpoising, -&c. as in the equatorial, and when circles are not desired this will -be the most economical, handiest, and easily managed instrument, as -it partakes of the equatorial form or motions. The telescope tube is -well balanced, and the declination movement is easy and free, and -fitted with a clamp screw, so that when the instrument is turned on -an object that object can then be followed by an endless screw. - -It can be made to suit any latitude, and by the addition of foot -screws on a level floor, can be got into suitable adjustments, and -if it has to be removed (for the stand is very little heavier than -an ordinary Alt-Azimuth), “guides” can be provided, so that it can -go very approximately into the exact place again when brought out -for use. They are made, when desired, with a revolving body and fine -screw-motion in declination. - - - THE “POPULAR REFLECTOR.” - - WITH ANGLE-BLOCK STAND.—(FIG. 4.) - -The speculum is 5-1/4 inch diameter, and carefully figured, and is -recommended as a very useful instrument. The size and power of this -telescope has been adopted as that most likely to meet the means and -requirements of a large number of amateurs. Many prefer to commence -astronomical observations with “_something inexpensive_,” and are -led to begin with the popular and well advertised 3 inch refractor -at £5. These, except in a few chance cases, are sure to prove -disappointing, and are perhaps the cause of their giving up any -further attempts to follow up the subject of astronomy, which may -otherwise be so pleasantly and profitably pursued with a reliable -instrument. - -It is well known that a less aperture than 4-1/2 inches is -insufficient to give the observer a satisfactory idea of the varied -and most interesting details of the planets, and the ever-changing -outline and tone of the belts and markings of Jupiter, Saturn, and -Mars. Large apertures bring out _details_ when the smaller ones can -only show a _general outline_. The former also delineates more with -a lower power, providing the focus is of proper length. - -The planets and nebulæ cannot be seen to advantage without aperture -and focal length. The field of view is then flat, the object is -properly illuminated, with sharp and crisp definition, and is also -_much_ less subject to annoyance from tremour, through the necessity -for constant adjustments in the fields, &c., as is the case with -small apertures, for the object is magnified more in proportion. A -certain magnifying power is necessary in order that the object may -be sufficiently large to scrutinize; and this, whether the aperture -is large or small, must be from about 150 to about 300 times for the -planets. But 150 on a 3 inch is a high power for the quantity of -light obtained, to say nothing of the separate consideration and -advantage of long focus. Aperture is a quality or function of the -telescope considered separately from light or focus. For instance, -suppose a 5 inch speculum is so thinly silvered that it gives the -exact degree of light as a 2-1/2 inch refractor, the defining and -separating power of the 5 inch aperture would be very superior to -the 2-1/2 inch. - -The “Popular Reflector,” with 5-1/4 inch speculum of 5, 5-1/2, to 6 -feet focus, will be found very suitable, and if its illuminating -power is not greater than a 4-1/2 inch refractor, its defining and -dividing powers are superior. By choosing the above focal lengths, -according to circumstances, the observer can reach the eye-tube, -while standing erect for objects in the zenith, and it is a great -mistake to suppose that the shorter the tube the more handy it -becomes. These foci will not require a “Barlow” lens to flatten the -field. The “Barlow” is very useful for short foci when the aperture -is considerable, as it improves the imperfect correction for -spherical error, but this is much better corrected in the mirror -itself than by a “Barlow” lens, which cannot be used without more -than one disadvantage. There must, by its insertion, be some loss of -light, which can ill be spared with small instruments, and when used -to obtain magnifying power there is some disturbance of colour, and -this subtracts from the beauty and purity of the definition of a -reflector. There is _nothing_ equal to a good eye-piece to obtain -_power_, and _flatness_ of field by the _curve_ of the speculum. - -With the “Popular Reflector” and an outlay of a few shillings on -some popular books, such as the excellent work “Celestial Objects,” -by the Rev. T. W. Webb, and some first-class publications by Mr. R. -A. Proctor, especially his smaller star atlas, &c., the amateur can -compare the work he is then capable of doing with a large and -expensive refractor which might be beyond his reach. - - - THE ADJUSTMENTS OF THE EQUATORIAL. - -When the inexperienced amateur purchases an equatorial with circles, -he should not be without the third edition of “Chamber’s Handbook of -Descriptive Astronomy,” which, besides being an excellent book in -other respects, is a really practical guide to the use and -application of the Equatorial, and is indispensable to the beginner. -He will there find the fullest details of the adjustments to any -degree of exactness. Besides many other matters he will be -instructed in the use and application of apparatus to the perfect -Equatorial, including all kinds of eye-pieces, micrometers, &c., -&c., as well as other optical instruments and accessories. - -The Equatorials described in this catalogue are provided with every -means of adjustment. The cradle contains powerful screws to set the -line of sight at right angles to the declination axis, and shifting -screws to place the polar axis in the meridian and to the correct -elevation, and with care and a few experiments with the adjustments, -and by observations of some catalogue stars, the various adjustments -will soon be correctly made, and the verniers set accordingly. - - - THE CASSEGRAIN. - -This is a form of reflecting telescope but little known. This is -rather strange, since it is a very much better principle than the -“Gregorian,” so well known to the old observers. Herschel says it -admits of a theoretically perfect telescope. Compared with the -“Newtonian” it has its advantages and disadvantages. Its principal -advantages are, first, the shortening of the tube, which in large -telescopes is sometimes very important. Secondly, the observer has -not to _ascend_ to the eye-tube, the observations being made at the -lower end, as with a refractor. The “Cassegrain” has a flatter field -of view, owing to the action of the curve of the second reflector -causing the rays to travel twice the distance, and, adding the -element of magnifying power, the eye-pieces need not be composed of -small lenses. The adjustments are perhaps a little more trouble, as -the line of collimation must be carefully attended to, this -requiring only a little more care can soon be accomplished, and then -the definition of a good “Cassegrain” is very pleasing. - -Amongst its disadvantages is the necessity for the observer to gaze -upwards as with refractors, which, when the object is at a -considerable altitude, is distressing, this is one of the reasons -why the “Newtonian” is so pleasant to use, on account of the natural -and easy position of the observer. The eye-piece being a fixture, it -is not quite so convenient to use some of the accessories of the -telescope. But there are means to overcome these drawbacks, and so -make the “Cassegrain” even more handy than the “Newtonian.” - -I have mounted an 18 inch speculum of 12 feet focus in the -“Cassegrain” form, so that objects at any altitude could be observed -with the greatest possible ease. A plane was fixed near the large -mirror to receive the rays from the convex reflector and to throw -them out to the side, illuminating apparatus were fixed here, and -micrometers, &c. used, as if it were a “Newtonian”; the tube was -thus made shorter, and the flat field of a long focus realised, but -there would be a little loss of light in consequence of an extra -reflection, but this, with a large aperture (and the fact that the -“Cassegrain” gives a little more light than the “Newtonian”) can -better be spared, considering the convenience gained. The observer -is not elevated for any altitudes, and a large telescope is actually -occupying less room than a small one. It can be used with or without -the diagonal. - -I have, by request, fitted the “Cassegrain” with means for two -observers to view the same object at the same time. A perforated -plane was arranged to receive a portion of the converging rays and -throw them to the side of the tube into an eye-tube, and the -remainder passing on to the eye-tube at the proper place, two images -are thus formed, and can be magnified at will and viewed -simultaneously. The perforated plate was so arranged that it could -be removed at pleasure. - - -[ Illustration: FIG. 1. ] - - -[ Illustration: FIG. 2. ] - - -[ Illustration: FIG. 3. ] - - -[ Illustration: FIG. 4. ] - - - THE ALT-AZIMUTH STAND. - - Fig. 3. - -The Telescope, being balanced on trunnions, can be moved from an -elevation approaching the zenith to an almost horizontal position. -In order that it may be secured anywhere between these extremes, -attached to the upper part of the telescope is an iron rod, which, -sliding through the end of the arm of the stand, can be there -clamped. The telescope will now be clamped in _Altitude_. As the -progression of celestial objects will apparently be very slow, -resource must be had to the smoothness of motion obtained by a -screw. The upper end of the Altitude rod is therefore tapped to -receive a long screw with a large milled head, jointed to the -telescope body; by revolving this head the telescope is raised or -depressed accordingly as the screw is unscrewed, or the reverse. It -is necessary that the screw should be withdrawn some way from the -rod before clamping it, preparatory to following an object which has -passed the meridian, or is setting; as perhaps, just when the -clearest vision is obtained, the observer may be annoyed by the -screw action being suddenly stopped by the milled head coming in -contact with the top of the rod. - -When viewing objects near the zenith, and the focus long, the handle -attached to the clamp will be found useful, as it can thereby be -reached without leaving the finder. The handle may be so placed that -a downward push should clamp, and an upward pull release. - -The second motion in the Alt-azimuth Stand, namely, _Azimuth_, is -obtained as follows:—The strong iron disc which forms the upper -fitting of the legs has its surface accurately turned. On this -revolves an iron disc, rather less in diameter, to which the -trunnions which support the telescope are attached. The main axis of -this disc passes through the centre of the lower disc, and then -through a hollow bearing tube, a continuation of it. All these -fittings having been most carefully turned and ground together, -great steadiness, combined with facility of horizontal movement, is -ensured. In order that this motion can be communicated as evenly as -possible, resource must be again had to a screw which is thus -applied. Just within the circumference of the lower disc is a narrow -groove, turned to such a depth that the ring which is thus separated -from the main disc is still firmly held to it by the uncut portion. -An iron clamp grooved to this ring holds the nut of a long screw, -the plain end of which is jointed to the upper disc. When this clamp -is fixed to the ring, any motion given to the screw will act on the -upper disc, and cause it to revolve, and thus the whole telescope -will be slowly moved in _Azimuth_. The advantages of this plan are -many, the most important being the rapidity and ease with which the -telescope can be shifted from one object to another, even to those -in contrary directions; all that is necessary being to release the -clamp and turn the telescope to the object required. The clamp being -carried round with the upper disc, can be fixed directly the desired -position is obtained, when the screw is at once in action. Should it -happen, whilst following an object, that the screw becomes exhausted -from the joint and clamp coming together, the clamp should be -released, and the screw turned sufficiently in the reverse way to -bring it into action, the weight on the upper disc keeping the -telescope meanwhile in position. If this operation be rapidly -performed, the whole length of screw can be brought in play before -the object has left the field of view of the finder, and can thus be -easily refound with the higher power of the telescope. Motion is -applied to the screw by means of a Hook’s joint, named thus from its -inventor. This joint being furnished with a long handle, enables the -observer, by means of it, to move the Telescope in _Azimuth_ at any -rate, and without removing his eye from the eye-piece. - -It will be seen from the preceding remarks that by means of the -vertical and horizontal screw motions, the telescope, when clamped, -can be moved in any direction with the greatest facility, permitting -a celestial object to be observed with high powers for a -considerable time, and with the greatest pleasure and comfort to the -observer. - - -[ Illustration: decorative ] - - - - -[ Illustration: decorative ] - - ADJUSTMENTS. - - -These instruments are always sent out in correct adjustment, and -with moderate care during transit, and afterwards, will remain so, -but as the performance of the instrument greatly depends on the -accuracy of the adjustments the following instructions will enable -the observer not only to ascertain whether they are perfect, but -also to render them so if found defective. These adjustments are by -no means difficult, and will be easily understood by attention to -the following remarks:— - -Into the draw tube screw the “adjusting piece,” which is a small -brass circle with a hole in its centre about 1/20 of an inch in -diameter. (The draw tube should be in about the same position as -when at focus with an eyepiece.) Place the large mirror in its cell -in the tube or body of the telescope, taking care that the three -bayonet-joint pins are correctly placed, that is, with grooves -pointing downward. They will be found to drop easily into their -corresponding holes; care must however be taken that the grooves -have gone well home. Both the speculum and small mirror, or “flat,” -must be uncovered. On looking through the aperture of the adjusting -piece, if the mirrors are in correct adjustment their reflections -will be seen as follows:—the small oval mirror being placed at an -angle of 45° will appear circular, and reflected exactly in the -centre of this circle will be seen the bright image of the large -mirror with a dark round spot in its centre, as shown by Fig. 1. -This dark spot is the double reflection of the “flat,” and should be -concentric with both the bright reflection of the large mirror and -the circular outline of the “flat.” All these should also be -perfectly concentric with the circle given by the stop in the draw -tube. Should these circles not be all central the adjustments are -not perfect and must be rectified as follows. - - - To adjust the “Flat” or small diagonal Mirror. - -If the bright reflection of the large mirror is seen as a perfect -circle, but not exactly in the centre of the “flat,” the latter -requires adjustment; for this purpose loosen the milled head screw -at the middle of the back of the “flat” which in large instruments -is made sufficiently heavy to act as a counterpoise to prevent -vibration. This will allow the “flat” to be rotated by the hand -vertically with respect to the tube of the telescope. Bring the -bright circular reflection of the mirror exactly central in this -direction, and fasten in position by screwing up the milled head -screw or counterpoise. If the circular reflection is quite central -no further adjustment is required, but if not, then, after -completing the vertical adjustment, as described, proceed to make -the horizontal adjustment by turning in one or other direction the -milled head of the horizontal adjustment screw, situated in front of -the vertical adjustment screw; this will bring the circle of light -exactly into its proper horizontal and central position, and the -adjustments are then completed. - -If the bright reflection of the large mirror is _not_ seen as a -perfect circle, and the small dark spot not in the centre, the -speculum is out of adjustment, and consequently the adjustment of -the “flat” is best performed by removing the large mirror or -speculum with its cell and so arranging the body of the telescope -that on looking through the “adjusting piece” a large sheet of white -paper spread on the ground a short distance from the open end -appears as a white circle of light reflected in the “flat.” Now -bring this white circle exactly into the centre of the flat -precisely in the way described above, and on this being accomplished -replace the speculum uncovered with its cell, and proceed. - - - To adjust the large Mirror or Speculum. - -[ Illustration: Fig. 1. Fig. 2. Fig. 3. Fig. 4.] - - -When in perfect adjustment the large mirror viewed through the -“adjusting piece” should appear as before stated, as a complete -bright circle, with the image of the “flat” as a smaller dark -circular spot exactly in the centre (Fig. 1). Should this not be the -case it must be rectified by means of the three adjusting screws at -the back of the cell. Proceed as follows:—First unloose (by a few -turns) the small clamping screws which pass through the larger -hollow adjusting ones; then, on looking through the adjusting piece -the relative position of the image of the “flat” should be carefully -noted. If the dark spot is nearer the bottom, and consequently more -of the top part of the bright circle is seen (Fig. 2), the mirror -reflects too much of the upper part of the tube, and therefore the -top of the mirror leans too far back and must be pushed more forward -by screwing in the top adjusting screw (_a_ Fig. 1) a little at a -time till the dark spot is central. Should the spot be towards the -top (Fig. 2 _inverted_), the reverse holds good and the top -adjusting screw (_a_) must be _un_screwed or the two other adjusting -screws turned in. Should the spot be towards the left (Fig. 3), -screw up the right adjusting screw (_c_ Fig. 1). If towards the -right (Fig. 4), screw up the left adjusting screw (_b_ Fig. 1). The -dark spot is always furthest away from that part of the tube which -is too much reflected, and from the adjusting screw that must be -turned in to correct it. - -When all the adjustments are considered perfect, as in Fig. 1, the -small screws are to be clamped up to keep the adjusting screws in -position. If, after the greatest care has been taken in adjustment, -a flare should appear on looking at a star (say of the second -magnitude) with an eye-piece of a high power, and the diffraction -rings are not quite concentric, it can generally be rectified by -turning the large screws of the mirror round in cell a little at a -time. If this does not remove the flare the adjustment of the -“flat”. is not sufficiently correct and must be altered by means of -the screws at its back. If the flare is at the top or bottom of the -star the “flat” must be very slightly revolved by the hand, after -unloosing the middle screw, and when correct reclamping it. If at -either side, namely, in the direction of the major axis of the -“flat” and in a line with the tube, the “flat” must be altered by -the long screw. It is always advisable to leave the telescope for a -short time undisturbed, especially if, on first looking at a star, a -flare should appear, as these appendages often vanish when the -instrument has been for a short time in the air. These adjustments -may at first appear somewhat difficult, but are rendered remarkably -easy by observing how the different screw movements alter the -positions of the reflections. A useful test as to the correctness of -the adjustments may be obtained by viewing a star with a high power -eye-piece out of focus. When in the centre of the field it should -appear as a bright luminous circle with a circular dark spot in the -centre, the size of the bright circle diminishing as the focus is -approached and the dark spot remaining central. - -The cell mount of the large mirror can be removed from the tube and -replaced without disturbing its adjustments, but it is very -advantageous if the entire instrument can always be left undisturbed -when not in use in an observatory of light construction, having a -skeleton revolving dome, covered with well oiled canvas or calico, -and made with a wide opening and large shutters. The Rev. E. L. -Berthon has described in the “English Mechanic,” October 13th, 1871, -an observatory of this kind most admirably suited to shelter a -reflector, as the temperature inside would be as nearly as possible -that of the external air, and no annoyance from damp would be -experienced. Where an observatory is not practicable the telescope -might be protected by a close fitting covering of like make. Both -the large and small mirrors should be protected by their covers -(with which they are provided), when not in use, especially if left -in the open air. The larger sized tubes have a door large enough to -admit the cover and so allow of its being put on the large mirror -without the necessity of the speculum being removed from the body of -the telescope. - - - To Adjust the “Finder.” - -Direct the telescope to any bright star (the Pole-star being by far -the best, as it has very little apparent motion), and bring this -star into the centre of the field of a low-power eye-piece. Now -adjust the “finder” by means of the three screws bearing on it, till -the star is bisected by the cross wires seen in the focus of the -eye-lens of the “finder.” Change the low eye-piece to a high one, -and perfect the adjustment as before described. Any well-defined -terrestrial object at a distance can be employed in the day-time to -roughly adjust the “finder,” leaving the final adjustments to be -made by a star. - - - The use of Stops. - -A stop 1/4 inch less in diameter than the speculum, is often useful, -in order to cut off the internal reflections of the tube. - -With mirrors 6-1/2 inches in diameter and under stops are seldom -required; with the 6-1/2 inch a 5 inch stop may however sometimes be -used with advantage on very bright objects. With an 8-1/2 inch -mirror a 7 inch stop often improves planetary definition. - -With larger sizes than 8-1/2 inches several stops of different -diameter may be used, when experiment will determine what size is -best suited to the condition of the atmosphere, and the character -and brilliancy of the object to be observed. These stops can be -easily cut out of thin cardboard and afterwards blacked with -Indian-ink or lampblack. - - - On Observing the Sun. - -No larger aperture than 4-1/2 inches should be employed for solar -observation, except with a specially constructed “Solar Eye-piece,” -as even a 4-1/2 inch speculum often concentrates sufficient heat to -crack the coloured glass of an ordinary sun-cap, if exposed for any -length of time. - -If a “solar eye-piece” is employed, the whole aperture may be used. -In viewing the moon a slightly tinted glass is often most useful, -especially to persons with weak sight, as it takes off a great deal -of the glare. - - - TO SILVER AND POLISH THE SPECULA. - -The cost of silvering is trifling, and with cleanliness and ordinary -care very little difficulty will be experienced. The apparatus and -chemicals required consist of the following articles:— - -APPARATUS.—A Silvering Vessel:—This should be a flat-bottomed -circular glass or glazed dish, 1 inch or more larger in diameter than -the speculum to be silvered, and sufficiently deep to allow of a stratum -of fluid of an inch or rather more between the face of the mirror and -the bottom of the dish, the top of the mirror being nearly level with -the edge of the dish. - -A Mixing Vessel:—A 40 oz. glass measure will answer well, but care -should be taken not to scratch the sides whilst stirring, or the -glass is liable to fly. Should a measure not be procurable, any -receptacle of sufficient size may be used, but a glass one is best, -as it will allow of the action of ammonia being better observed. - - A Box of Scales and Weights. - A Glass funnel and filtering paper. - Two Glass Rods for Stirring purposes. - A Five-ounce Glass Measure. - A Test tube 3/4 or 1 inch in diameter. - Some Clean Cotton Wool. - Some very fine Wash-leather. - A Support of Wood, on which to cement the speculum, described - further on. - -All these articles should be perfectly clean and free from dust, and -the Glass ones well rinsed with distilled water just before using. - - CHEMICALS.—Nitrate Silver. - Potash pure, precipitated by Alcohol. - Sugar of milk powdered. - Nitric Acid, pure. - Liquor Ammoniæ. - Distilled water, pure. - Pitch. - Fine Rouge. - Turpentine. - -Procure a strip of wood an inch or so less in diameter than the -mirror, and of sufficient length to rest securely on the opposite -sides of the silvering vessel. Pour on this piece of wood some -melted pitch, and whilst it is still hot, place on it the back of -the speculum moistened with a little turpentine; when cold, reverse, -and lay the cemented speculum face downwards, suspended in the dish. -Should the distance between the face of the mirror and the bottom of -the dish be less than an inch, raise the mirror by means of thin -wedges, placed between the strip of wood and the edges of the dish. -But if, on the contrary, the mirror should be considerably more than -an inch away from the bottom, which will occur if the silvering -vessel is very deep, the speculum must be cemented to a block of -wood of sufficient height, screwed to the suspending strip, instead -of being cemented directly to the strip itself. - -When the speculum is properly placed in the dish, namely, with the -front surface about one inch from the bottom, pour in water till the -fluid reaches about 1/4 inch up the side of the mirror. Measure this -quantity, as it will indicate the total amount of the silvering -solution required to be prepared. - -To prepare the Silvering Solutions—make 3 standard solutions as -follows— - - No. 1. {Nitrate Silver in crystals 100 grains. - {Distilled water 4 oz. - - No. 2. {Potash pure by alcohol 1 oz. - {Distilled water 25 oz. - - No. 3. {Powdered sugar of milk 1/2 oz. - {Distilled water 4 oz. - - -Solution No. 3 must be made just before using. The others will keep -if the distilled water employed in their preparation is pure, and -the solutions, when made, are kept in glass stoppered bottles. - -Suppose it is desired to silver an 8-1/2 inch mirror, proceed as -follows:—the total amount of solution required having been -ascertained, as before described, pour 2 ozs. of solution No. 1 into -the mixing vessel previously well washed and rinsed with distilled -water, and cautiously add Liquor Ammoniæ. A grey precipitate will be -formed; continue to add the ammonia, drop by drop, till the -precipitate is just dissolved and the solution becomes clear. The -solution should be well stirred with a glass rod whilst adding the -ammonia. Now add 4 oz. of solution No. 2, and re-dissolve the brown -precipitate which is produced with Liquor Ammoniæ as before -described. There will now be about 6-1/2 oz. of solution. Subtract -the 6-1/2 oz. from the total amount previously ascertained to be -required, and the remainder will be the amount of distilled water to -be added. Add half this quantity of distilled water to the 6-1/2 oz. -of solution, and add a drop at a time of solution No. 1, till there -is a slight precipitate, which cannot be re-dissolved by a -considerable amount of stirring (say for 2 or 3 minutes); then add -the remaining half of distilled water and cover up from dust the -vessel containing the solution, so as to allow the slight -precipitate to settle. There is a point of importance to be attended -to, namely, that no more ammonia is employed than is absolutely -necessary. The total amount of ammonia required in the 6-1/2 oz. of -solution is about 2 drs. - - - To Clean the Mirror. - -Fill the end of a test tube with cotton wool, leaving plenty outside -the tube. Having poured a small quantity of strong nitric acid on -the front of the mirror, rub the acid well all over the front and -sides with the cotton wool brush. Place the speculum under a water -tap for a few minutes till the acid is washed away, and finally well -rinse with distilled water; then place it in the silvering vessel -(previously thoroughly cleansed) and pour in distilled water till it -reaches 1/8 inch up the side of the speculum. - - - To Mix the Solutions. - -The precipitate having settled, pour into a clean vessel all that is -clear of the solution, leaving about 2 oz. behind, which will be -turbid with the precipitate, and therefore useless. The total amount -will be afterwards made correct by the addition of the same quantity -of solution No. 3. Having filtered solution No. 3, warm it to about -100° F. by allowing the bottle to stand in warm water, or by heating -in a small flask. When everything is quite ready, add 2 oz. of the -filtered solution No. 3, whilst warm, to the clear solution -described above, and thoroughly mix. - - - To immerse the Mirror. - -Remove the mirror from the distilled water, taking care not to touch -the surface of the mirror, and wipe the back and edge with some -clean cotton wool. If this precaution is not taken the water is -liable to drain down the sides of the mirror whilst silvering, and -cause streaks at the edge of the film. Having poured away the -distilled water in the silvering vessel, substitute the mixed -solutions, and directly the solution becomes slightly inky, gently -immerse the mirror, taking care that no air bubbles, or specks of -any kind, remain between the surface of the mirror and the solution. -The mirror should not be removed from the bath until all the silver -has been exhausted from the solution. This may be known by the -solution being clear below the silver film on the surface of the -liquid. The time required will vary from 45 minutes on a hot -summer’s day, to 90 minutes when the thermometer shows a low degree -of temperature. In the latter case it is better to have the -silvering bath in a warm room. - -Immediately the mirror is removed from the bath, the silvered -surface should be well washed by allowing ordinary water to flow on -it from a tap, for five minutes or more, then finally rinse with -distilled water and place the mirror to dry with the silvered -surface resting on some blotting paper. It is as well if the mirror -can be left undisturbed for a day or two, as the film will be -firmer, but it may be polished, if desired, after drying for a few -hours. - - - To Polish the Silvered Surface. - -Make a couple of polishing pads by filling two pieces of very soft -wash-leather about six inches square loosely with cotton wool, and -tie them into balls. Gently remove any dust that may have settled on -the film with some loose cotton wool, and then go over it with one -of the pads in small circular strokes for about 15 minutes. This -will consolidate the film and fit it for polishing. Spread a little -of the finest rouge on a sheet of writing paper, and impregnate the -other pad with it. Go over the film with the rouged rubber with the -same circular strokes till it is perfectly polished, which will take -another 15 minutes or so. Never commence with the rouged pad, as the -surface may be injured. When once the film has been consolidated it -will remain so, and can be repolished many times with the rouged pad -should it get tarnished. The pads should be kept from dust in -wide-mouthed bottles for future use. With care the film will last -for a long time, especially if it is not allowed to get damp, and -consequently the mirrors should never be brought uncovered from the -cold air to a warmer temperature. - -The “flat” may be silvered and polished in the same way as the -speculum, using a smaller appropriate vessel for the silvering -solution. - - - To Separate the mirror from the Wooden Support. - -Should the mirror be attached directly to its support, insert a -chisel between them, when one or two gentle blows will cause them to -separate, but, should the mirror be cemented to a block, stand the -mirror on edge, when a slight tap on the block will detach it. -Scrape off any pitch that remains on the back of the mirror, using -finally some turpentine to wipe it clean. Great care should be taken -not to finger the film. - - - Martin’s process for Silvering. - -Prepare four solutions of any quantity. Keep in stoppered bottles. - -_Solution 1._—Dissolve 175 grains of pure nitrate of silver in 10 -oz. of distilled water. - -_Solution 2._—Dissolve 262 grains of pure nitrate of ammonia in 10 -oz. of distilled water. - -_Solution 3._—Dissolve 1 oz. avoirdupois of pure caustic potash -(prepared by alcohol) in 10 oz. of pure distilled water. - -_Solution 4._—Dissolve 1/2 oz. avoirdupois of pure sugar candy in 5 -oz. of distilled water, then add 32 grains of tartaric acid and boil -in a flask or other clean glazed vessel for 10 minutes, when cool -add 1 oz. of alcohol and then dilute with distilled water, so as to -make up the volume to 10 oz. - -For silvering use equal parts of each, mix solutions 1 and 2 -together, and 3 and 4; when the mirror is ready mix the whole -together in the silvering vessel and quickly suspend the mirror. - -In the summer time, if the solution cannot be kept in a very cool -place, the mirror must be quite ready to be placed in the bath, as -the solutions turn instantly when mixed together. - - -[ Illustration: decorative ] - - - - - SELECTED TESTIMONIALS - - - 11, WELLINGTON PARK TERRACE, BELFAST, - _July 7th, 1876_. - -DEAR SIR,—The night before last was a clear night and I got the -6-1/2 inch out, and though the air was not good when using a 3-1/4 -inch refractor, I was much pleased with the performance of the -mirror. - -Mr. W. came up about 11 o’clock and stayed till one, we got it on -Saturn, and although rather low it was really a fine sight, Mr. W. -was much pleased. - -The moon being near to full we could not do much with faint points -of light. - - Yours sincerely, W. E. PARKINSON. - -Mr. G. CALVER. - - * * * * * - - 74, HAGLEY ROAD, EDGBASTON, - _November 27th, 1876_. - -DEAR SIR,—I have now got the 5-inch speculum fairly into adjustment, -and I am well pleased with it. Last night, the sky being clear for a -short time, I turned it on the moon, the definition of minute -craters was all that could be desired. I hope to have better -opportunities. - - I am, Yours truly, F. G. L. - -Mr. G. CALVER. - - * * * * * - - 74, HAGLEY ROAD, EDGBASTON, - _November 21st, 1876_. - -DEAR SIR,—I have mounted the 5-inch speculum on a simple equatorial -and it works well. I am sure it will turn out a fine glass and I -shall not regret the time and pains I have spent over it. It is not -quite in adjustment yet, when it is it will afford me a treat. I -have just tried it once on the Orion Nebula and Trapezium, details -of Nebula well seen, and the 5th star in Trapezium easy. - - I am, Yours truly, F. G. L. - - * * * * * - - DOWLAIS, - _December 27th, 1876_. - -DEAR SIR,—I have thoroughly tested the speculum, and am pleased to -tell you it stood its trials well. - - Yours truly, D. C. C. - -Mr. G. CALVER. - - * * * * * - - ST. DENIES, SOUTHAMPTON, - _August 14th, 1876_. - -DEAR SIR,—With reference to your enquiry as to the performance of -the 6-1/2 in. reflector. I have pleasure in stating that I am -perfectly satisfied. It readily divides the test objects and shows -delta Cygni with as low a power as 160. The comes to Sirius may be -considered an atmospheric test, perhaps, but I have repeatedly seen -it. Your stand I find very steady and convenient; altogether I can -fairly say that I consider that I have an instrument of considerable -power at a comparatively small cost. I now find the attempt to -observe with a refractor the reverse of pleasant. - -The owner of a 3-inch refractor, after using my reflector, writes -that he envies me its power, ease, definition, and comfort in -observing. - - I am, Dear Sir, Yours truly, A. H. S. - - - * * * * * - - HILGAY RECTORY, - _February 20th, 1877_. - -DEAR MR. CALVER,—The sky last night cleared up in places and enabled -me to try the 10-inch; there was a slight haze, but I never saw -discs so perfect and neat. I felt certain it would divide anything -divisible. Clouds came up and put an end to work. - - I am, Dear Sir, Yours sincerely, St. V. B. - - * * * * * - - 25, HAMILTON TERRACE, ST. JOHN’S WOOD, - _May 26th, 1877_. - -MY DEAR SIR,—I have much pleasure in informing you that the very -first time I saw Saturn through your 6-1/2-inch reflector, the -definition of the planet was far superior to anything I had observed -before, even with a good 4-1/4-inch refractor; and on another -occasion I was greatly pleased with the clear and easy view of the -“Comes” to ε Boötis. But even without including the stars, the views -of Saturn and Jupiter through my 6-1/2-inch mirror do _alone_ (in my -opinion) well repay the cost of the telescope. - - Yours truly, WM. L. LANCASTER. - -Mr. G. CALVER. - - * * * * * - - 37, EATON RISE, EALING, - _February 13th, 1877_. - -DEAR SIR,—I like the 18-inch speculum, and I think it as good as it -can be, and if the larger one is as good I shall be delighted with -it. It gives beautiful star images: I see Sirius as a brilliant dot, -a glorious object without ray or flares of any kind. I never saw it -so well before. - - Yours truly, A. A. C. - -Mr. G. CALVER. - - * * * * * - - 37, EATON RISE, EALING, - _February 3rd, 1877_. - -DEAR SIR,—I have tried the 18-inch speculum on some tests, and -especially the satellites of Uranus, and it appears fine, very fine. -What I have done in photography promises well. - - Yours truly, A. A. C. - -Mr. G. CALVER. - - * * * * * - - SOUTHEND ON SEA, - _March 17th, 1877_. - -MY DEAR SIR,—Circumstances have prevented me from making many -observations of late, but I am more confirmed in my opinion that my -telescope of your make is a very fine instrument. (A 6-1/2-inch.) I -heartily wish you success, which I feel sure you will obtain, as you -are so painstaking and turn out a thoroughly reliable article. I -shall be curious to compare my brother’s telescope with mine. - - Sincerely yours, J. L. L. - -Mr. G. CALVER. - - * * * * * - - GORLESTONE, - _March 30th, 1875_. - -DEAR SIR,—You have asked me for my opinion of the 10-inch speculum. -I have tested it, and can speak in the highest terms of its -performance. Definition in good air is as near perfection as it is -possible to imagine. Such tests as ζ Cancri, γ^2 Andromedæ are well -divided. The 6th star in the trapezium stands out well clear of its -brighter neighbour. On the moon any power may be applied, only -lessening the light, but retaining sharpness of outline. I have -tried a good many telescopes, but never yet saw these 10-inch -mirrors surpassed. The image of a star disc expanded on each side of -the focus is of as nearly the same appearance as possible. - - Yours truly, W. P. MATTHEWS. - - * * * * * - - 416, BRIXTON ROAD, - _April 20th, 1875_. - -DEAR SIR,—In compliance with your request, I send particulars of my -observation on φ Draconis. It was about 12.45 this morning when I -directed my 8-1/2-inch to this object, the air was very unsteady, in -fact not nearly so good as some nights lately, and I well divided it -with a power of 450. I then used a 6-1/2-inch stop, which I found -very much increased the steadiness of definition. - - Yours very respectfully, P. H. - - * * * * * - - _10th July, 1874_. - -DEAR SIR,—I tried the 6-1/4-inch mirror I had from you for the first -time last night. The night was not a good one for definition, but I -was very much pleased indeed with its performance. - - Yours truly, C. H. W. - - - * * * * * - - HONITON RECTORY, - _Dec. 23rd, 1874_. - -DEAR SIR,—I have been wishing to tell you how pleased I am at the -performance of your mirror. Many thanks for your kind offer to -change the mirror if needful, but I think it could hardly be better -than it is, its performance on different double stars is most -excellent. I have examined nearly 100 of these since September with -the 6-1/2-inch, most of them very difficult, but the mirror came out -well under all tests. I send you a list of some of the objects I -have examined. - - Yours truly, H. SADLER. - - -π _Aquilæ._—Divided, power 80. 14-^m. Webb might have been rated -15-^m. [6-^m., 7-^m. 1″·5.] - - A B C A-B A-C -δ _Aquilæ._—[3-1/2, 16, 14, 96″, 194″.] Comites easy, 3 other -excessively minute ones, not shown by the 11-in. refractor with -which P. Smyth measured B.C. Found 6-1/2-in. Sept. 12th, est. ± 17 -mag. 100″ to 110″. - -ζ _Persei._—My friend “Linea” sees two minute stars with 4·28-in. -Wray, not in Webb, and Wray himself sees an excessively difficult -companion. I see these 3 stars easily with 6-1/2-in. (Wray says his -companion is “very difficult with 7-in. refractor, really only a -glimpse star, even with the best atmosphere”), and have added two -more stars to the group. - -β _Delphini._—Close double discovered by Burnham, est. 0″·7. In -contact 6-1/2-in., power 430. - -η _Coronæ Borealis._—Very low, long past meridian, in contact 164 -power. - - _a_ _b_ _c_ B _a-b_ _a-c_ _b_-B -β _Equalei._—[5-1/2, 13, 14, 16: 35″, 50″, 3″.] 16-^m. not -seen, Webb 9-1/3-in. Just divided 164, easy 430 power. Two other -companions (not seen by Smyth or Webb?) - -μ _Andromedæ._—The 16-^m. comes, a very difficult test, easy; other -comites (not seen by Smyth or Webb?) seen. - -Companion to Vega pretty easy, Nov. 12th, 37m. after sunset. - - _a_ b _c_ B _a-b_ _b_-B _a-c_ -_P._ 178 xx. _Delphini._—[7-1/2, 8, 16, 9: 14″·3, 0″·7, 20″.] -Smyth 16 by evanescent glimpses; easy 164 power, 6-1/2-in. 8-^m. -well elongated in direction of 230°, power 430. - - - * * * * * - - REGENT ROAD, GREAT YARMOUTH, - _October 24th, 1874_. - -DEAR SIR,—Doubtless you have anticipated hearing from me, relative -to the 10-inch telescope, but the weather has been so indifferent -for delicate astronomical work that it has not yet had a fair trial. - -In middling good air, when I could use a reduced aperture with -satisfactory results, the star images reminded me of the 6-1/2-inch, -which I considered so superb a mirror that nothing could surpass its -fine performance either on stars or planets. - -With the recollections of the truly splendid views I had of Jupiter -last season, I count much to see him with the 10-in. - - Yours truly, H. BLYTH. - - * * * * * - - REGENT ROAD, GREAT YARMOUTH. - _February 2nd, 1875_. - -DEAR SIR,—In reply to yours, I have not had a really good night to -test severely the defining powers of the 10-inch mirror, but judging -from the few difficult objects I have seen, the mirror promises -well, and I have no doubt of its excellence. - -I had a fine view of the grand nebula in Orionis, and noticed that -the 5th and 6th stars were quite plainly seen, when the aperture was -reduced to 5-1/2-inches. I consider your stand a great improvement, -being very convenient and steady. - - Yours truly, T. AYERS. - - -[ Illustration: decorative ] - - - - - CATALOGUE - - OF - - Silvered Glass Reflecting Telescopes, &c., - - AND THEIR ACCESSORIES, - - JUNE, 1877. - - -G. Calver, while introducing the list of prices below, has the -greatest confidence in calling the attention of the practical -astronomer and the amateur to the very moderate prices charged, -considering the principle and style of the mounting, and the firm -and well-fitted arrangements in all the parts. - -The principle of the mounting is that which is the most convenient -to use, and with the greatest degree of steadiness with the easiest -movements. Fig. 1 is the _best mounting ever applied to the -Reflecting Telescope_. - -He is also able to state, that with his processes and special -facilities for working large specula, and substantially mounting -them, he is prepared to construct automatic equatorials of large -sizes, and with any special arrangements that may be required. - - -SILVERED GLASS EQUATORIAL TELESCOPES, very substantially and -well-fitted, as (Fig. 2). All these equatorials have revolving body. - - £ s. d. - 5-1/4 inch Speculum from 4 to 6 feet focus, with 8 inch - hour circles reading to 5 seconds of time, and - declination circle reading to 1 minute, two powers 30 0 0 - - 6-1/2 inch Speculum of from 5 to 6-1/2 feet focus, - 10 inch rotating hour circle, reading to 5 seconds of - time, and declination circle to 1 minute of arc, with - 3 powers--100 to 500 45 0 0 - - 8-1/2 inch Speculum as above with 12 inch circles, - 4 powers 70 0 0 - - 9-1/4 inch Speculum as above with 12 inch circles, - 5 powers 78 0 0 - - 10 inch Speculum as above with 12 inch circles, - 5 powers 90 0 0 - - 12-1/2 inch Speculum as above with 12 inch circles, - 5 powers 120 0 0 - - -Any of the above sizes mounted on Stand (Fig. 1), from 5 per cent. -extra. - -Fig. 1 is a very excellent stand, and admirably suited for large -instruments with clock power, and is convenient to remove and adapt -to different latitudes, &c., see p. 20. - - £ s. d. - 14, 15, to 16 inch Speculum mounted as (Fig. 1), with - rotating hour circle reading to 5 seconds and - declination to 1 minute (the diameter of the circles - never less than that of the Speculum and often larger) - - 8 powers from 50 to 700, with first-rate and very - powerful driving clock, first-class instrument, - and complete from £220 to 330 0 0 - - 17, 18, to 20 inch Speculum, with 10 powers, - from 50 to 800, including Kellners, Huyghenians, - and Achromatics, from £350 to 500 0 0 - - 22 to 24 inch Speculum from £550 to 750 0 0 - - 30 inch Speculum, with 12 powers and position - micrometer, and transit eye-pieces 1000 0 0 - -Prices will be forwarded for special arrangements in any of these -large sizes. - - -The CASSEGRAIN TELESCOPE is a very convenient form, and perfect in -performance, and in this form a very large instrument requires -comparatively a much smaller observatory, and is more conveniently -worked. They can be made so that the image is thrown out at the side -and viewed diagonally, as with the Newtonian, see p. 35. Cassegrains -from 5 per cent. extra. - - £ s. d. - Driving Clocks for 6 to 10 inch 35 0 0 - 〃 〃 12 〃 18 〃 40 0 0 - 〃 〃 20 〃 30 〃 50 0 0 - - -The EDUCATIONAL REFLECTOR is a plain and very steady and -satisfactory instrument, mounted on (Fig. 2) stand, without circles, -has revolving body, and made so as to be portable. - - £ s. d. - 6-1/2 inch Speculum (clear aperture), and of 4 to - 6-1/2 feet focus, has screw-motion in right ascension, - 2 powers 25 0 0 - - 8-1/2 inch speculum as above, with 3 powers 40 0 0 - -Any of these equatorials can be, according to wish, fitted without -the circles, &c., and dispensing with that which is not needed, so -as to reduce the price to suit circumstances and convenience. - - -The POPULAR REFLECTOR, with Angle-Block Stand, with endless -screw-motion to follow the stars with equatorial motion. - - £ s. d. - 5-1/4 inch Speculum, of from 4 to 6 feet focus, - with 2 powers 15 15 0 - - - (See p. 20, Fig. 4). - - -Reflecting Telescope on Alt-azimuth Stand, as described at page 13, -fitted with Silvered Glass Speculum, and provided with Two -Eye-pieces. - - £ s. d. - 5-1/4 inch Speculum, 4 to 6 feet focus 19 10 0 - 6-1/2 〃 〃 4 〃 6-1/2 〃 24 0 0 - 8-1/2 〃 〃 〃 6-1/2 〃 33 10 0 - 10 〃 〃 〃 7 〃 45 0 0 - 12-1/2 〃 〃 〃 8 〃 70 10 0 - - -Reflecting Telescope on Cradle Stand, and provided with Two -Eye-pieces. - - £ s. d. - 5-1/4 inch Speculum, 4 to 6 feet focus 21 0 0 - 6-1/2 〃 〃 4 〃 6-1/2 or 5 feet focus 27 0 0 - 8-1/2 〃 〃 〃 6-1/2 〃 〃 36 0 0 - 10 〃 〃 〃 7 〃 〃 49 0 0 - 12-1/2 〃 〃 〃 8 〃 〃 76 0 0 - - - SILVERED-GLASS SPECULA (_unmounted_). - - THE FINEST QUALITY GUARANTEED. - - £ s. d. - 5-1/4 inch diameter 4 to 6 feet focus 5 0 0 - 6-1/2 〃 4 〃 6-1/2 〃 6 5 0 - 8-1/2 〃 〃 6-1/2 〃 13 0 0 - 10 〃 〃 7 〃 25 0 0 - 12-1/2 〃 〃 8 〃 38 10 0 - 14 〃 〃 8 〃 55 0 0 - 16 〃 〃 10 〃 75 0 0 - 18 〃 〃 12 〃 100 0 0 - 20 〃 〃 12 〃 130 0 0 - 22 〃 〃 15 〃 165 0 0 - 24 〃 〃 18 〃 200 0 0 - 26 〃 〃 〃 250 0 0 - 30 〃 〃 〃 300 0 0 - 36 〃 〃 〃 400 0 0 - - - SILVERED-GLASS DIAGONAL MIRRORS (_unmounted_). - - FINEST QUALITY GUARANTEED. - - £ s. d. - 1 inch in the minor axis, or narrowest diameter of the ellipse 1 0 0 - 1-1/2 ditto ditto ditto 1 10 0 - 2 ditto ditto ditto 2 0 0 - 2-1/2 ditto ditto ditto 2 10 0 - 3 ditto ditto ditto 3 3 0 - 3-1/2 ditto ditto ditto 4 4 0 - 4 ditto ditto ditto 5 5 0 - - - SILVERING AND POLISHING SPECULA. - - £ s. d. - 5 inch 0 6 0 - 6-1/2 〃 0 8 0 - 8-1/2 〃 0 10 6 - 10 〃 0 15 0 - 12-1/2 〃 1 1 0 - Diagonal Planes 3s. to 0 5 0 - - - ASTRONOMICAL EYE-PIECES. - - OF BEST QUALITY. - -Huyghenian Construction of the following magnifying powers on a 6-1/2 -feet focus object-glass:— - - £ s. d. - 35, 60, 90, 130, 180, 200 each 0 15 0 - 320, 450 1 1 0 - 610 1 7 6 - - All fitted with Sunshades. - - - Improved Achromatic, Ramsden’s Construction. - - £ s. d. - 90, 130 each 1 0 0 - 189, 250 1 10 0 - 320, 450 1 12 6 - 610, 750 1 15 0 - 850 2 2 0 - - - Kellner’s Construction. - - £ s. d. - 30, 58, and 80 each 1 5 0 - - - SOLAR EYE-PIECES. - - £ s. d. - Solar Diagonal, with true plane 1 12 6 - Dawes’ Solar Eye-piece 8 0 0 - Barlow’s Lens 1 1 0 - Day Erecting Eye-piece 1 5 0 - Coloured Eye-piece Cap for observing the Moon 0 4 0 - Transit Eye-pieces, for use with Equatorials £1 1s. to 2 2 0 - Adjusting Piece 0 2 6 - - - MICROMETERS. - - £ s. d. - Parallel Wire Micrometer 5 5 0 - Position Micrometers £8 8s. to 15 15 0 - Double Image Micrometer 8 8 0 - Illuminating Apparatus from 8 0 0 - - -_Good Instruments will be taken in exchange and liberally allowed -for._ - - - GEORGE CALVER, - - HILL HOUSE, WIDFORD, - - CHELMSFORD, ESSEX. - - -[ Illustration: decorative ] - - - - - APPENDIX TO Hints on Silvered Glass Reflecting Telescopes. - - -The object of this pamphlet is to give some new and additional -advice not contained in my “Hints on Silvered Glass Reflecting -Telescopes,” on silvering and adjusting them; and, as the results of -continued and recent experience, it is hoped they may be useful, and -may prove an acceptable appendix to my little book of Hints. - -The first thing to do after fastening the mirror to the wooden -support, is to suspend it in the dish in which it is to be silvered, -and so to adjust it that there shall be one inch between the bottom -of the mirror and the bottom of the dish; then pour in water to come -a quarter of an inch up the sides of the glass—the quantity thus -found is to be measured, and will be the exact amount of the bath -when all the solutions are mixed; this will prevent any hitch at an -important moment, and the glass can be immersed without delay or -disturbance. - -Next proceed to wash the surface with nitric acid, taking care that -the acid does not run down the sides, as it is not so easily removed -from the fine ground sides as it is from the polished surface. After -gently but thoroughly rubbing the surface, add a little water, and -again go over; then wash all off, and take a large piece of cotton -wool and well sponge the surface and sides with plenty of water, and -suspend in a dish or plate with water in it. - - - TO PREPARE THE SOLUTIONS. - -The plan I adopt is this,—I dissolve a large quantity of the -chemicals required in one-fourth or one-eighth the quantity of water -employed by Martin, so that in a Winchester quart stoppered bottle I -can keep a large supply ready. I make the silver and ammonia -solutions eight times the original strength, the potash and sugar -solutions four times only, as these latter solutions require more -water to properly prepare them. - -Enough solutions to silver a 6-1/2 in. mirror, eight times are to be -made. - -Dissolve 1400 grains of nitrate of silver in 10 ounces of water (it -may be clean fresh rain water filtered, if distilled is difficult to -obtain, and it will act very well), but do not put the silver into -the exact 10 ounces, but make up to exact 10 ounces after the silver -is dissolved. It is thus condensed eight times. Do the same with the -nitrate of ammonia, by dissolving 2096 grains. - -Next prepare the potash and sugar solutions, condensed to four times -the initial strength. - -Dissolve 8 ounces of potash, and make up to exactly 20 ounces of -water; if this is dissolved in a glass measure it will evolve -sufficient heat to break it; it is safely done in a clean white jug. -Lastly, dissolve 4 ounces of white sugar candy with 416 grains of -tartaric acid, and boil ten minutes in a clean glazed vessel; when -cold, add 8 ounces of alcohol, and make up to 20 ounces with water. - -Do not use the solutions till all are of the same temperature, nor -on the same day as they are made; and do not silver until the glass, -and _everything_ used are of the same temperature. To insure this, -get the mirror ready and suspend in water, with the solutions all -collected in the room in which the silvering is to be done, and let -them remain until next day. - -Now, remembering how many ounces were needed to leave one inch of -solution under the surface and a quarter of an inch up the sides of -the 6-1/2 in. mirror, using a vessel about 2 inches larger in -diameter than the mirror, pour into a glass measure 10 drams of the -silver solution, add next 10 drams of the ammonia, then 20 drams of -potash; if the potash turns the mixture thick, pour it backwards and -forwards into another vessel, or stir it with a strip of glass, or a -glass rod, for half a minute; if the mixture does not turn clear -(which it will not do if it has turned very thick on adding the -potash), add cautiously, drop by drop, some of the ammonia solution, -agitating it till it just clears; do not filter unless there are a -great many floating particles through using unfiltered water, the -floating particles cause minute black spots, which are, however, of -little consequence, as their action is only the loss of so much -light. Measure 20 drams of the sugar—and the mirror being ready to -dip (the amount with the sugar added must be the required quantity -previously ascertained), add the sugar, stir well, and immerse when -the mixture begins to turn dark ink colour. - -When silvered, well wash the mirror with water of the _same_ -temperature as itself (it may be safely sponged with a lump of -cotton wool), and stand it on its edge on blotting-paper to dry. -Drops of water standing long on it when drying will cause stains, -which will not readily polish off, and indeed these should not be -polished, as it is not right to polish one part more than another; -so it is best to leave them, or if this is objected to, re-silver -the mirror, when standing it in the sun and wind will soon dry the -surface, but be careful not to let the dust settle on it. - -Do not warm anything; if the weather is cold, everything may be left -many hours in a warm room, but the _equal_ temperature is of the -utmost importance—the actual temperature is not of much consequence. -More failures are due to unequal temperature than any other cause in -the hands of the amateur. One careful and clever amateur informed me -that he had failed six times, but on the seventh, by attending to -the advice given above, the silvering was a perfect success, and all -seemed then as easy and certain as it had before seemed uncertain -and difficult. - - - THE FINAL ADJUSTMENT OF A NEWTONIAN REFLECTOR ON A STAR. - -The adjustments being as near correctness as can be seen by the -usual method of procedure—of which the fullest details are given in -my little book, the telescope is ready to try on a star. If all does -not appear satisfactory on first observing the star, do not disturb -the adjustments for a while, but wait until the air is steadier, -when perhaps it will be quite satisfactory. If not, turn it on a -bright star; if the adjustments are at fault, the black shadow of -the flat, when the star is out of focus, will not be central in the -slightly expanded image, but will be on one side of the expanded -disk. Use a power of 200 to 300. - -If the black spot is near one of the sides corresponding with major -axis of flat, the side screw will put it right. We will assume that -the black spot is nearer the edge of flat nearest the mirror, and -that the eyepiece is beyond the focus proper for the star; then -_unscrew_ the side screw. If the spot is nearer the other edge of -flat, _screw it up_ little by little, taking the hand out of the -telescope each time, and see what the effect is. The movement and -alteration can be watched while the hand is in the tube. If the spot -is seen either the top or bottom of the flat that is in the -direction of the diameter of tube, the middle screw must be slightly -loosened and the flat revolved. Let the screw just bite so as to -hold, and tighten it when correct. The definition of a star should -now be perfect, and by daylight observe the adjustments; it may -appear that the reflected circles are not concentric, but if the -definition on a star is good in still air, consider the appearances -correct adjustments, except that the reflection of the black spot on -mirror may be made concentric. It sometimes happens that a -persistent and rigid centreing of the reflections on _face of flat_ -are not the positions for best definition, perhaps from a slight -optical eccentricity somewhere in the instrument. - -The adjustments of a reflector are soon familiar and easy, and there -is this to be said, the adjustments can be manipulated upon to any -amount without the slightest injury to the instrument; there is no -danger of any sort, and the instrument can be perfected in its -adjustments by daylight, and this makes it both pleasant and -convenient to leisurely work at, for experience sake. The instrument -need not be out of doors either, unless an artificial star from a -black pin’s head—which is better in _strong_ sunlight than a -thermometer bulb—is to be used instead of a star at night, if so let -it be as high as possible. - -Be careful not to screw up the small screws too tight, as the fine -threads are liable to be spoiled and the screw made useless. - - - THE DURABILITY OF SILVER FILMS. - -It sometimes happens that the films do not last so long as they are -expected to do, that is, they sometimes lose their splendid lustre -sooner than they should do. When it is remembered that though the -process of silvering is an easy operation to perform, yet it is a -delicate chemical one, in which good results are best obtained when -several good conditions all meet together, this is not to be -wondered at, but it is certain the instructions given above will -reduce the chances of failure to a minimum, and imperfect results -will be rare. The silver usually lasts a long time. I know silver -films that have been in use for ten years. - -Do not suppose that a long and badly worn surface will fail to show -the detail and colours of the planets, the fullest detail also of -the moon or the stars. It is surprising how long a silvered surface -will continue to do its full work while its appearance is much -deteriorated; picking up faint points of light that are near the -very limit of its aperture and power, seem alone to require the -perfect and fresh film. I know silver films that have been in use -for seven years and have not been ever re-polished. - -That the Silvered Glass Reflecting Telescope is giving satisfaction -and is capable of performing the best work of a telescope—being -durable, convenient, and perfectly efficient—may be gathered from -the selection of a very large number of satisfactory and most -gratifying letters received by the writer; and it has been most -encouraging to find in all cases the observer is most eager to -express his entire satisfaction, and especially so where so many -have been prejudiced, in the absence of experience, with feelings of -doubt as to their _complete_ efficiency, and many, again, having -used fine refractors. - -It will be seen, too, that many have commenced with a small sized -reflector, and gone on increasing their optical power until they -have obtained a large and powerful instrument, imposing indeed as -compared with what was considered a powerful instrument less than -half a century ago, and at a fraction of its cost. - -The gratifying success which I have achieved, attested by the -universal satisfaction given by the instruments supplied by me, is -the result of unwearying labour and untiring patience; every -speculum, large or small, being figured with my own hands. - -I have every facility for setting up instruments, and for testing, -working, and regulating them in every part on celestial objects. The -optical parts are, therefore, not merely tested separately and -subsequently mechanically adjusted, but each and every instrument is -put together and optically and mechanically tested, as a whole, -before it is permitted to leave the workshop. - -Every speculum is most carefully figured, and confidently guaranteed -perfect _to the extreme edge_. - -The employment of “stops” of any kind is quite unnecessary, except -on bad nights, when the aperture must of course be suited to the -degree of steadiness of the air. For, whether the instrument be a -reflector or a refractor, the aperture and power used must of -necessity be limited by the atmospheric conditions under which -observations are made. - -It should, however, be remembered that the reflector having not only -a much larger aperture than a refractor of same focal length, and, -being open to the influence of the external air, is not only -affected by a relatively larger column of air, but is affected in a -different way. It frequently happens that reflectors of 6 in. to 18 -in. aperture, are of same focal length as 5 or 6 inch refractors. - -These considerations will explain the occasional use of stops, and -the expediency of having them ready to use when they can be of -temporary advantage. There are nights in our climate when a 6 inch -aperture may be the largest that can be used with satisfaction or -advantage; indeed it used to be accepted as an incontrovertible fact -that an aperture of G inches was the largest that could be used on -_average_ nights in our country. But there are also not a few nights -when large apertures, bring great gain to their fortunate -possessors. - -It is obvious that, _mutatis mutandis_, these conditions must affect -refractors as well as reflectors, with perhaps this difference, that -in the case of a reflector the rays pass down the tube and infringe -on the speculum as a column of parallel rays; consequently all the -external rays of the column travel near the surface of the tube for -its entire length. This being so, it is obvious that until the -internal and external air and the metal tube have time to equalize -in temperature, it will be advantageous, in all work requiring high -powers, such as the examination of difficult double stars, to “stop -off” for a time the _extreme edge_ by a small diaphragm. - -This will occur especially on frosty nights after a warm fine day. -When the telescope is in the open air, ice will not infrequently -form on the top of the tube, and the effect will be at once obvious -in the irregularity of a star image at the edge corresponding to the -top of the tube. The use of a small diaphragm will at once obviate -this defect and give perfect images. On such nights a wooden tube is -preferable to a metal one. It has been frequently my experience, -when finally testing an instrument, that when I have been -dissatisfied with its performance, and in order to trace the cause -of the apparent defect, have removed the mirrors to my wooden -testing tube, I have found every defect removed, and imperfect -replaced by perfect images. The explanation is simple. Not only is -the wooden tube much larger in diameter than the specula, but wood -is of itself less sensible than metal to differences of temperature. - -To make the tubes some 2 or 3 inches larger in diameter than the -speculum would cure this evil, but it would involve various -inconveniences and expenses in structure, which would more than -outweigh the advantage of overcoming a hindrance which is after all -only temporary and occasional. - -The experiment of perforating or ventilating tubes has now been -thoroughly and exhaustively tried, with the result that there is a -fairly general consensus of experienced opinion against their use. -The advantages are merely theoretical; the disadvantages are grave. -Among them are the admission of cross reflections in every -direction, the admission of dust, imperfect protection of the -mirrors, and a decided loss of strength and rigidity to the tube. - - - - - CATALOGUE OF Silvered Glass Reflecting Telescopes, &c., - - AND THEIR ACCESSORIES, - - OCTOBER, 1880. - - -G. Calver, while introducing the list of prices below, has the -greatest confidence in calling the attention of the practical -astronomer and the amateur to the very moderate prices charged, -considering the principle and style of the mounting, and the firm -and well-fitted arrangements in all the parts. - -The principle of the mounting is that which is the most convenient -to use, and with the greatest degree of steadiness with the easiest -movements. Fig. 1 is the _best mounting ever applied to the -Reflecting Telescope_. - -He is also able to state that, with his processes and special -facilities for working large specula, and substantially mounting -them, he is prepared to construct automatic equatorials of large -sizes, and with any special arrangements that may be required. - -Fig. 1 is a very excellent mounting, and admirably suited for large -instruments, especially when clock-power is applied; and as now -manufactured by G. CALVER, is the most complete and reliable, its -details of construction being such as to give the greatest freedom -of motion and steadiness. For large sizes the top of polar axis -works on friction rollers; and, as a superior German stand, its -arrangements and means of astronomical as well as optical -adjustments, are such as to make it an instrument of precision. - -To ensure these qualities many well considered -arrangements—entailing careful and expensive workmanship—must be -provided; details that do not easily admit of description in an -ordinary catalogue, or can be shown in an engraving, but are duly -appreciated by the observer, who will find their value by practice. - -The cradle bar is compound with fine screw movement for perfecting -collimation, but to make this adjustment perfect, the telescope tube -must be centred in a powerful lathe, and the cradle and solid metal -rings in which the tube revolves (see illustration) has to be turned -and fitted with true flanges or working bearings; the mechanical and -optical centres will then coincide. - -The hour circle, to be truly divided, must be truly made, and is a -solid wheel of good substance, working on a secondary axis, and -rotated by mechanical means, truly concentric, with a very strong -polar axis. - -The declination circle has fine tangent screw movement, with double -action for setting the readings by milled-headed screw, and also -long driving rod from the eye-piece to set and adjust the object in -centre of field. There is slow hand motion, by a separate wheel in -right ascension. - -The clock-power is connected and disconnected instantly, by touching -with the finger a small lever, conveniently placed at a small door -in the clock case. The clock will go at exactly the same rate, -whether it is driving the telescope or not, and the latter begins to -move at the proper speed the instant the lever is moved, and the -connection of the telescope and clock made, which is instantly -effected by another lever. - - £ s. d. - 14, 15, to 16 inch Speculum mounted as (Fig. 1), with - rotating hour circle reading to 5 seconds and decimation - to 1 minute (the diameter of the circles never less than - that of the Speculum, and often larger) - - 8 powers, from 50 to 700, with first-rate and very powerful - driving clock, first-class instrument, - and complete from £250 to 350 0 0 - - 17, 18, to 20 inch Speculum, with 10 powers, from 50 to 800, - including Kellners, Huyghenians, and Achromatics, - from £360 to 500 0 0 - - 22 to 24 inch Speculum from £600 to 700 0 0 - - 30 inch Speculum, with 12 powers and position micrometer, - and transit eye-pieces 1000 0 0 - - -Prices will be forwarded for special arrangements in any of these -large sizes. - - £ s. d. - Driving Clocks for 6 to 8 inch 35 0 0 - 〃 〃 12 〃 18 〃 40 0 0 - 〃 〃 20 〃 30 〃 50 0 0 - - -These are fitted with governor, regulator, and self-adjusting break. -They are made of gun metal and steel, and every wheel is cut. They -drive with most excellent regularity, and are, when desired, made to -beat seconds on a bell. The whole is enclosed in a mahogany frame, -with glass panels. - - -Fig. 2 admits of equal completeness, &c., but the stand is not so -convenient for clock-power. - - -SILVERED GLASS EQUATORIAL TELESCOPES, very substantially and well -fitted as (Fig. 2). All these equatorials have revolving body. - - £ s. d. - 5-1/4 inch Speculum, from 4 to 6 feet focus, with 7 inch - hour circles, reading to 5 seconds of time, and - declination circle reading to 1 minute, 2 powers 30 0 0 - - 6-1/2 inch Speculum, of from 5 to 6-1/2 feet focus, - 10 inch rotating hour circle, reading to 5 seconds of - time, and declination circle to 1 minute of arc, with - 3 powers--100 to 500 45 0 0 - - 8-1/2 inch Speculum, as above, with 10 inch circles, - 4 powers 70 0 0 - - 10 inch Speculum, as above, with 10 inch circles, - 5 powers 90 0 0 - - 12-1/2 inch Speculum, as above, with 10 inch circles, - 5 powers 120 0 0 - - -The EDUCATIONAL REFLECTOR is a plain and very steady and -satisfactory instrument, mounted on (Fig. 2) stand, without circles, -has revolving body, and made so as to be portable. - - £ s. d. - 8-1/2 inch Speculum as above, with 3 powers 40 0 0 - - -The POPULAR REFLECTOR (Fig 3), with Angle-Block stand, with -endless screw-motion to follow the stars with equatorial motion. - - £ s. d. - 5-1/4 inch Speculum, of from 4 to 6 feet focus, - with 2 powers 15 15 0 - - -These are also made with rotating body. - - -Reflecting Telescope on Alt-azimuth Stand, fitted with silvered -glass Speculum, and provided with two eye-pieces. - - £ s. d. - 5-1/4 inch Speculum, 4 to 6 feet focus 19 10 0 - 6-1/2 〃 〃 5 〃 6-1/2 〃 25 0 0 - 8-1/2 〃 〃 〃 6-1/2 〃 33 10 0 - 10 〃 〃 〃 7 〃 50 0 0 - 12-1/2 〃 〃 〃 8 〃 70 10 0 - - - Silvered-Glass Specula (_unmounted_). - - THE FINEST QUALITY GUARANTEED. - - £ s. d. - 5-1/4 inch diameter, 4 to 6 feet focus 5 0 0 - 6-1/2 〃 4 〃 6-1/2 〃 6 5 0 - 8-1/2 〃 〃 6-1/2 〃 13 0 0 - 10 〃 〃 7 〃 25 0 0 - 12-1/2 〃 〃 8 〃 38 10 0 - 14 〃 〃 8 〃 55 0 0 - 16 〃 〃 10 〃 75 0 0 - 18 〃 〃 12 〃 100 0 0 - 20 〃 〃 12 〃 130 0 0 - 22 〃 〃 15 〃 165 0 0 - 24 〃 〃 18 〃 200 0 0 - 26 〃 〃 〃 250 0 0 - 30 〃 〃 〃 300 0 0 - 36 〃 〃 〃 400 0 0 - - - Silvered-Glass Diagonal Mirrors (_unmounted_). - - FINEST QUALITY GUARANTEED. - - £ s. d. - 1 inch in the minor axis, or narrowest diameter of the ellipse 1 0 0 - 1-1/2 ditto ditto ditto 1 10 0 - 2 ditto ditto ditto 2 0 0 - 2-1/2 ditto ditto ditto 2 10 0 - 3 ditto ditto ditto 3 3 0 - 3-1/2 ditto ditto ditto 4 4 0 - 4 ditto ditto ditto 5 5 0 - - - Silvering and Polishing Specula. - - £ s. d. - 5 inch 0 6 0 - 6-1/2 〃 0 8 0 - 8-1/2 〃 0 10 6 - 10 〃 0 15 0 - 12-1/2 〃 1 1 0 - Diagonal Planes 3s. to 0 5 0 - - - ASTRONOMICAL EYE-PIECES. - - OF BEST QUALITY. - -Huyghenian Construction of the following magnifying powers on a 6-1/2 -feet focus object-glass:— - - £ s. d. - 35, 60, 90, 130, 180, 200 each 0 15 0 - 320, 450 1 1 0 - 610 1 7 6 - - All fitted with Sunshades. - - - Improved Achromatic, Ramsden’s Construction. - - £ s. d. - 90, 130 each 1 0 0 - 189, 250 1 10 0 - 320, 450 1 12 6 - 610, 750 2 2 0 - 850 2 2 0 - - - Kellner’s Construction. - - £ s. d. - 30, 58, and 80 each 1 5 0 - - - Solar Eye-Pieces. - - £ s. d. - Solar Diagonal, with true plane 1 12 6 - Barlow’s Lens 1 1 0 - Day Erecting Eye-piece 1 5 0 - Coloured Eye-piece cap for observing the Moon 0 4 0 - Transit Eye-pieces, for use with Equatorials £1 1s. to 2 2 0 - Adjusting Piece 0 2 6 - - -_Good Instruments will be taken in exchange, and liberally allowed -for._ - - - GEORGE CALVER, - - HILL HOUSE, WIDFORD, - - CHELMSFORD, ESSEX. - - - LETTERS AND EXTRACTS FROM THE FOLLOWING GENTLEMEN. - - _From_ W. E. PARKINSON, Esq. - - 11, WELLINGTON PARK TERRACE, BELFAST, - _July 7th, 1876_. - -DEAR SIR,—The night before last was a clear night, and I got the -6-1/2 inch out, and though the air was not good when using a 3-1/4 -inch refractor, I was much pleased with the performance of the -mirror. - -Mr. W. came up about eleven o’clock and stayed till one; we got it -on Saturn, and although rather low it was really a fine sight—Mr. W. -was much pleased. - -The moon being near to full we could not do much with faint points -of light. - - Yours sincerely, - (_Signed_.) - - -Mr. G. CALVER. - - * * * * * - - _From_ F. G. LEMMAN, Esq. - - 74, HAGLEY ROAD, EDGBASTON, - _November 27th, 1876_. - -DEAR SIR,—I have now got the 5 inch speculum fairly into adjustment, -and I am well pleased with it. Last night, the sky being clear for a -short time, I turned it on the moon; the definition of minute -craters was all that could be desired. I hope to have better -opportunities. - - I am, - Yours truly, - (_Signed_.) - - -Mr. G. CALVER. - - * * * * * - - (_From the same Gentleman._) - - 74, HAGLEY ROAD, EDGBASTON, - _November 21st, 1876_. - -DEAR SIR,—I have mounted the 5 inch speculum on a simple equatorial, -and it works well. I am sure it will turn out a fine glass, and I -shall not regret the time and pains I have spent over it. It is not -quite in adjustment yet, when it is it will afford me a treat. I -have just tried it once on the Orion Nebula and Trapezium, details -of Nebula well seen, and the 5th star in Trapezium easy. - - - I am, - Yours truly, - (_Signed_.) - - * * * * * - - _From_ D. C. CARMICHAEL, Esq. - - DOWLAIS, - _December 27th, 1876_. - -DEAR SIR,—I have thoroughly tested the speculum, and am pleased to -tell you it stood its trials well. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - ST. DENIES, SOUTHAMPTON, - _August 14th, 1876_. - -DEAR SIR,—With reference to your enquiry as to the performance of -the 6-1/2 inch reflector, I have pleasure in stating that I am -perfectly satisfied. It readily divides the test objects, and shows -delta Cygni with as low a power as 160. The comes to Sirius may be -considered an atmospheric test, perhaps, but I have repeatedly seen -it. Your stand I find very steady and convenient; altogether I can -fairly say that I consider that I have an instrument of considerable -power at a comparatively small cost. I now find the attempt to -observe with a refractor the reverse of pleasant. - -The owner of a 3 inch refractor, after using my reflector, writes -that he envies me its power, ease, definition, and comfort in -observing. - - I am, dear Sir, - Yours truly, - A. H. S. - - * * * * * - - _From_ W. L. LANCASTER, Esq. - - 25, HAMILTON TERRACE, ST. JOHN’S WOOD, - _May 26th, 1877_. - -MY DEAR SIR,—I have much pleasure in informing you that the very -first time I saw Saturn through your 6-1/2 inch reflector, the -definition of the planet was far superior to anything I had observed -before, even with a good 4-1/4 inch refractor; and on another -occasion I was greatly pleased with the clear and easy view of the -“Comes” to ε Boötis. But even without including the stars, the views -of Saturn and Jupiter through my 6-1/2 inch mirror do _alone_ (in my -opinion) well repay the cost of the telescope. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - From A. A. COMMON, Esq., F.R.A.S. - - 37, EATON RISE, EALING, - _February 13th, 1877_. - -DEAR SIR,—I like the 18 inch speculum, and I think it as good as it -can be, and if the larger one is as good I shall be delighted with -it.[1] It gives beautiful star images; I see Sirius as a brilliant -dot, a glorious object without ray or flares of any kind. I never -saw it so well before. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - -Footnote: - -[1] This alludes to the 37 inch, then under consideration; and I may -now add that Mr. Common is perfectly satisfied with it, and -“consider it a _very fine_ mirror.” - - - * * * * * - -(_From the same Gentleman._) - - 37, EATON RISE, EALING, - _February 3rd, 1877_. - -DEAR SIR,—I have tried the 18 inch speculum on some tests, and -especially the satellites of Uranus, and it appears fine, very fine. -What I have done in photography promises well. - - Yours truly, - (_Signed_.) - -Mr. G. CALVER. - - * * * * * - - _From_ J. L. LANCASTER, Esq. - - SOUTHEND-ON-SEA, - _March 17th, 1877_. - -MY DEAR SIR,—Circumstances have prevented me from making many -observations of late, but I am more confirmed in my opinion that my -telescope of your make is a very fine instrument (a 6-1/2 inch). I -heartily wish you success, which I feel sure you will obtain, as you -are so painstaking and turn out a thoroughly reliable article. I -shall be curious to compare my brother’s telescope with mine. - - Sincerely yours, - (_Signed_.) - -Mr. G. CALVER. - - * * * * * - - _From_ The REV. W. P. MATTHEWS. - - GORLESTONE, - _March 30th, 1875_. - -DEAR SIR,—You have asked me for my opinion of the 10 inch speculum. -I have tested it, and can speak in the highest terms of its -performance. Definition in good air is as near perfection as it is -possible to imagine. Such tests as ζ Cancri, γ^2 Andromedæ are well -divided. The 6th star in the Trapezium stands out well clear of its -brighter neighbour. On the moon any power may be applied, only -lessening the light, but retaining sharpness of outline. I have -tried a good many telescopes, but never yet saw these 10 inch -mirrors surpassed. The image of a star disc expanded on each side of -the focus is of as nearly the same appearance as possible. - - Yours truly, - (_Signed_.) - - * * * * * - - 416, BRIXTON ROAD, - _April 20th, 1875_. - -DEAR SIR,—In compliance with your request, I send particulars of my -observation on φ Draconis. It was about 12.45 this morning when I -directed my 8-1/2 inch to this object, the air was very unsteady, in -fact not nearly so good as some nights lately, and I well divided it -with a power of 450. I then used a 6-1/2 inch stop, which I found -very much increased the steadiness of definition. - - Yours very respectfully, - P. H. - - * * * * * - - _July 10th, 1874_. - -DEAR SIR,—I tried the 6-1/4 inch mirror I had from you for the first -time last night. The night was not a good one for definition, but I -was very much pleased indeed with its performance. - - Yours truly, - C. H. W. - - * * * * * - - _From_ H. SADLER, Esq., F.R.A.S. - - HONITON RECTORY, - _December 23rd, 1874_. - -DEAR SIR,—I have been wishing to tell you how pleased I am at the -performance of your mirror. Many thanks for your kind offer to -change the mirror if needful, but I think it could hardly be better -than it is, its performance on different double stars is most -excellent. I have examined nearly 100 of these since September with -the 6-1/2 inch, most of them very difficult, but the mirror came out -well under all tests. I send you a list of some of the objects I -have examined. - - Yours truly, - H. SADLER. - - * * * * * - -π _Aquilæ._—Divided, power 80. 14-^m. Webb might have been rated -15-^m. [6-^m., 7-^m. 1″·5.] - - A B C A-B A-C -δ _Aquilæ._—[3-1/2, 16, 14, 96″, 194″.] Comites easy, 3 other -excessively minute ones, not shown by the 11 inch refractor with -which P. Smyth measured B.C. Found 6-1/2 in. Sept. 12th, est. -± 17 mag. 100″ to 110″. - -ζ _Persei._—My friend “Linea” sees two minute stars with 4·28 in. -Wray, not in Webb, and Wray himself sees an excessively difficult -companion. I see these 3 stars easily with 6-1/2 in. (Wray says his -companion is “very difficult with 7 in. refractor, really only a -glimpse star, even with the best atmosphere”), and have added two -more stars to the group. - -β _Delphini._—Close double discovered by Burnham, est. 0″·7. In -contact 6-1/2 in., power 430. - -η _Coronæ Borealis._—Very low, long past meridian, in contact 164 -power. - - _a_ _b_ _c_ _n_ -β _Equalei._—[5-1/2, 13, 14, 16: _a-b_ 35″, _a-c_ 50″ _b_-B 3″.] -16-^m. not seen, Webb 9-1/3 in. Just divided 164, easy 430 power. -Two other companions (not seen by Smyth or Webb?) - - -μ _Andromedæ._—The 16-^m. comes, a very difficult test, easy; other -comites (not seen by Smyth or Webb?) seen. - -Companion to Vega pretty easy, Nov. 12th, 37m. after sunset. - - - _a_ _b_ _c_ B _a-b_ _b_-B _a-c_ -_P._ 178 xx. _Delphini._—[7-1/2, 8, 16, 9: 14″·3, 0″.7, 20″.] -Smyth 16 by evanescent glimpses; easy 164 power, 6-1/2 in. 8-^m. -well elongated in direction of 230°, power 430. - - * * * * * - - _From_ H. BLYTH, Esq. - - REGENT ROAD, GREAT YARMOUTH, - _October 24th, 1874_. - -DEAR SIR,—Doubtless you have anticipated hearing from me, relative -to the 10 inch telescope, but the weather has been so indifferent -for delicate astronomical work, that it has not yet had a fair -trial. - -In middling good air, when I could use a reduced aperture with -satisfactory results, the star images reminded me of the 6-1/2 inch, -which I considered so superb a mirror that nothing could surpass its -fine performance either on stars or planets. - -With the recollections of the truly splendid views I had of Jupiter -last season, I count much to see him with the 10 inch. - - Yours truly, - (_Signed_.) - - * * * * * - - _From_ T. AYERS, Esq. - - REGENT ROAD, GREAT YARMOUTH, - _February 2nd, 1875_. - -DEAR SIR,—In reply to yours, I have not had a really good night to -test severely the defining powers of the 10 inch mirror, but judging -from the few difficult objects I have seen, the mirror promises -well, and I have no doubt of its excellence. - -I had a fine view of the grand nebula in Orionis, and noticed that -the 5th and 6th stars were quite plainly seen, when the aperture was -reduced to 5-1/2 inches. I consider your stand a great improvement, -being very convenient and steady. - - Yours truly, - (_Signed_.) - - * * * * * - - _From_ THE REV. A. P. ARNOTT. - - AM STREET, EDINBURGH, - 8-1/2″ ALT. AZIMUTH. _February 25th, 1878_. - -DEAR SIR,—I have found no difficulty in putting up the instrument, -and the adjustments do not appear to be hard to manage. It is a very -beautiful instrument, and the case with which its movements are made -is extraordinary. - - I am, dear Sir, - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ J. SLATER, Esq. - - TOWN HALL SQUARE, BOLTON, - _September, 1877_. - -DEAR SIR,—Last night, through a break in the clouds, I managed to -get my first look with the telescope (an 8-1/2″). I turned it on -delta Cygni. The definition was rather fluttering, but a 6-1/2 in. -stop enabled me to see it well with 170; the first time I have ever -divided this star[2] the colour was obvious. I then tried Pi Aquilæ, -which was beautifully divided. I am much pleased with the instrument -and its performance, and when I do get a fine night I expect great -things from my 8-1/2 in. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - -Footnote: - -[2] The observer had used an 8-1/2″ reflector, however, for some -years. - - * * * * * - - (_Another Letter from the same Gentleman._) - - BOLTON, - _December, 1877_. - -DEAR SIR,—As to the adjustments of the 8-1/2 in., I have had very -little trouble, and I have tried it again on a few objects, a few -nights ago, and with a 7 in. stop, and a power of 300, I split Zeta -Cancri and the close double _sf_ Castor; with the full aperture they -were very easy. I have also split Gamma Andromedæ with 7 in. stop, -and on a very fine night. I believe 6-1/2 in. would do it. - -I just got a look, on November 7th, at the shadow of Titan, on -Saturn’s disc; but was driven inside before I could observe the time -of its passing the centre. I have seen the Moon on two occasions -with the finest definition I ever saw. Altogether I am highly -pleased and proud of my telescope. I have not yet tried many faint -objects; still I saw the Crab nebula in Taurus much nearer Lord -Rosse’s drawings than Herschel’s, as given by Chambers. - - I am, dear Sir, - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ G. BILLINGTON, Esq. - - WELLWICK, WENDOVER, - _July 19th_. - -DEAR SIR,—I am perfectly satisfied with the mirror, a 6-1/2 in., now -that I have tried it with _your_ flat, and I have no difficulty in -seeing star disks when the air is good. - - I am, dear Sir, - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. S. TOMLINSON. - - CARLISLE TERRACE, MALAHIDE, 1877. - -DEAR SIR,—I write to tell you that I was very much pleased indeed -with last night’s observations with your 18″. The star disks were -good and well condensed, and stray light well gathered in, and when -a good night comes I have no doubt of it. - -The markings on Mars were well defined. I find the advantage of a -large aperture shows itself. I have no doubt I picked up the outer -Satellite, but cannot be certain till to-night, if I am permitted to -see it. The only objection I have is its short focus, as it -seriously complicates my labours in matching the secondary convexes -for the Cassegrain form—if you can help me in the matter of a longer -focus; if not I am satisfied and thankful, and must only proceed to -the testing process inch by inch. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From the same Gentleman._ - - CARLISLE TERRACE, MALAHIDE, - _November 6th, 1877_. - -MY DEAR SIR,—I had intended to write to you before, but, from -various duties, I have been obliged to postpone it from day to day. -I have been working steadily, however, with your 18″, and I like it -the more I use it. The figure is wonderfully perfect, and the -definition of planetary details and markings most admirable. - -On four occasions I have seen the Satellites of Mars; of these I can -feel quite certain of two, for it was held sufficiently long to -perceive a marked change in position angle. The intensely small -companion to _a_^2 Capricorni I have well seen, cleanly separated, -and that in strong daylight. I am anxious for a good night to attack -τ Orionis. During these observations I have been using the telescope -as a Newtonion during my work at the convexes; but my labours have -been pleasantly intercepted by the magnificent and gorgeous views of -the star clusters and nebulæ—more beautiful and startling than I -could possibly have imagined, and surpassing far anything I had ever -pictured to my mind. - - Yours very truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ MAJOR-GENERAL ADDISONS, C.B. - - MELTON, 1878. - -SIR,—You will be glad, I know, to hear that I had a very -satisfactory evening with the 18 in. equatorial yesterday, and the -clock kept it in the field without vibration of any sort. - - Yours obediently, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ J. T. STEVENSON, Esq. - - AUCKLAND, NEW ZEALAND, 1877. - -DEAR MR. CALVER,—Since writing to you I have been observing with the -6-1/2 in. equatorial telescope, and I have much pleasure in -informing you that I am greatly pleased with its performance. The -definition of the planets is beautiful. - -I am happy to inform you that I have found a most minute crater in -Picard _a_, it is much smaller than the test crater (mentioned in -Cel. Obj., by WEBB, page 83) in same formation. I wrote to Mr. WEBB -about it, and since then I have seen it distinctly; it is another -proof of the excellence of your telescope. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ J. C. HARKER, Esq. - - KENDAL, 1878. - -DEAR SIR,—After four months use of the telescope I had of you (an -8-1/2 in.), I am able to tell you that I am much pleased with its -great excellence. The definition of the mirror is very fine, and its -performance on the Moon being superb, with so high a power as 700. -The stand I find very steady and convenient, and the screw motions -very smooth. - - I am, dear Sir, - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ E. A. FRY, Esq. - - BIRMINGHAM, 1878. - -DEAR SIR,—I write to tell you that I like the telescope very much -(5-1/4 in. equatorial); the mirror is, as you say, a gem, and it is -quite a treat to look at the mysteries above. Saturday was a -glorious night, though the Moon was rather too bright. Altogether I -am quite satisfied with the telescope, and the finish is first-rate. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. J. M. COATES. - - THE GODDARDS, MOULTON, SPALDING. - -DEAR SIR,—I am able to report satisfactorily on the mirror. On -Saturday night last I divided μ^2 Boötis, power 300, with the -8-1/2″. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From the same Gentleman._ - - THE GODDARDS, MOULTON, SPALDING, - _January 18th, 1879_. - -DEAR SIR,—I have been waiting for opportunities for testing -observations, but there have been few nights when the weather and -leisure time have concurred in giving me a chance. However, I can -say this much, that in lunar work I have had much pleasure from the -excellent definition given by the 8-1/2 in. I had some fine views of -Copernicus under high powers not long since, and am anxiously -awaiting opportunities for closer study of a particular region. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ FRANK C. DENNETT, Esq. - - 10, TERMINUS TERRACE, SOUTHAMPTON, - _December 10th, 1878_. - -DEAR SIR,—The 5-1/4 in. is beautiful. Last night there was a slight -frosty fog, and the air was very very steady, in fact I only -remember about one such night since November, 1875. Saturn was sharp -and hard, and the shadow of the ring intensely black, with all -powers from 168 to 372, and even with 550, obtained by removing the -field lens of 372. The very broad polar caps presented a bluish -grey, rendered more visible by contrast with a fine penumbral line -of a very slightly brownish grey close to the south border of the -ring, and which seemed to be diffused on the south border (_i.e._, -the line was diffused). I should fancy the line here referred to was -less than 1″ of arc broad; considerably less! - -Turning from Saturn, I looked at ε Arietis, the present distance of -which is only 1″·17; with all powers, from 213 upwards, the -components were visible as two tiny points of light thoroughly -divorced, and with 288 power, reminding one of ε^2 Lyræ, as seen -with 120 on a 2-3/4 in. achr., only the discs seemed freer from -rings. - -Later in the evening, when the definition was more fluttery, η -Orionis shared the same fate as ε Arietis, being well separated with -the same powers. The present distance of this star is only 1″·12. -Recently I divided this star with a 4-1/2 in. stop on the mirror, -power 288. I have seen ε Arietis well with the same stop too. I -think I have told you before that the 5-1/4 in., with 372 power, was -successful in dividing ε Equulei, whose distance is only 0″·97. This -is the closest star I have yet divided. - -The Moon is fine too! Last night I caught a large ring close to the -terminator, near the south pole (for the Moon was only about 2-1/2 -hours past full), which was looked at over the northern wall; the -view was beautiful, such gradations of light and shade, interspersed -with greys of different hues, which render the object scarcely -describable. The finest clefts and tiniest craterlets are rendered -visible. It is a puzzle to me how anyone in the possession of £16 or -£17 can go buying a refractor of from 2-1/2 to 3-1/4 in. aperture in -preference to a silvered mirror. It must be from sheer ignorance of -the beautiful views given by the latter, which is superior in every -way. - -I do not think there is any other point on which I need touch now, -so I must beg to remain, - - Dear Sir, - Yours very truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. JEVON J. MUSCHAMP PERRY, M.A., F.R.A.S. - - ST. PAUL’S VICARAGE, ALNWICK, - _December 20th, 1879_. - -MY DEAR SIR,—I was very much interested in reading your account of -the grinding and polishing of the 37″ speculum, in this week’s E. -M., copied from the “Monthly Notices” of the R. A. S. Had I only the -wealth of my neighbours, I would give you an order for the largest -telescope you could construct. Every really good night gives me some -fresh proof of the great excellence of the 6-1/2 in. For instance, -on December 15th, I clearly and steadily saw _Enceladus_. I could -not, of course, have recognized it had it not been that this month’s -“Observatory” gave diagrams of the positions of the five inner sats. -of Saturn, for 8 p.m. on every day of this month, and a careful -search for Encel. was fully rewarded by a clear and steady view of -it at that time in its proper place. On this feat the Rev. T. W. -WEBB warmly congratulated me, and said that he had seen Encel. with -his 9-1/3″ “With,” but only when Saturn was hidden behind a bar. I -saw it clearly in exactly its proper position _without_ hiding the -planet—power 400. I have also had several good views of Mr. WEBB’S -new nebula in Cygnus, and have sent him drawings of the field, which -he confirms. I have also made some fine drawings of Jupiter and -Mars. On December 12th, 11 p.m., I tackled σ 749 (in Taurus), a very -difficult test for my aperture, only 0·8″, and _perfectly_ split it -with power 300. The night was superb. When you consider that the -theoretical limit of dividing power for a 6-1/2″ is only 0·7″, I -think I have done well to split a star 0·8″ in our climate. - - Faithfully yours, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ G. FERGUSON, Esq., M.D. - - ALTIDORE VILLA, PITTVILLE, CHELTENHAM, - _June 26th, 1879_. - -MY DEAR SIR,—I write to tell you that Thursday night being very -fine, I had a good opportunity of trying the (an 8-1/2″) mirror on -Jupiter, although this planet was lower than it should have been to -give the fairest trial. Well, I can only assure you that, despite -the low position, I never saw it so beautifully before. The contour -of the clouds, particularly one orange one, _sf_, the delicate -lines, and the colouring of the belts, were all seen with a -sharpness which I had believed scarcely attainable, and proving that -the result of your labours has been to effect a very real -improvement in the speculum. Notwithstanding the good planetary -definition, the air was not tranquil enough for seriously attacking -really difficult doubles; still I was much pleased with its -performance on π Aquilæ. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From the same Gentleman._ - - _July 27th._ - -Yours of July 25th duly to hand. I have the speculum adjusted in the -“pink” of accuracy. Star discs are small and round, with concentric -diffraction rings, and quite rayless except in the case of the very -brightest stars, and even with these the rays are very trifling. But -for the diffraction rings, which I fancy a really good night will -mostly get rid of, I should, I feel sure, have readily divided η -Coronæ. - -The Moon last night, though very low, was seen with most gratifying -ease and sharpness. Altogether you will gather that I am more than -satisfied. I am greatly gratified, and will look forward to great -pleasure as the season advances. - - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. JEVON J. MUSCHAMP PERRY, M.A., F.R.A.S. - - ST. PAUL’S VICARAGE, ALNWICK, - _August 8th, 1880_. - -MY DEAR SIR,—You have indeed surpassed yourself, and your -performance is better than your promise. The mirror arrived safely -yesterday, and when I unpacked it I was exceedingly pleased with its -surface and general appearance, but of course that was a small -matter. I waited till night to test it on a close double. - -I first tried it on ε Boötis, and although the night was very bad -the definition it gave of the two stars with 450 and full aperture -was admirable, considering the night. After waiting a long time for -clouds to clear away, I turned the tube on that exceedingly -difficult test, δ Cygni. I consider this star to be one of the -severest tests for any telescope, reflector, or refractor of any -aperture in the whole heavens. The performance of the mirror upon -it, with the _full_ aperture, was simply MAGNIFICENT, the small -close companion standing out clearly, well separated from its bright -primary, although the night was not good. I hardly thought it -possible that the fine 18 inch mirror could be excelled. I am bound -to confess that you have excelled it. - -To say that I am extremely grateful to you is a very poor expression -of my feelings. I am pleased and delighted _beyond measure_ with the -_super-excellent_ 18-1/4 in. mirror you have sent me. Clouds put an -end to my work last night, but I have no doubt that in the future -every night’s observations will only accumulate proofs of the -exquisite perfection of your handiwork. - - Very faithfully yours, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _Extract from the “English Mechanic,” September 3rd, 1880_. - -“The telescope is a silvered-glass Newtonian reflector. There are -two specula, one of 18 in. in diameter, figured by the celebrated -WITH, in 1878, and is a magnificent specimen of that eminent maker’s -handiwork. In fact, I am given to understand that Mr. COOPER KEY -considered that he had attained perfection in this mirror, and of -all men then living there was no better judge than he. The other is -18-1/4 in. in diameter, and was originally figured by Mr. COOPER KEY -himself; but it was only perfect over the interior 14 in., and, -consequently, while the whole aperture could be used on the moon and -planets, this was all of it that could be used upon close doubles. I -determined, therefore, to give this mirror to Mr. CALVER to -re-figure, and, if possible, excel Mr. WITH’S speculum. - -“In order that Mr. CALVER might know the extent of his task, I -returned home through London and Chelmsford, bringing with me both -mirrors, which Mr. CALVER tested for himself at the centre of -curvature. After the trial was over, he pronounced Mr. WITH’S mirror -to be an exceptionally fine one, and heartily congratulated me on -the possession of such a beauty. I left the other one with him to be -re-figured. - -“And now to come to the most important part of all—the performance -of the 18 in. WITH mirror upon close double stars. Since the -telescope came into my possession, I have never had a thoroughly -good night; but even on an indifferent one, I have, with the _full -aperture_, divided such extremely difficult tests as η Coronæ, μ^2 -Herculis, and δ Cygni; and these results were quite sufficient to -show me that the very high reputation of its excellent maker is -worthily and proudly maintained by this mirror. In fact, the -definition it gave one night, when the Scottish Astronomer Royal was -staying with me of ε Boötis, was such, that the Professor -told me I could not expect any telescope, reflector, or refractor, -upon the same object, and on the same night, to do better. However, -I knew Mr. CALVER was doing his very utmost in the honourable and -praiseworthy competition to attain perfection, to give me a mirror -which would bear favourable comparison with Mr. WITH’s. It reached -me about a week ago, and on the same evening I had an opportunity of -testing it upon what I consider to be one of the very severest tests -that can be applied to any telescope, reflector or refractor, in the -northern heavens, that is δ Cygni[3]. To make the test as severe as -possible, I applied the _whole_ aperture at once, with a power of -347. I was surprised and delighted _beyond measure_ with the result. -The small star stood out clearly separated from its bright primary, -and there were two or three diffraction rings round the latter -_unbroken_ in any part, and _equally_ bright all round. I consider -the results which Mr. CALVER has attained in this mirror cannot -possibly be excelled. I measured this star on two separate nights, -and the mean results are pos. 328·9°, dist. 1·29″. I also measured, -with this mirror, another difficult star of the same character, -namely, with a bright primary and small companion, ζ Herculis, with -the following results, pos. 111·3°, dist. 1·03″. I make the -companion of this star a dusky blue, or lavender. As regards the -light-grasping power of the mirror, I turned it, the other night, on -the minute pair between β^1 and β^2 Capricorni, which Sir John -Herschel gave as the test of the power of any telescope and eye to -observe the satellites of Uranus. The night was very unsteady, but -the minute stars of the pair were most conspicuous.” - -Footnote: - -[3] NOTE.—Mr. Perry does not mean that δ Cygni is one of the most -difficult tests as regards the distance of the components. He has -often seen it with his 6-1/2″ Silvered Glass Reflector, and even -with a stop of 5-1/2″ on the same instrument; but he was surprised -at the beauty and perfection of the images—perfect and unbroken -rings—and he knew that these appearances could not have presented -themselves had not the _figure_ of the mirror been of extraordinary -perfection. - - * * * * * - - _From_ THOS. WESTLAKE, Esq. - - FORDINGBRIDGE, - _June, 1880_. - -MY DEAR SIR,—I wrote you last evening, and afterwards had my first -trial with the telescope (a 10 in. Alt Azimuth). It turns out a -beauty. I had the full aperture on all the evening, the air was very -good, but not perfect. Alpha Lyræ was a splendid object with all -powers, a most brilliant disk with just indications of circular -rings—no wings or rays. I turned it on B Cygni, and the beauty of -the mirror stood revealed, both stars _perfect disks_, with _just_ -the indication of perfect circular defraction rings, and the colours -were most beautiful,—nothing could possibly appear finer. I consider -the mirror perfect, and the stand most convenient and smooth and -pleasant in action. - - I am, dear Sir, - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. E. H. LOWE. - - WILMCOTE, - _April, 1880_. - -DEAR Mr. CALVER,—I have been exceedingly occupied, as there has been -a succession of gloriously fine nights, and I have secured the -Nebulæ diagrams I so much wished for Mr. WEBB,—and I would allow -nothing to divert me from them—but I put it (a 15-1/2 in. -equatorial) on Zeta Cancri, and it was shown beautifully. I have -been working at the driving clock, and it is under capital control. -The great cluster 13 M., is a glorious object, and magnificently -resolved. I see all I look for on the Moon. - - Faithfully yours, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ W. SCOTT, Esq. - - SOUTHWICK, SUNDERLAND, - _July, 1880_. - -DEAR SIR,—Dr. Blacklock, and also Mr. Bell, have seen the mirror, -and pronounce it perfect, and it performs admirably. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ JAMES L. MCCANCE, Esq., F.R.A.S. - - PUTNEY HILL, S.W., - _October 23rd, 1879_. - -DEAR Mr. CALVER,—Weather and circumstances have been very -unfavourable for doing much work with the 10 inch reflector since I -got it from you. But with what I have seen with it I am very -pleased; in good weather the views of the planets, Jupiter and his -“Great Red Spot” especially, were grand. The mounting (equatorial) -is quite satisfactory, and the movements work smoothly and well. - - Yours very truly, - (_Signed_.) - - * * * * * - - _From_ FRANK C. DENNETT, Esq. - - 10, TERMINUS TERRACE, SOUTHAMPTON, - _July 11th, 1880_. - -DEAR SIR,—It is a long time ago that you asked me to write and let -you know how I liked the 9-1/2 in. I have not done so because I -wished first to see what was to be made of the planets. - -Last night, at 14^h 30^m, I had my first peep at Jupiter through it, -and a grand sight it was; his belts coloured, though not so vivid as -they are sometimes, were fine in the extreme; and, though the power -employed was only the achromatic 215, the satellites were beautiful -little discs, one especially showing the tint of its surface. - -A quarter of an hour later Saturn was in the field of view; a grand -object, several of the satellites surrounding it. A little cloud was -coming up over it, so I could not see so much of this planet as I -should have liked, though amply sufficient to prove incontestably -its high class. - -With double stars I have not attempted much, though on turning it on -μ^2 Boötis I found the division easy, the components being -respectively slightly orange, and dullish white or pale yellow. - -I have had lots of grand views of the wonders of the “star depths,” -clusters, and nebulæ, notably 57 Messier, the Ring nebula, in Lyra, -the great Orion nebula, and the marvellous Whirlpool nebula, the -chief outlines of which were well shown. - -Hitherto the instrument has been chiefly employed on the Moon, the -details of which come out hard and sharp in a beautiful manner, with -all powers up to 515, the highest I have. - -The 5-1/4 in. pleased me very much, but the 9-1/2 in. delights me. - - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ G. W. ROYSTON PIGOTT, Esq., M.D., F.R.S., F.R.A.S., &c. - - EASTBOURNE, - _July, 1880_. - -DEAR SIR,—There has been but little weather for testing. Mr. Slack -has been staying with me; we are delighted with the 8-1/2″, and -think it a great beauty. I can hardly imagine any mirror much -better, and I expect great precision from it. I find its focus to be -65 inches, not quite so much as my fine Dollond glass, of 3-3/4 in. -aperture. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ A. WILLIAMS, Esq. - - CLARENCE LODGE, GRAFTON STREET, BRIGHTON, - _August, 1880_. - -DEAR SIR,—I had intended writing to you to express my pleasure in my -5-1/4 in. telescope nearly a month ago, but thought I would have -another look first, and the more I look the more I am pleased with -it. I cannot now imagine anyone preferring a 3-1/4 in. refractor to -one of these. I have had no trouble at all with the adjustments, and -if was to get out of adjustment, it could be rectified in about five -minutes. Notwithstanding the unsettled weather (giving on an average -only about one fine night in eight), I have examined nearly two -hundred double stars in the last three months, some of them very -difficult. I enclose a list of a few, which will show very well the -capabilities of the instrument. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - - LIST OF OBJECTS SEEN WITH A 5-1/4 INCH REFLECTOR. - -ξ Scorpii (5, 5·2 : 1″·3) clearly divided, 186 power. - -ε Lyræ. Debilissima quite easy; 186, 380 powers, though nearly half -Moon. - -μ^1 Herculis (9·4, 10 : 1″·04). Just divided, 380. Very difficult, -owing to its faintness. - -Vega. 11 mag., comes quite plain, only 47 minutes before sunrise; -powers 110, 186. - -φ Virginis (5, 13 : 4″·27). Comes plain, 110, 186. - -ν Scorpii, the close pair (4, 5 : 0″·8), well elongated, power 380; -small star (7, 8 : 1″·68) divided, 110. - -ζ Boötis (4, 5 : 0″·88) elongated 186, notched 380, and with field -lens removed, all but divided. - -δ Cygni (3, 9 : 1″·63), 9 plain, 380 on several occasions. - -Antares is chiefly an atmospheric test, but the companion was quite -plain on one occasion out of the three I looked for it. - -μ^2 Boötis (7·5, 8 : 0″·63), considerably elongated, 380. - -η Coronæ (5·5, 6 : 0″·62), slightly elongated, 380; but not nearly -so much as μ^2 Boötis. - -56 Herculis (6·5, 13 : 15″), 13 easy, all powers. - -π Aquilæ (6, 7 : 1″·5). Just separated, 110; 14 mag. (Webb), just -visible, 186. - -ε Equulei. Just divided, 186 (5·7, 6·2 : 1″·0). - - * * * * * - - _From_ D. TRAILL, Esq. - - BALHAM, - _March 5th, 1880_. - -MY DEAR SIR,—I have at present to use the telescope, an 8-1/2 in. -Alt Azimuth, out of a window, and so at a great disadvantage,—in -fact cannot give it a fair trial till I have the house ready for it, -meanwhile I use it as occasions offer, and have been highly pleased -with it. The views of the nebular in Orion resemble those of BOND’S -drawings, and shows immeasurably above my 3-1/4, of course. I think -the mirror is _perfect_ from its performance on large stars whose -disks are small and round, without rays of any kind. I am -_thoroughly_ satisfied. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ HUGH AULD, Esq. - - GROSVENOR CRESCENT, EDINBURGH, 1879. - -DEAR SIR,—I have not been able to do much with the telescope, a -5-1/4″ Angle-block, through the unfavourable weather for the last -four months, but with the opportunities I have had its performance -is satisfactory; it performs well on stars with high powers, and I -am sure it will bear a good deal higher. I am very well pleased with -the instrument. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. THOS. H. BROWNE, F.R.M.S., &c. - - THE CEDARS, HIGH WYCOMBE, - _February 26th, 1880_. - -DEAR SIR,—I am exceedingly pleased with the action of your mirror -(an 8-1/2 in. equatorial); what I have seen of sun, moon, planets, -and stars is splendid. I have had a good many telescopes, but never -liked one so well as the one you have sent me. For the first day or -two I could do nothing but admire it. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. CONYBEARE W. BRUCE, M.A., F.R.A.S. - - ST. NICHOLAS’ RECTORY, CARDIFF, - _July 17th, 1880_. - -MY DEAR Mr. CALVER,—You will have been looking for some news of me -and the 12″. I have purposely delayed writing until I was _really_ -in a position to say something definite, and you had already heard -my high opinion of the mirror. Let me at once say that that opinion -has been _more_ than justified. The mirror is a _beauty_. Canon -Beechey and I got Jupiter and Saturn at 2.30 a.m. yesterday -morning—and they were _superb_. The Canon declared enthusiastically, -that he “had _never_ seen Saturn anything like that,” and the last -thing he said to me on the subject as I parted with him to-day was, -“You have got a very fine instrument indeed.” - -If I _could_ add anything more in its praise it would be this, that -I am really going to build a _new observatory_ for it—my present one -is too small and cramped. - - Sincerely yours, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - (_Another Letter from the same Gentleman._) - - ST. NICHOLAS’ RECTORY, NEAR CARDIFF, - _August 28th, 1880_. - -DEAR Mr. CALVER,—The 12 inch surpasses all my expectations. It has -the perfection in figure of the incomparable 10 inch of yours, which -was its immediate predecessor, and, in grasp of light, the -difference in favour of the 12 inch over the 10 inch, a most -brilliant mirror, is simply astounding. - -You may equal the 12 inch—I am sure you will, whenever you make -one—but you will not easily beat it. Mr. SADLER, the well-known and -most accurate observer, has himself worked with it, and writes of it -to me as follows:—“The 12 in. is considerably superior in light -grasping power to H2’s 20 ft. reflector, and the figure is a much -finer one.” - -Your skill and kindness has placed me in the front rank among -amateur observers, as far as the possession of optical means is -concerned. - -I was afraid that the German Stand would not prove so convenient as -the “Berthon Equestrian Stand,” but I find it is and far more so. -The German Stand is, for anything above 6 in., decidedly more solid -and dependable, and far simpler in any case. The merit of the -“Berthon Equestrian” form is that it requires a smaller observatory. - - Very sincerely yours, - (_Signed_.) - - * * * * * - - _From_ W. S. FRANKS, Esq., F.R.A.S. - - LEICESTER, - _August 19th, 1880_. - -MY DEAR SIR,—As the 11-1/4 in. speculum has now been in constant use -for two years, I may be allowed to express my opinion, with all due -deliberation, as to its performance. It has been tried upon almost -every conceivable object during that period with uniformly good -results. Having also a 5 in. achromatic of the finest quality, I -have been enabled to make a great number of comparisons between the -two instruments, mounted side by side: and for fine definition, -smallness of star discs, and freedom from chromatism, the reflector -always came out victorious. Upon first mag. stars, nothing could -surpass the purity of the image in the reflector; but on immediately -turning to the achromatic, the outstanding violet rays were -painfully obtrusive. For red stars, which are generally small, the -reflector seems especially adapted. I used to be rather sceptical -about the tints of red stars, but must confess, after seeing the -magnificent tints of scarlet and crimson which are shown by a large -aperture, that my doubts have vanished. Definition upon the planets, -in good air, is simply perfect. The detail on Jupiter and Saturn -reminds one of an exquisite engraving. The far distant Neptune is -perfectly defined as a tiny disc, and his moon steadily visible with -the 11-1/4 in. The faculæ and other Solar detail are brought out -with wonderful distinctness. As for the Moon, the crispness of -definition, absence of colour, and immense number of minute objects -shown by the reflector, leave nothing to be desired. The gorgeous -views of clusters and nebulæ: the brilliantly crowded fields in the -galaxy: and the endless variety of star colours and groups are -almost beyond description, and must be seen to be appreciated. I -could fill pages about objects seen with the 11-1/4 in. mirror, but -will not occupy more space, save to thank you for having placed such -perfect instruments within the reach of amateurs. - - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ W. S. FRANKS, Esq., F.R.A.S. - - LEICESTER, - _September, 1878_. - -MY DEAR SIR,—Mr. BURNHAM is undoubtedly “facile princeps,” as an -observer of double stars. Few men could have discovered such -difficult objects as he has done with a 6″ O.G. Perhaps, it ought to -be conceded that the usual formula for separating power, requires -modification in his case, as Pogson’s formula for “minima visible” -did with Mr. J. W. WARD, of Belfast. Apropos of this topic: do you -recollect Mr. WARD’S discovery of several additional comites to Pxx -452 Cygni? One of them Mr. BURNHAM only succeeded in seeing with the -greatest difficulty with the 6″ O.G. when the large star was hidden -by a bar. I can see this easily enough with the 11-1/4 in. mirror, -in presence of the brighter star. - -When you have any intending customers halting between two opinions, -or in any way dubious about reflectors—if you will refer them to me, -I shall have great pleasure in giving my testimony. Reflectors have -been very badly used, and in justice to them I shall not hesitate to -express my honest conviction whenever it is asked. - - Yours sincerely, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ PHILIP VALLANCE, Esq. - - COBB COURT, - _August 24th, 1880_. - -DEAR Mr. CALVER,—I write to thank you for the very excellent 12 in. -mirror you have worked for me. You know that I have excellent -opportunities of testing it by those of With, Bird, Grubb, Murrell, -and others; and also for definition by a capital 6 in. refractor—and -I can truly say it is surpassed by none, and equalled by few: it is -quite my favourite instrument, and has been a source of much -enjoyment ever since I have had it, and again I thank you for -working me such a treasure. - - I am, dear Mr. Calver, - Yours very truly, - (_Signed_.) - - * * * * * - - _From_ THE REV. N. S. GODFREY, F.R.A.S. - - ST. BARTHOLOMEW’S VICARAGE, SOUTHSEA, - _August 23rd, 1879_. - -MY DEAR Mr. CALVER,—The 12 inch mirror which you worked and mounted -for me gives me perfect satisfaction. I think I have had sufficient -experience with telescopes to justify the expression of a confident -opinion, as at one time or other I think I have had 13 achromatic -telescopes by Tulley, Dollond, Simms, Jones, Cooke, and Murrell, of -apertures varying from 2-1/4 to 7 inches, besides 5 graduated -instruments. The one I had before yours was an 8-1/8 With-Browning, -which With himself considered of as fine a figure as he could work, -and it certainly was a beautiful mirror. Your 12 inch is quite as -perfect in its definition. I have soon on particularly fine evenings -Castor as two discs, without wing or ray. I have separated with ease -ω Leonis, μ^2 Boötis, η Coronæ, γ^2 Andromedæ, and many others of -that class. The planets Jupiter and Saturn on fine nights are -magnificent. I could go into more detail, but I think the facts I -have mentioned are sufficient to convey an idea of the performance -of the mirror; and I am particular in expressing my opinion thus -strongly, because great names, with no experience of reflectors, are -committed to their condemnation. I am certain that under favourable -circumstances they are _quite_ equal in their performances to -refractors. I know my 8-1/8 With was in every way equal to my 7 inch -Troughton and Simms, and in light superior, when newly silvered. And -when the cost is taken into consideration, Cooke’s price of a 10 -inch O.G. being £390, without mounting, and your price for a 10 inch -mirror being only £25, I can only express my astonishment that there -can exist any doubt in the scientific mind upon the subject. - -May I say that I shall be pleased to answer any questions that any -of your intending purchasers may wish to ask. I cannot speak too -highly of your mirror. I may add that through the courtesy of its -possessor, A. A. COMMON, Esq., I saw Jupiter magnificently last year -in the 37″ mirror you worked for him. - - I am, my dear Mr. Calver, - Very truly yours, - (_Signed_.) - - * * * * * - - _From_ THE REV. CANON ST. VINCENT BEECHEY, F.B.A.S., &c. - - HILGAY RECTORY, DOWNHAM, NORFOLK, - _June 22nd, 1880_. - -MY DEAR CALVER,—I really do not know how to thank you enough for all -the pleasure you have given me, and for the excellence of the work -you have done for me. I like it more and more! It is really as near -perfection as I could ever hope for. Even at my old age of nearly -eighty, I divide 1 second stars like two dots, and I used a power of -700 on my 8 foot. What mirror but of great excellence would bear -that power, and give you two _dots_! I would not part with it for -anything. When I have been to Mr. Bruce he is coming to me, and we -will try your 12″ against my 10″. He will have light, a great thing -for me, but I do think he _cannot_ have _better_ definition. I hope -nothing will prevent his getting the 12″ in time for us to adjust -and try it. My telescope, now complete, is a gem, and has given me -more pleasure than any other scientific thing. - - Ever most sincerely yours, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ THE REV. J. GWYNNE HUGHES, F.M.S., &c. - - MALDON, - _August 30th, 1880_. - -MY DEAR SIR,—All my friends are greatly pleased with the 10 in. -reflector you have made me—with its general appearance and superior -workmanship. - -The wheel at the foot adds very much to its portability. Tho form of -its stand is the best I have seen, giving a complete sweep without -any fresh adjustment. The cell bearing the reflector drops into its -place without any difficulty, even in the dark. The trials I have -made of the instrument have revealed a clearness and sharpness of -definition far superior to anything of the kind I had seen before. - -I am indebted to you for a source of boundless scientific interest -at very moderate expense. With best wishes, - - I remain, dear Sir, - Yours sincerely, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ WALTER GOODACRE, Esq. - - SEVEN SISTERS ROAD, - _May, 1879_. - -DEAR SIR,—Respecting the 5-1/4 in., I am very much pleased with it; -the definition is very good. - -As a proof of the excellence of the mirror, I may mention that I -have on several occasions seen the small crater on the floor of -Helicon, given by WEBB as a test for a 9 in. mirror. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - (_Another Letter from the same Gentleman._) - - * * * * * - - _From_ REV. N. S. GODFREY, F.R.A.S. - - _October 4th, 1880_. - -DEAR SIR,—I have been able to get to use my telescope again, and -send you some of my seeings. - -_June 17th._—α Herculis. Still night, 10 p.m., light haze. Discs -beautifully small. Occasionally without any interference ring. - -_July 18th._—ζ Herculis. Beautifully seen, power 496, Cooke. - -_August 16th._—Well seen. 6.15 p.m., daylight. - -_August 26th._—σ Cygni. Misty. Adjustment beautiful. Small star well -seen. ψ Cassiopeæ. Double blue star well divided. - -_August 27th._—ζ Herculis. Both stars occasionally without ray or -ring. Two brilliant dots. Colour of companion, pale lilac. Full -aperture. Power 207, Cooke. 6 p.m. - -_August 30th._—Dumb bell nebula. Well seen. Not at all regular in -outline, as given by H. Several glimpse stars. - -_September 24th._—ζ Herculis. Very fine definition. The difficulty -with this star is clearly atmospheric. I saw it well with 8 in. -aperture, power 207: but the interference ring became thicker and -continuous all round: probably owing to the larger size of my -diagonal prism, nearly 2-1/2 in. - -The planets are now again simply magnificent. All the usual details -I see splendidly with full aperture of 12 in., and as sharp as if -cut out with a penknife. The great red spot is a wonderful -phenomenon. What is it? - -The great nebula in Andromeda, seen through your fine nebula power -is a mysterious sight. You seem to be looking at something which -eludes your vision (except the nucleus); you see it, and yet, -ghost-like, you see through it. I long for the great nebula in -Orion. I did not see it last year on account of illness. - - Believe me, - My dear Mr. Calver, - Very truly yours, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ T. HILDERSLEY, Esq. - - 33, BERWICK STREET, W., - _September 9th, 1880_. - -DEAR SIR,—I am very pleased to be able to tell you that I have got -the 8-1/2 in. mirror into very fair adjustment—only a slight flare -on one side, which I could correct if clouds had not come up, and I -have not had a chance since, as my time is very limited. - -As it was, comes to Wega came out fine; also debilissima, ε Boötis, -μ^2 Boötis, was well seen. Stops made little improvement. -Nebula in Andromeda very fine, both large and small. Clusters in -Hercules well resolved, and Jupiter and Saturn, though low down, -were grand; all is clear with any power or aperture. - - Yours truly, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ REV. E. A. FISHBOURNE, M.A. - - MINOR CANONRIES, LLANDAFF, - _October 4th, 1880_. - -MY DEAR SIR,—I send you my cheque, which I do with the greatest -pleasure. I am delighted with, the telescope and with its -capabilities. - - - Faithfully yours, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ WALTER GOODACRE, Esq. - - LONDON, N., - _September 6th, 1880_. - -MY DEAR SIR,—The 8-1/2 in. mirror which you supplied me with during -the early part of the year, I have recently mounted as a simple -equatorial. And although I have had but few opportunities at present -of using the same, they have been sufficiently numerous to prove, -beyond doubt, the great excellence of the mirror. - -On the evening of the 3rd September, I had some beautiful views of -Jupiter and Saturn. In the former the deep coppery hue of the -equatorial belts, and of the large spot, at present visible on his -southern hemisphere, were strikingly conspicuous, as were many -minute details of his surface. In the case of Saturn, the gradations -of light on the rings, and the delicately tinted belts on the planet -itself, also the sharply defined shadow of the planet on the rings -were easily seen. - -Its definition of the most minute portions of the moon’s surface, -leaves nothing to be desired, whilst its performance on double stars -is equally satisfactory. - - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ G. W. ROYSTON PIGOTT, Esq., M.D., F.R.S., F.R.A.S., &c. - - EASTBOURNE, - _October 4th, 1880_. - -DEAR SIR,—Your figuring of the 8-1/2 in. mirror has given me much -satisfaction. I see ε^1 ε^2 Lyræ, and such stars, with 600, very -sharp and clear. - -I have counted six satellites of Saturn, and the equatorial belt is -very distinct. - -Last week I happened to observe Jupiter, when a black shadow of a -satellite was beautifully notching the limb, at the same time a -small pearl-like satellite was about to egress from the belts. I -saw, also, six spherical cloudlets on the belts. - - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - _From_ REV. S. MILLS, M.A. - - DROYLSDEN, - _September 4th, 1880_. - -DEAR Mr. CALVER,—To-night was an exceptional fine night for the -stars, the like of which I have not had since I got your 6-1/2 in. I -had for a week or so been observing with my home-made 8 in., but -to-night again recurred to _yours_, and its _superiority_ was all -the more _conspicuous_ and _gratifying_. It quite _delighted me -afresh_! - -Looking at β Cygni, I picked up two, to me, new stars, which I think -fully completes Mr. SADLER’S tests in this field. I mark the newly -found ones in red. This only shows that one has only patiently to -watch and wait for a suitable observing night to get one out of a -certain aperture it is capable of showing. - -The vast superiority of your 6-1/2 in. _has quite put my 8 inch out -of court_!! - - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - (_From the same Gentleman._) - - DROYLSDEN RECTORY, MANCHESTER, - _July 30th, 1880_. - -MY DEAR SIR,—I can scarcely express to you the pleasure I have had -in the use of your splendid 6-1/2 in. mirror. Both in light grasping -power, and in the separation of close double stars, it has far -surpassed my highest expectations, and quite removed the prejudice I -entertained against reflectors. Though possessing a first-class -4-1/4 in. refractor, equatorially mounted in my observatory, I now -use your mirror in preference, in the more difficult and delicate -observations, and with it am able to master the most severe tests -given for 6-1/2 in. aperture. I would not be without one of your -finely figured mirrors, now that I have tested their merits, for any -consideration. - - (_Signed_.) -Mr. G. CALVER. - - * * * * * - - (_From the same Gentleman._) - - - THE RECTORY, DROYLSDEN, - _October 4th, 1880_. - -MY DEAR SIR,—I have given your 8-1/2 in. mirror one night’s trial, -and it has greatly pleased me. Its resolution of the great cluster -in Hercules was truly magnificent, and the clearness with which it -brought out stars very difficult with 6-1/2 in. was highly -gratifying. I hope to turn it to very good account. - - Yours faithfully, - (_Signed_.) -Mr. G. CALVER. - - - [ Illustration: (FIG. 1.) ] - - - [ Illustration: (FIG. 2.) ] - - - [ Illustration: (FIG. 3.) ] - - - Printers: WARREN HALL & LOVITT, 88, Camden Road, London. N.W. - - -TRANSCRIBER’s NOTE: -The constelation Boötis also appears in the original spelt as Bootis -and Böotis these have all been corrected to Boötis without being -noted below. Also variations in hyphenation remain unchanged. - -The following typos are corrected in this e-text and are shown within -the text as - - Original Replaced by - Page 4 Many of the speculas Many of the specula - Page 7 on the pitch or the pitch - Page 24 and proceed and proceed. - Page 28 kept is glass kept in glass - Page 31 See p. 27, Fig. 4 See p. 20, Fig. 4 - Page 39 (prices for 8-1/2 then 6-1/2) (prices for 6-1/2 then 8-1/2) - (appendix) - Page 14 Rev. W. P. MATTHEWS REV. W. P. MATTHEWS - Page 24 (a 10 in. Alt Azimuth. (a 10 in. Alt Azimuth). - Page 32 Rev. E. A. FISHBOURNE, M.A. REV. E. A. FISHBOURNE, M.A. - - - - - -End of the Project Gutenberg EBook of New Edition of Hints on Silver-Glass -Reflecting Telescopes Manufactured b, by George Calver - -*** END OF THIS PROJECT GUTENBERG EBOOK SILVER-GLASS REFLECTING TELESCOPES *** - -***** This file should be named 53494-0.txt or 53494-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/3/4/9/53494/ - -Produced by Eric Hutton and the Online Distributed -Proofreading Team at http://www.pgdp.net - - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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