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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/75519-0.txt b/75519-0.txt new file mode 100644 index 0000000..913e6c2 --- /dev/null +++ b/75519-0.txt @@ -0,0 +1,1808 @@ + +*** START OF THE PROJECT GUTENBERG EBOOK 75519 *** + +Transcriber’s notes: + +The text of this e-book has been preserved as in the original, except +for the correction of a single typo (ests → test), the insertion of a +missing footnote anchor and the shifting of footnotes below the relevant +paragraph. + +Italic text is denoted by _underscores_, and bold text by =equals= symbols + + + + + THE HUNTERIAN LECTURES + + ON + + COLOUR-VISION AND COLOUR-BLINDNESS + + + + + THE + + HUNTERIAN LECTURES + + ON + + COLOUR-VISION AND + COLOUR-BLINDNESS + + + _Delivered before the Royal College of Surgeons of England + on February 1st and 3rd, 1911_ + + + BY + + PROFESSOR F. W. EDRIDGE-GREEN + + M.D.Durh., F.R.C.S.Eng. + + BEIT MEDICAL RESEARCH FELLOW + + + KEGAN PAUL, TRENCH, TRÜBNER _&_ Co., Ltd. + 43 GERRARD STREET, LONDON, W. + 1911 + + + + +PREFACE + + +As there are many who are interested in the subject of vision and +colour-blindness who are not acquainted with the structure of the eye, +I will give a few details so that these persons may be able to consider +the problem from the point of view of these lectures. + +The eye is very similar to a photographic camera, and an actual image +is formed on the back of the eye just as it is on the plate of the +photographic camera or on the view-finder. The eye possesses a lens and +also an iris which acts as an adjustable stop and regulates the size +of the pupil. The membrane at the back of the eye upon which the image +is formed is called the retina. The retina has several layers, but the +sensitive layer consists of two elements called, from their shape, rods +and cones. The problem therefore which has to be considered is, how is +the light which forms the image on the sensitive layer of the retina +transformed into visual impulses? + +Those who are interested in the subject will find further details in my +book on _Colour-Blindness and Colour-Perception_ in the International +Scientific Series. In that book there are three plates which show how +the colour-blind see colours. + +I have been annoyed to find that unauthorised persons have made +lanterns professing to be mine but grossly inaccurate. The sole makers +are those mentioned on page 53 in this book. + + F. W. Edridge-Green. + + The Institute of Physiology, + University College, + Gower Street, London. + + + + +CONTENTS + + +LECTURE I + +THE THEORY AND FACTS OF COLOUR-VISION AND COLOUR-BLINDNESS + + PAGE + + THE VISUAL PURPLE THE ESSENTIAL FACTOR IN VISION 11 + + 1. Anatomical Evidence 11 + + 2. Physiological Analogy with other Body Cells 12 + + 3. The Relation between the Foveal and the Extra-Foveal + Regions 13 + + 4. The varying Sensibility of the Fovea 13 + + 5. Chemical Analogy 14 + + 6. Disappearance of Lights falling upon Fovea 15 + + 7. Illusion of Moving Light 16 + + 8. Purple After-Image 17 + + 9. Currents seen in the Field of Vision not due to the + Circulation 17 + + 10. Pressure Figure 19 + + 11. Macular Star 19 + + 12. Entoptic Appearance of Cone Mosaic 20 + + 13. Visual Acuity 21 + + THE EVOLUTION OF THE COLOUR-SENSE 26 + + THE FACTS OF COLOUR-BLINDNESS 34 + + 1. Defects of Light-Perception 35 + + 2. Defects of Colour-Perception 38 + + THE TWO MAIN VARIETIES OF COLOUR-BLINDNESS: + + 1. Dichromic Vision 40 + + 2. Trichromic Vision 42 + + + LECTURE II + + THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW + + 1. Object of a Test for Colour-Blindness 44 + + 2. The Requirements of a Test for Colour-Blindness 45 + + 3. Persons to be Excluded 47 + + 4. The Construction of a Test for Colour-Blindness 48 + + 5. The Lantern Test 53 + + 6. Other Tests for Colour-Blindness 66 + + + + +LECTURE I + +_Delivered on February 1st_ + + +GENTLEMEN,--Colour-blindness is not a good name for the condition +to which it is applied, and still worse is the use of the term +red-blindness or green-blindness. In the majority of cases of +colour-blindness there is no blindness to colours in the ordinary +acceptation of the term; a green, red, or yellow light produces a very +definite sensation of colour. Those who confuse red and green do so, +not because they see red as green or green as red, but because both +give rise to a similar sensation of colour. The word light must be +used in the sense of referring to those waves which excite the organ +of vision. Because two stimuli excite a sensation of light, it does +not follow that they are similar. We cannot, for instance, distinguish +by the eye polarised light from non-polarised light. We have to +distinguish between the physical stimuli by their physical properties +apart from their effect on the organ of vision. I propose to divide the +subject into two parts, and in this lecture to deal with the theory and +facts of colour-vision and colour-blindness, and in the second lecture +with the detection of colour-blindness from a practical point of view. + + + + +I. THE THEORY AND FACTS OF COLOUR-VISION AND COLOUR-BLINDNESS + + +The following is the theory which I have propounded in order to explain +vision and colour-vision. A ray of light impinging on the retina +liberates the visual purple from the rods and a photograph is formed. +The rods are concerned only with the formation and distribution of +the visual purple, not with the conveyance of light-impulses to the +brain. The ends of the cones are stimulated through the photo-chemical +decomposition of the visual purple by light (very probably through the +electricity which is produced), and a visual impulse is set up which is +conveyed through the optic-nerve fibres to the brain. The character of +the stimulus differs according to the wave-length of the light causing +it. In the impulse itself we have the physiological basis of the +sensation of light, and in the quality of the impulse the physiological +basis of the sensation of colour. The impulse being conveyed along +the optic nerve to the brain, stimulates the visual centre, causing +a sensation of light, and then passing on to the colour-perceiving +centre, causes a sensation of colour. But though the impulses vary +in character according to the wave-length of the light causing them, +the retino-cerebral apparatus is not able to discriminate between the +character of adjacent stimuli, not being sufficiently developed for the +purpose. At most, seven distinct colours are seen, whilst others see +in proportion to the development of their colour-perceiving centres, +only six, five, four, three, or two. This causes colour-blindness, the +person seeing only two or three colours instead of the normal six, +putting colours together as alike which are seen by the normal-sighted +to be different. In the degree of colour-blindness just preceding +total, only the colours at the extremes of the spectrum are recognised +as different, the remainder of the spectrum appearing grey. Though my +own opinion is that the ordinary form of congenital colour-blindness +is caused by a defective development of the portion of the brain which +has the function of the perception of colour, we must not exclude any +portion of the retino-cerebral apparatus, defect of which would have +exactly the same result. It will be noticed that the theory really +consists of two parts, one concerned with the retina and the other with +the whole retino-cerebral apparatus. I shall in these lectures use the +word cerebral in this sense. I am not aware of a single fact which does +not support this theory, and I have used it to predict facts which have +subsequently been rediscovered by others and now form a part of our +common knowledge. + + +THE VISUAL PURPLE THE ESSENTIAL FACTOR IN VISION + +I will now state very briefly the evidence which supports the view that +the visual purple is the essential factor in the retina which enables +it to transform light into visual impulses. + +1. _Anatomical._--In the fovea of the retina only cones are to be +found. Immediately external to this each cone is surrounded by a ring +of rods. The number of rings of rods round each cone increases as the +periphery is reached. The outer segments of the cones are situated in +a space which is filled with fluid. The external limiting membrane +retains this fluid in its place. I find[1] four depressions or canals +which lead into the larger depression of the external fovea. These +canals appear to have smaller branches, and serve to conduct the +visual purple into the part of most acute vision. The cones which are +present in the fovea have very long outer segments which would present +a greater surface for photo-chemical stimulation. The visual purple +is only to be found in the rods and not in the cones. I determined to +ascertain whether the visual purple could be seen between the cones +in the fovea. I have examined under the microscope the retinas of two +monkeys which had been kept previously in a dark room for forty-eight +hours. The yellow spot was the reddest part of the whole retina, and +the visual purple was seen to be between and not in the cones.[2] + +[1] _Journal of Physiology_, vol. xli, p. 274. + +[2] _Transactions of the Ophthalmological Society_, 1902, p. 300. + +2. _Physiological analogy with other body cells._--It is far more +probable that the rods should produce a secretion which would affect +other cells rather than themselves. The liver cells do not form bile +in order to stimulate themselves, and the internal secretions are +produced to affect other parts of the body. I am not aware of a single +instance in which a cell produces a secretion which has the function +of stimulating the cell producing it. The visual purple is regenerated +in the rods by the pigment cells in connection with them. + +3. _The relation between the foveal and the extra-foveal regions._--As +the fovea only contains cones, if any of the older theories of the +relative functions of the rods and cones were true we should expect +to find qualitative differences between the foveal and extra-foveal +regions. This is not the case, but as we should expect if the visual +purple were the visual substance, all the phenomena which have been +attributed to the visual purple should be found in the fovea. Von +Tschermak, Hering, Hess, Garten and others have found the Purkinje +phenomenon, the variation in optical white equations by a state of +light and dark adaptation, the colourless interval for spectral lights +of increasing intensity, and the varying phases of the after-image in +the fovea only gradually diminished. + +4. _The varying sensibility of the fovea._--The fovea is in some +conditions the most sensitive part of the whole retina, and with other +conditions the least. Helmholtz has recorded some of these facts +and regarded them as quite inexplicable. We have, however, an easy +explanation of the facts on the assumption that when there is visual +purple in the fovea this is the most sensitive part of the whole +retina, but when there is none there time must elapse before it can +diffuse into the spot, and in the meantime it is insensitive to light. +I have devised several experiments which show the visual purple flowing +into the foveal region. The following simple experiment shows this very +well. If on awaking in the morning the eyes be directed to a dull +white surface, as for instance the ceiling, the region of the yellow +spot will appear as an irregular black spot, and light will appear to +invade this spot from without inwards. If the eyes be now closed and +covered with the hands, purple circles will form round the centre of +the field of vision and gradually contracting reach the centre. When +the circle reaches the centre it breaks up into a star-shaped figure +and becomes much brighter. It then disappears and is followed by +another contracting circle. Now it will be noticed that if one eye be +opened when the circle has broken up, a brilliant rose-coloured star +much brighter than any other part will be seen in the centre of the +field of vision. This has the exact hue of the visual purple. If we +wait until the star has disappeared before opening an eye, the macular +region appears as a black spot as before. This conclusively shows that +the central portion of the retina is sensitised from the peripheral +portions. + +5. _Chemical analogy._--The visual purple gives a curve which is very +similar to that of many other photo-chemical substances. We know that +with photo-chemical substances the chemical effect is not proportional +to the intensity of the light. That is, a different curve is obtained +with weak light from that which is formed with light of greater +intensity. It is reasonable, therefore, to suppose that the visual +purple which is formed by the pigment cells under the influence of a +bright light would be somewhat different in character from that which +is formed in darkness. Again, from the chemical analogy which I have +just given, even if the visual purple were of the same character, we +should not expect similar curves with different intensities of light. +It is probable that both factors are in operation. This deduction +gives an explanation of the Purkinje phenomenon, or the fact that when +the eyes are adapted to darkness the point of greatest luminosity is +shifted more towards the violet end of the spectrum. Some dichromics +who have shortening of the red end of the spectrum have a luminosity +curve which is very similar to that of a normal-sighted person with a +spectrum of lesser intensity. We have only to assume in these cases +either that the receiving nervous apparatus is less responsive, or +that the visual purple formed at one intensity of light is similar to +that formed at a lower intensity by a normal-sighted person. We also +have an explanation of other conditions, such as erythropsia, or red +vision, white objects appearing more or less red. If we suppose that +the eye has remained in a state of light adaptation, the visual purple +produced being more sensitive to the red rays, objects appear of this +colour. As we should expect, erythropsia is frequently associated +with hemeralopia, or difficulty in seeing in the twilight, the eyes +being adapted to light and not to darkness. In green vision the eyes +have probably remained in a condition of more or less adaptation to +darkness, and are therefore more sensitive to the green rays. + +6. _Disappearance of lights falling upon fovea._--If the cones are not +sensitive to light, a ray of light falling upon the fovea alone and not +upon the adjacent portion of the retina containing rods should produce +no sensation of light, provided that there is not already any visual +purple in the fovea. It has been known to astronomers for a long time +that if a small star in a dark portion of the sky be steadfastly looked +at, it will disappear from view, whilst other stars seen by indirect +vision remain conspicuously visible. The following simple experiment +shows the same thing. If a piece of black velvet about three feet +square on a door have a pin put in the centre, and the source of light +be behind the observer, the pin will be brightly illuminated; and on +looking at it (the observer not being too close) and keeping the eye +quite still, the pin will disappear, the visual substance diffused +into the fovea centralis being used up and not renewed. When viewed by +indirect vision it is impossible to make it disappear in this way. When +I have taken great care to have very dark surroundings and have used +only one eye, I have made moderately bright lights disappear in this +manner. These facts have been attributed to a defective sensibility +of the fovea for feeble light. The important point, however, that the +light is at first most clearly seen by the fovea and only subsequently +fades, has been overlooked. If these facts were due to a defective +sensibility of the fovea, the star or light would not be visible at +first. + +7. _Illusion of moving light._--If a small light be looked at fixedly +in a dark room, it will appear to move until it comes apparently so +close that it could be grasped. The reason of this is that the eye +moves so that the light falls upon a more peripheral part of the +retina. I find that the movement takes place as if some photo-chemical +substance acted under the influence of gravity. For instance, when +standing the light appears to travel upwards; resting the head on one +side, it appears to travel in the opposite direction. The light appears +as if we were looking straight at it, and the eye, which is covered up, +remains directed straight at the object. When the second eye is opened +two images of the light are seen, and the image which is seen with the +periphery of the first eye rapidly coalesces with that seen directly by +the second eye. + +8. _Purple after-image._--A positive after-image of a purple (rose) +colour can be obtained after white light or any spectral colour. It +will be noticed that when there is little light during the subsequent +observation the colour of an after-image inclines to blue or green, +when there is more light towards purple or red. + +[Illustration: Fig. 1.] + +9. _Currents seen in the field of vision not due to the +circulation._--It occurred to me that if there were canals in the +retina which promoted the easy flow of the visual purple into the +fovea, we ought to obtain evidence of the currents flowing along these +channels entoptically. I found that this was the case, and that the +currents could be seen in numerous ways.[3] If one eye be partially +covered with an opaque disc whilst both eyes are directed forwards in +a not too brightly illuminated room, and special attention be paid to +the covered eye, an appearance of whirling currents will be seen with +this eye (see Fig. 1). These currents appear to be directed towards the +centre, and have a very similar appearance to a whirlpool which is fed +by four main branches. These again are fed by smaller branches which +continually change their paths. On closing both eyes all the portion +in which the whirling currents are seen appears as dull purple. These +currents cannot be due to vessels, because we know that the centre +of the retina corresponding to the point where the greatest movement +is seen is free from vessels. The appearance is also very different +from that of the movement of blood in vessels. The currents can also +be seen in the light, in the dark, through yellow-green glass, and +with intermittent light. The main branches form a star-shaped figure +with four rays. The currents carry the visual quality, colour, and +brightness from whence they come into an after-image. They also tend +to move an after-image towards the centre. The currents behave as if +they ran in definite channels, but could also overrun, on any further +stimulus, the banks of the channels. For instance, a thin, bright line +with a little more light appears as a broad band, and the central star +figure will enlarge into a rhomboid, oval or disc. Movements of the +eyes affect the broad currents in the outer part of the field of vision. + +[3] _Journal of Physiology_, vol. xli, p. 269. + +10. _Pressure figure._--Pressure on the front of the eye causes the +star-shaped figure to be seen, and this changes into a rhomboid with a +little more pressure. + +11. _Macular star._--It occurred to me that we ought to obtain evidence +of the canals in the retina in cases where the outflow from the retina +is obstructed, as by tumour. I find this is the case; the star-shaped +figure given by Sir Victor Horsley in his paper on tumour of the +frontal lobe[4] is almost exactly the same as that seen subjectively. + +[4] _British Medical Journal_, 1910, p. 556. + +[Illustration: Fig. 2.] + +12. _Entoptic appearance of cone mosaic._--Appearances corresponding +to the cone mosaic of the retina may be seen in several ways[5] (see +Fig. 2). The appearance seen corresponds to the cone distribution of +the retina as viewed from its outer side, the portions occupied by rods +appearing as dark spaces. + +[5] _Journal of Physiology_, vol. xli, p. 226. + +13. _Visual acuity._--Visual acuity is most acute with the fovea, and +diminishes from within outwards. It corresponds very fairly with the +cone distribution of the retina. On the other hand, there is not one +single fact which points to the rods as being light-sentient organs. +This is well recognised by those best qualified to judge.[6] I could +give many more facts in support of the view that the visual purple is +the visual substance, and I have not yet had brought to my notice any +fact which is not readily explicable on that hypothesis. There may be +other photo-chemical substances in the retina, but there is not the +slightest evidence that such is the case. I regard the visual purple as +the sole visual substance. We could, of course, split the visual purple +into innumerable simpler photo-chemical substances, each of which has +its own absorption curve, having its maximum in some particular part of +the spectrum. It is difficult to say at present exactly how the visual +purple acts as a stimulus transformer, but this is because so many +plausible hypotheses immediately occur to us. It is very probable that +light acting upon the visual purple is, according to its wave-length, +absorbed by particular atoms or molecules, the amplitude of their +vibrations being increased. These vibrations may cause corresponding +vibrations in certain discs of the outer segments of the cones, which +seem especially constructed to take up vibrations. We know that when +light falls on the retina it causes an electric current. We know how +the telephone is able through electricity to convey waves of sound, +and something similar may be present in the eye, the apparatus being +especially constructed for vibrations of small wave-length. The current +of electricity set up by light may cause the sensation of light, and +the vibrations of the atoms or molecules the sensation of colour. + +[6] _Nagel. Physiol. des Menschen_, vol. iii, p. 107. + +In all vital processes there is a condition of katabolism or chemical +change in the protoplasm, and an anabolic or building-up process, in +which the protoplasm is restored to its normal state. We have therefore +to consider two definite processes in the visual purple--namely, a +breaking down of the visual purple photo-chemically by light and its +restoration by the pigment cells and rods. Under ordinary conditions +of light, and during the whole of the daytime, the visual purple is +continually being bleached and reformed. It is obvious, therefore, +that when the eye has been kept in the dark and is then exposed to +light, an observation taken immediately will not be comparable with +one taken a few seconds afterwards, because in the first observation +we have only to consider the katabolic change; whilst in the second +observation the anabolic change has to be considered as well, as the +visual purple has to be reformed for subsequent seeing. There appears +to be very little evidence in ordinary circumstances of this anabolic +process; for instance, if we fatigue the eye with sodium light in a +dark room, and then immediately examine a spectrum, we find that +though all the yellow has disappeared there is no increase in the blue; +in fact, the blue seems rather diminished than otherwise. Again, there +is not the slightest diminution in either the red or green, showing +conclusively that yellow cannot be a compound sensation made up by a +combination of red and green. We must therefore explain in another way +the apparent trichromatism of normal colour-vision, which is so well +known to every photographer, especially those who are concerned with +colour photography. If my theory of the evolution of the colour-sense +be the correct one, and we have cases of colour-blindness corresponding +to every degree of the evolutionary process, we have an explanation +of the facts. In past ages all saw the rainbow made up of only three +colours--red, green, and violet. When a new colour (yellow) appeared +between the red and green, it is obvious that a mixture of red and +green would give rise, not to red-green, but to the colour which had +replaced it--namely, yellow. The retina, therefore, corresponds to a +layer of photo-chemical liquid in which there are innumerable wires +each connected with a galvanometer. When light falls upon a portion of +this fluid the needle of the galvanometer corresponding to the nearest +wire is deflected. The wires correspond to the separate fibres of the +optic nerve, and the galvanometers to the visual centres of the brain. + +Cases of colour-blindness may be divided into two classes, which are +quite separate and distinct from each other, though both may be +present in the same person. In the first class there is light as well +as colour loss. In the second class the perception of light is the +same as the normal-sighted, but there is a defect in the perception +of colour. In the first class certain rays are either not perceived +at all or very imperfectly. Both these classes are represented by +analogous conditions in the perception of sounds. The first class +of the colour-blind is represented by those who are unable to hear +very high or very low notes. The second class of the colour-blind is +represented by those who possess what is commonly called a defective +musical ear. Colour-blind individuals belonging to this class can be +arranged in a series. At one end of this series are the normal-sighted, +and at the other end the totally colour-blind. The colours appear +at the points of greatest difference, and I have classified the +colour-blind in accordance with the number of colours which they see in +the spectrum. The normal-sighted may be designated hexachromic; those +who see five colours, pentachromic; those who see four, tetrachromic; +those who see three, trichromic; those who see two, dichromic; and +those who see none, totally colour-blind. There are many degrees +included in the dichromic class. There may or may not be a neutral +band, and this is widest in those cases approaching most nearly to +total colour-blindness. I have recorded a case of a patient who was +colour-blind with one eye.[7] It is an interesting fact that for +form vision the colour-blind eye was much the better of the two, and +he could recognise fine lines in the spectrum with this eye which +were not visible to the other. He saw the two ends of the spectrum +tinged with colour and the remainder grey. It will be noticed that +his colour sensations were limited to the extreme red and the extreme +violet--namely, those colours which present the greatest physical +contrast to each other. Neither the red nor the violet appeared of the +nature of a primary colour, but gave the impression that they were +largely diluted with grey. A theory of colour-vision must account for +a case of this kind, and also for the other varieties and degrees of +colour-blindness. The trichromic are a very important class, and any +theory must account for the fact that they see yellow as red-green, and +blue as violet-green. As we should theoretically expect, when there is +shortening of the spectrum the centres of the colours are moved towards +the unshortened side. + +[7] “Colour Blindness and Colour Perception,” _International Scientific +Series_, p. 196. + +I will now show on the screen some representations of pictures painted +by colour-blind persons. The upper picture is the copy, and the one +below is the one which has been painted by the colour-blind artist. +He has been given a selection of colours on plates, and from them +has selected the one which he thought appropriate in each case. It +will be noticed that the mistakes made are characteristic of the +colour-perception of the person painting them. Whenever I show these +pictures, I am asked why it is that these characteristic mistakes +should be made, and that the true colour of the object is not used +instead? This undoubtedly would be the case if the artist were allowed +to match the colours by directly comparing them. But he is not able to +do this; he looks at the copy and decides upon the colour of an object, +and then looks for the colour with which to paint it. + +A man rarely uses a hue which he does not see as a definite colour, and +therefore it has been quite possible for me to pick out those who are +more or less colour-blind in the exhibitors of the picture gallery. For +instance, if a trichromic have to paint a yellow object he will decide, +after looking at it, whether it be a red or green in his estimation, +and represent it accordingly. He will be greatly influenced by the +nature of colours in its immediate proximity, because simultaneous +contrast is increased in the colour-blind. Thus he will certainly +represent a yellow which is adjacent to a red as green, and a yellow +which is adjacent to a green as red. + + +THE EVOLUTION OF THE COLOUR-SENSE + +There can be no doubt that an evolution of the colour-sense has taken +place: the only point is how and when did this occur. It is obvious +that in those low forms of animal life in which the most rudimentary +sense of sight exists there can be no sense of colour. The animal which +can only perceive light and shade can only discriminate in a rough way +between varying intensities of the stimulus. It is obvious, therefore, +that the sense of light must have been developed first and then the +sense of colour. The sense of sight must have been first developed +for those waves which produce their maximum effect upon the sensitive +protoplasm. The next process of development would be for the protoplasm +to become sensitive to the waves above and below those which produced +the primary stimulus. In the physical stimulus which produces the +sensation of light there are two factors to be considered, the length +of the wave and its amplitude: the greater the amplitude within certain +limits the greater the intensity of the sensation. The wave-length of +the physical stimulus is the physical basis of the sensation of colour. +How did the sensations of colour first arise? Let us suppose that the +physiological effect of the physical stimulus differed according to the +wave-length of the physical stimulus. + +Let us consider that the eye has reached a stage in which it has +become sensitive to a fair range of the spectral rays; that is to +say, evolution has proceeded to the extent of making the protoplasm +sensitive to rays of light considerably above and below those which +first caused a sensation of light. We now have an eye which is +sensitive to the greater part of the rays which form the visible +spectrum. It is, however, an eye which is devoid of the sense of +colour; no matter from what part of the spectrum the rays be taken the +only difference which will be appreciated will be one of intensity. +I however mentioned that in the physical stimulus there were two +variables, wave-length and amplitude of the wave. Let us now suppose +that a fresh power of discrimination was added to the eye and that it +became able to discriminate between different wave-lengths of light. +What would be the most probable commencement of development of the +sense of colour? Undoubtedly to my mind the differentiation of physical +stimuli which were physically most different. That is to say, the eye +would first discriminate between the rays which are physically most +different in the visible spectrum, the red and the violet, that is +presuming the eye had become sensitive to this range. It is probable +that it had not, and there has been a steady evolution as to the extent +of the spectrum perceived as well as to colour. We have examples of +this in those cases of defective light-perception in which there is +shortening of the red or violet end of the spectrum. + +Let us now work out the evolution of the colour-sense on the assumption +that the rays which are physically most different, namely, red and +violet, were those which were first differentiated. We know that the +various rays differ in their effects on various substances; the red +rays are more powerful in their heating effects, whilst the violet +rays are more active actinally, as is well known by the readiness with +which they act upon a photographic plate, which is scarcely affected by +red light. We should now have an individual who would see the spectrum +nearly all a uniform grey of different degrees of luminosity, but with +a tinge of red at one end and a tinge of violet at the other. There is +a great deal of evidence to show that this is how the colour-sense was +first developed. For instance, in the degree of colour-blindness just +preceding total the spectrum is seen in this way. I have also examined +a woman who became totally colour-blind, apparently through disease +of the ear. I examined her when she had recovered a certain amount of +colour sensation; her sensations were confined to the extreme red and +violet. As the colour sense developed it was not necessary that the +rays should differ so much in refrangibility before a difference was +seen, and so the red and violet gradually invaded the grey or neutral +band, until at a certain point they met in the centre of the spectrum. +Such cases are called dichromics. + +The next stage of evolution of the colour-sense is when the +colour-perceiving centre is sufficiently developed to distinguish three +main colours in the spectrum. The third colour, green, appears in the +centre of the spectrum, that is, at the third point of the greatest +physiological difference. In accordance with the prediction of the +theory, I found a considerable number of persons who saw the spectrum +in this way, about 1·5 per cent of men. The trichromic see three main +colours in the spectrum--red, green, and violet. They usually describe +the spectrum as consisting of red, red-green, green, green-violet, and +violet. They do not see yellow and blue as distinct colours, and are +therefore in continual difficulty over them. There are very few of the +tests in general use which can detect them, especially if names be not +used. They will usually pass a matching test with ease. An examination +with the spectrum shows that their colour-perception is less than the +normal in every part, though the curve has the same general shape. +The three trichromics described in my recent paper[8] on “The Relation +of Light-Perception to Colour-Perception” each saw ten consecutive +monochromatic patches in the spectrum instead of the eighteen or +nineteen seen by those who see six colours in the spectrum. It is easy +to show that the trichromic are dangerously colour-blind. They will +mark out with my colour-perception spectrometer a patch containing +greenish yellow, yellow, and orange-yellow, and declare that it is +absolutely monochromatic. When tested with coloured lights they find +great difficulty with yellow and blue. Yellow is continually called red +or green. + +[8] _Proceedings of the Royal Society_, vol. B 82, 1910, p. 458. + +There are several other degrees of colour perception, and it may +be well to say a word or two about them, though I class all above +the trichromic with the normal-sighted for practical purposes, as +they are not dangerously colour-blind, and can always, in ordinary +circumstances, distinguish signal lights correctly. In the next +stage of evolution four colours are seen in the spectrum, and the +fourth colour appears at the fourth point of greatest physiological +difference, namely, at the orange-yellow of the hexachromic or +six-colour people. These persons I have designated “tetrachromic,” +because they see four distinct colours in the spectrum, that is, red, +yellow, green, and violet. They do not see blue as a definite colour, +and are continually classing blues with greens; they usually prefer +to call blue, purplish green. In the next stage of evolution there +appeared those who see five colours in the spectrum--red, yellow, +green, blue, and violet, blue being now recognised as a definite +colour. These are the pentachromic group. These people pass all the +tests in general use with ease. They, however, have a definitely +diminished colour-perception compared with the normal, or those who see +six colours in the spectrum. They mark out in the spectrum only fifteen +monochromatic patches instead of eighteen. They cannot see orange as a +definite colour; for instance, they can never tell whether a strontium +light, which is red, or a calcium light, which is orange, is being +shown them. + +In the next stage of evolution orange is recognised as a definite +colour, and thus we get the hexachromic or normal group, and, as +we should theoretically expect, the yellow of the pentachromic is +now split up into two colours--orange and yellow. The last stage of +evolution which we appear to have reached are those who see seven +colours in the spectrum, and the additional one is called indigo. These +constitute the heptachromic group, and this seventh colour appears at +the exact point at which it should appear, according to my theory, +namely, between the blue and violet. Persons belonging to this class +have a marvellous colour-perception and memory for colours. They will +indicate a certain shade of colour in the spectrum, and then next day +will be able to put the pointer at precisely the same point, a feat +which is quite impossible to the ordinary normal-sighted person. They +see a greater number of monochromatic patches in the spectrum than the +hexachromic, but the curve has the same form. The marking out of the +heptachromic does not appear correct to those who see six colours; for +instance, the blue appears to invade the green, and the indigo does +not appear a definite colour at all. If, however, we bisect the blue +of the seven-colour man, and then bisect his indigo, on joining the +centres we get the blue of the six-colour man, showing most definitely +that the blue has been split up into two fresh colours. It will be +noticed that there is room for much further evolution, and we could go +on splitting up the spectrum indefinitely if only we had the power to +distinguish these finer differences, but as a matter of fact I have +never met with a normal-eyed man who could see more than twenty-nine +monochromatic patches in the spectrum, and there are really millions, +though by monochromatic patches I do not mean twenty-nine separate +colours. Not only are all the details of the process of the evolution +of the colour-sense supported by all the facts that we can obtain +from literature and museums, but the theory accounts for facts which +were previously inexplicable. The distinction between light-sensation +and colour-sensation is explained, and all facts of colour-mixing, +complementary colours, and simultaneous contrast. We can understand +how, as in many cases which have been recorded, a man may lose his +colour-perception and still have an unaltered sense of luminosity and +visual acuity. + +The explanation of complementary colours is a fundamental part of the +theory. It is obvious that the two colours of the dichromic are only +recognised as different because they are seen in contrast to each +other, and that when they are mixed they neutralise each other. It is +the same with the other colour-sensations, when they are developed they +replace the colours occupying their positions. Therefore green which +replaces the grey of the dichromic should be, and is, complementary to +the other two colour-sensations, red and violet combined. In the same +way when the yellow sensation replaces the red-green of the trichromic +it should be possible to compound it of both. Also, when the green +sensation is in a feeble state of development it will not have the +value that it has at a subsequent stage, and, therefore, yellow will be +a much redder colour to those persons than the normal, and in a colour +match of red and green forming yellow, more green will be required. + +Simultaneous contrast is also explained. When two colours are +contrasted each appears to be a colour higher or lower, as the case +may be, in the spectrum scale; that is to say, the close comparison +exaggerates the difference. As the colour-blind have fewer colours, +simultaneous contrast should be greater with them, and this I have +found to be the case. + +There may be some relation between the monochromatic patches and the +discs in the outer segments of the cones. These are about sixteen in +number in the guinea-pig. As in photography, the intensity of the +light is a very important factor in vision. With colours of moderate +intensity, the periphery of the retina is found to be colour-blind, +but this apparent colour-blindness disappears when more intense lights +are used. A person may have shortening of the spectrum with light +of moderate intensity, but when the light is increased be able to +recognise the spectrum to its normal limit. The change in steepness of +gradation, according to the intensity of the light, is well known to +photographers. The Purkinje effect, a change in maximum sensitiveness +of the eye according to the intensity of the light, is, in my opinion, +a photo-chemical effect. I find that the Purkinje effect is found +for small portions of the retina if a black object has been situated +in the corresponding part of the field of vision. The yellow pigment +which is found in the yellow spot probably acts like the yellow screen +in photography, which, by absorbing the blue and violet rays of the +atmosphere, renders visible that which would otherwise be invisible. +This is further borne out by the fact that hunters in India are able to +hunt later in the day than usual by using spectacles glazed with golden +yellow glass. + + +THE FACTS OF COLOUR-BLINDNESS + +When we consider the path along which a visual impulse has to pass, +and that each cell has probably some special function in connection +with that impulse, it is not surprising that we meet with a large +number of different defects of colour-perception and light-perception. +Defects of light-perception are quite distinct from defects of +colour-perception. + +1. _Defects of light-perception._--The person having the defect is +placed in a similar position to a normal-sighted person with those +particular rays removed or reduced to the same intensity. Defects of +light-perception may be caused by absorption or by some defect in +the visual purple or cerebro-retinal apparatus. The chief defect of +light-perception which is found is shortening of the red or the violet +end of the spectrum. Let us consider the influence of a shortened +spectrum upon colour-vision. The first evident fact is that bodies +reflecting only light, the rays of which occupy the missing portion of +the spectrum, appear black. + +Nearly all colours are compound; that is to say, the coloured body +reflects other rays than those of the colour seen. Thus a blue-green +glass may transmit the green, blue, and violet rays of the spectrum. +Let us suppose that we have a substance reflecting the green, blue, +and three-quarters of the violet, the colour of the body to a normal +person being green. Then if we had another substance which reflected +the whole of the violet, it would appear blue. But with a person who +could not perceive the terminal fourth of the violet the colour would +look exactly the same as the green one, and as he could not distinguish +between the two he would be in continual difficulty with blues and +greens. All coloured objects reflecting rays occupying the missing +portion appear darker than they do to the normal-sighted, and are +always matched with darker colours belonging to a point more internal. +Thus a dichromic with a shortened red end of the spectrum matches a red +with a darker green. + +It will be noticed that a shortened spectrum, especially if one +end only be affected, may interfere very little with the general +appreciation of shade. If, for instance, we take a case in which the +red end of the spectrum is shortened, so that only three-quarters +of the red of the normal-sighted is seen, then all bodies which +equally reflect or transmit these rays can be correctly compared, +because a similar portion of light has been removed from each. It is +only when one colour reflects or transmits the rays occupying the +shortened portion, and the other does not, that there is any definite +interference with the appreciation of shade. Again, if neither colour +reflects or transmits rays occupying the shortened portion of the +spectrum, there will obviously be no interference with the appreciation +of shade. + +A very common mistake due to shortening of the red end of the spectrum +is the confusion of pink and blue. If a person with considerable +shortening of the red end of the spectrum is shown a pink which is made +up of a mixture of red and violet, the red consisting of rays occupying +the missing portion of the spectrum, only the violet is visible to him, +and so the pink appears a violet without a trace of red. This pink is +therefore matched with a violet or blue very much darker than itself. + +Mistakes which are due to shortening of the spectrum may be remedied +if we subtract the rays occupying the missing portion from the colour +of confusion. For instance, if we take a blue and a pink which have +been put together as identical by a person with a shortened red end of +the spectrum, and look at them through a glass which is opaque to the +red, but transparent to the remaining rays of the spectrum, both will +appear alike in hue and shade. A person with considerable shortening +of the red end of the spectrum will look at a red light (which is so +dazzlingly bright to a normal-sighted person as to make his eyes ache +after looking at it closely for a few seconds), at a distance of a few +inches, and remark that there is nothing visible, and that the whole +is absolutely black. It is obvious that the light must consist only of +rays occupying the missing portion of the spectrum. The same remarks +which I have made for a shortened spectrum apply to cases in which +there is defect of light-perception through absorption or any other +cause. The person having the defect is placed in a similar position to +a normal-sighted person with those particular rays removed or reduced +to the same intensity. + +Another effect of shortening of the spectrum when it is sufficient to +interfere with the difference-perception which appears to be inherent +in the central nervous system, is that the colours appear to be moved +in the direction of the unshortened portion. For instance, we find the +neutral point of the dichromic, with shortening of the red end of the +spectrum, in most cases further towards the violet end of the spectrum +in comparison with a case in which the spectrum is of normal length. In +the same way a trichromic with a shortened red end of the spectrum has +the junction of the red and green nearer the violet end than in a case +where there is no shortening. + +The point that I specially wish to emphasise is that, though every +case in which there is defective light-perception can be explained +by a defective sensibility to light of certain wave-length, not a +single case of the very large number of persons that I have examined +can be explained on the older theories; that is, the defect of +light-perception cannot be explained on the assumption that there is +a defect in a light-perceiving substance which is sensitive to rays +of light from a considerable range of the spectrum. A large number +of cases in which there is shortening of the red end of the spectrum +escape detection when only the green test is used, as is usual +according to Holmgren’s instructions. + +2. _Defects of colour-perception._--The colour-blind have a diminished +hue-perception and see a less number of colours than the normal. All +the symptoms of colour-blindness are such as we should expect from want +of development of the retino-cerebral apparatus for the perception +for colour. This is evident even in the slighter cases which show a +diminished colour-perception compared with the normal. We find that +the colour-blind are much more dependent on the luminosity of the +colour than the normal-sighted; they require a stronger stimulus; they +fatigue more easily with colours than the normal-sighted; they have a +more marked simultaneous contrast; the visual angle subtended by the +coloured object requires to be larger, and they have a very bad memory +for colours. The diminution of colour-perception with a diminished +visual angle evidently depends upon several causes. It is very marked +when there is diminished light-perception for those rays which are +imperfectly seen. It is also dependent upon certain retinal conditions, +as in scotoma and allied conditions. There are colour-blind persons, +however, who are able to recognise colours under as small a visual +angle as the normal-sighted, and I have examined one dichromic (said +to be red-blind by a physicist) who recognised red easily through the +thickest neutral glass of my lantern, and who had no difficulty with +this colour at a distance. + +Apart from any other defect of light or colour-perception, every case +with which I have met has fallen naturally into one of the classes +I have given; that is to say, every person is either heptachromic, +hexachromic, pentachromic, tetrachromic, trichromic, dichromic, +or totally colour-blind. When I first gave this classification of +colour-blindness, the facts that I discovered were so at variance with +those generally stated that it was very difficult for those who were +not well acquainted with the subject to compare the two sets. The +general knowledge of the subject has, however, steadily increased, and +the facts which I had so great a difficulty in getting recognised now +form part of our common knowledge. It would be well, therefore, to +describe the two main varieties of colour-blindness which are of chief +practical importance, and to show the relation which they bear to the +writings of other persons. These two main varieties have dichromic and +trichromic vision. + +1. _Dichromic vision._--The cases which come under this head form +the class of the ordinary red-green blind. It is under this head +that nearly every one of the recorded cases may be classed. Vision, +as far as colour is concerned, is dichromic, the neutral point being +situated in the green of the normal-sighted at about λ 500. All the +colours on the red side tend to be confused with each other; therefore +red, orange, yellow and half of the green are seen as one colour, the +remainder of the green, blue and violet as the other. The luminosity +curve in uncomplicated cases is similar to the normal. There may be +shortening of the spectrum at either the red or the violet end of +a varying degree. All degrees of shortening of the red end of the +spectrum may be found. Dichromics with normal luminosity curves are +those which were formerly designated green-blind; but this designation +is not in accordance with the facts, because there is no defect of +light-perception in the green, and the so-called diagnostic mistakes, +as, for instance, putting a bright green with a dark red, are not made. +Cases of so-called red-blind are dichromics with shortening of the red +end of the spectrum. I have shown that the defective perception of +the red end of the spectrum will not account for the dichromic vision +which is found in these cases. We may also meet with shortening of the +spectrum with otherwise normal colour-perception. We also meet with +dichromic cases forming a series from almost total colour-blindness +to those bordering on the trichromic. Any theory must account for the +fact that there are varying degrees of colour-blindness in dichromic +vision, and why there is a large neutral band corresponding to the +colours of the centre of the spectrum in some cases, and in others the +neutral band is so small that the dichromic cannot mark it out. The +two colours seen by the dichromic are red and violet, though where no +distinction is seen between yellow and red, and blue and violet, the +brighter colour will often be selected; that is why so many dichromics +say that they see yellow and blue in the spectrum. Those who have had +practical experience of colour-blindness will know, however, that many +dichromics make no mistakes with the red test. The following will +give the normal-sighted the best idea of colour-blindness, and it is +how the dichromic see the spectrum, and explains why they are able to +distinguish between colours. Let him regard the dichromic as a man who +has two colours--red and violet and white. The purest red is at the +red end of the spectrum; this becomes less and less saturated as the +violet is approached until the neutral or white point is reached; then +violet comes into the white, and this increases in saturation to the +termination of the violet. The ordinary dichromic therefore sees green +as a much whiter and less saturated colour than red. + +2. _Trichromic vision._--These persons see three distinct colours +in the spectrum--red, green and violet. They describe the region +intermediate between red and green; that is to say, the orange and +the yellow as red-green, and blue as violet-green. It will be seen, +therefore, that their chief difficulty is distinguishing yellows and +blues. A yellow, for instance, which is situated next to a green will +be called red, and the same yellow when adjacent to a red will be +called green. There are various degrees of trichromic vision, varying +from those who are little better than dichromic to those who are +tetrachromic. The trichromic rarely find any difficulty with their +three main colours--red, green and violet. + +These cases have been described under the name of anomalous +trichromatics. This name is one which has been given to those persons +who in making a match between a yellow corresponding to the sodium +flame and a mixture of thallium-green and lithium-red make a mixture +which is different to that of the normal.[9] A man who puts too much +green in the mixture is called a green anomaly; whilst a man who puts +too much red in the mixture is called a red anomaly. The red anomaly +is only a trichromic with shortening of the red end of the spectrum, +and this may be as extensive as in any case of dichromic vision. I +have, however, described trichromic cases which had all the symptoms +attributed to the anomalous trichromatics, but they were not anomalous +trichromatics, as they made an absolutely normal match.[10] + +[9] _Proceedings of the Royal Society_, vol. B 76, 1905. + +[10] _Transactions of the Ophthalmological Society_, 1907, p. 255. +_Proceedings of the Royal Society_, vol. B 82, 1910. + + + + +LECTURE II + +_Delivered on February 3rd_ + +THE DETECTION OF COLOUR-BLINDNESS FROM A PRACTICAL POINT OF VIEW + + +I. _Object of a test for colour-blindness._--Tests for colour-blindness +are of two kinds; namely, those which are used for the purpose of +ascertaining the special phenomena of colour-blindness, and those which +are employed when the inquiry is made for some practical purpose. As +with visual acuity, it is necessary to fix an arbitrary standard. As we +do not wish to exclude a greater number than is absolutely necessary, +the object of the test should be to exclude dangerous persons and +dangerous persons only. These persons may have other duties to perform +which do not require them to possess a perfect colour-sense. I should, +however, like to see those persons who are specially qualified for +a certain position, occupy it, for instance, men who have to keep a +look-out on our most important ships being selected because of their +accurate colour-vision and visual acuity. I do not mean that I would +select only those men and reject the others, but that I should like +to see a second object of a test, namely, to select those who are +specially efficient so that the Captain might know on whom to rely in +conditions of exceptional difficulty. + + +II. _The requirements of a test for colour-blindness._--A test for +colour-blindness, when it is to be employed for some definite and +specific purpose, as, for instance, excluding dangerous persons from +certain callings, should be such as to show definitely that the +persons rejected are dangerous. It is very useful to demonstrate to +the men and their fellows that a rejected candidate is dangerous. The +colleagues of a rejected candidate would refuse to risk their lives +with a man who before their eyes called a red light, green. I was +expressing these views when a superintendent of a railway company, +who is using my lamp, told me that he had adopted this method with +great satisfaction to himself and to the men. A man, for instance, who +has been working twenty years on the railway has been rejected for +colour-blindness. He has complained bitterly to the superintendent, +at the same time declaring emphatically that he is normal-sighted. +The superintendent has replied, “You know red?”--“Yes.” “You know +green?”--“Yes.” “You will therefore agree that if you call green, +red; or red, green, you ought to be rejected. Bring two or three of +the other men in with you and I will test you.” The man has readily +agreed to this. The superintendent has then tested him by asking him to +name various coloured objects in the room, and knowing by experience +exactly the coloured objects which are miscalled by the colour-blind +readily exposes his defect. It is noteworthy that on some occasions a +colour-blind man has been tested by another person in the same room +without making any of the mistakes which he subsequently made, because +none but coloured objects which he could readily recognise were shown +to him. This is an example of the necessity of a practical knowledge +of colour-blindness in an examiner. On account of the arrangement of +signals by sea and land, it is necessary that persons employed in the +marine and railway services should be able to recognise and distinguish +between the standard red, green, and white lights in all conditions +in which they are likely to be placed. An engine-driver or sailor has +to name a coloured light when he sees it, not to match it. He has to +say to himself, “This is a red light, therefore there is danger”; and +this is practically the same as if he made the observation out loud. +Therefore, from the very commencement we have colour-names introduced, +and it is impossible to exclude them. The engine-driver is told that +red is a “danger” signal, green a “caution” signal, and white an “all +right” signal. Therefore, it is absolutely necessary that he should +know the meaning of these colour-names. A test should be such as to +make it impossible for the examinee to be coached through it. This is +one of the most important requirements of a test for colour-blindness +and one that is rarely fulfilled. Nearly every one of the tests in +general use fail on this account. + +A test should be one which can be carried out as rapidly as is possible +with absolute efficiency; of two equally efficient tests the one which +takes the least time must be selected. A test, therefore, should have +no unnecessary details which though of theoretical interest are not +concerned with the object in hand. The test should be made as easy and +as little complicated for the examiner as possible. + + +III. _Persons to be excluded._--We wish to exclude all those +individuals who are included in the following three classes: (1) +Those who see three or less colours in the spectrum. (2) Those who, +whilst being able to perceive a greater number of colours than three, +have the red end of the spectrum shortened to a degree incompatible +with their recognition of a red light at a distance of two miles. (3) +Those who are unable to distinguish between the red, green, and white +lights at the normal distance through defect or insensitiveness of the +cerebro-retinal apparatus when the image on the retina is diminished in +size. + +I will now explain why these three classes of persons should be +excluded. The first class includes the trichromic, the dichromic, and +the totally colour-blind, in accordance with the facts previously +stated. The trichromic never, in ordinary circumstances, mistake green +for red, but confuse yellow with green or red. Colour is a feeble +quality of objects to them, and nervousness or excitement may reduce +them to the condition of the dichromic. The dichromic are liable to +mistake a green light for red, and vice versa. It is very important +that persons belonging to the second class should be excluded, and +yet none of the ordinarily used tests detect them. The rays of red +at the extreme left of the spectrum are the most penetrating, as may +be seen by looking at a light or the sun on a foggy day, or through +several thicknesses of neutral glass. It is chiefly by these rays that +we recognise a red light at a distance; and it is therefore of great +importance that a sailor or engine-driver should be able to perceive +them. The third class contains persons who are able to distinguish +colours easily when they are close to, but fail to distinguish them at +a distance, owing to the nerve-fibres supplying the central portion +of the retina being impaired. As a light at a distance occupies +the central portion of the visual field, it is essential that the +corresponding portion of the retina should be normal. There are cases +of central scotoma for colours with perfect form-vision; these would, +therefore, not be detected by a test for visual acuity. This class also +includes those who without having a scotoma are unable to distinguish +between colours at the normal distance when the image on the retina is +diminished in size. + + +IV. _The construction of a test for colour-blindness._--In +the construction of a test for colour-blindness, the facts of +colour-blindness must be utilised so that the object and requirements +of the test are fulfilled. The following facts are of practical +importance. + +1. _Most colour-blind make mistakes with certain colours, but are +correct with regard to others._ This may be illustrated in the +following way. Let us take an ordinary dichromic, and, having given +him the set of wools belonging to the Classification Test, ask him to +pick out all the reds. On examining the pile of wools selected as reds, +it will be found that the majority are red, but in addition there will +be some browns and yellow-greens. If he be then told to pick out the +whole of the greens the greater number of those selected will be green, +but there will be also greys, browns, and reds. In each case, it will +be seen that the majority of wools are of the desired colour. + +If another dichromic be examined in the same way it will be found +that, though he may not make exactly the same mistakes, he will in all +probability pick out the same greens to put with the reds, and the same +reds to put with the greens. The same result will be obtained if the +colour-blind persons be asked to name a large number of colours. They +will in most cases name the colour correctly. It will be noticed that +the greens which were put with the reds when classifying the colours, +will be called red in naming them. It is evident that the same idea has +guided the colour-blind in each case. This shows that, though a person +may be red-green blind, he is not absolutely red-green blind in the +sense of being totally unable to distinguish between the two colours. +The fact that they are actually judging by colour may be demonstrated +by giving them coloured materials of different kinds, or by asking them +to name a large number of coloured objects. + +It will be seen that if we take a dichromic and ask him to name a +number of red and green wools, in the majority of instances he will +name them correctly. But as, almost invariably, the same wools are +chosen, for all practical purposes the same result would be obtained +by asking a person to name a few of these wools. What more decided and +brighter greens could we have than Nos. 76 and 94 of my Pocket Test? +yet these are two of the greens which are called reds by the dichromic. +We should have accomplished as much by asking a colour-blind person to +name Nos. 76 and 94 as if we had asked him to name a large number of +greens. The colours in a test should, therefore, be those which the +colour-blind are particularly liable to miscall. At the same time, +their nature should be unmistakable to the normal-sighted. + +2. _The colour-blind name colours in accordance with their +colour-perception, and thus show definitely to which class they +belong._ I have not come across a man who has guessed correctly when +examined with my test. A man who did guess would know that he was +incompetent. As the colour-blind are often not aware of their defect +they answer as they see, only guessing when they feel uncertain as to +the nature of the colour shown. There is probably more misapprehension +on this point than on any other in the practical testing of +colour-blindness. + +3. _Colours may be changed to the colour-blind, whilst leaving them +unaltered to the normal-sighted._ + +4. _The phenomena of simultaneous and successive contrast are much more +marked for the colour-blind than for the normal-sighted._ Two colours, +which have not changed in the slightest degree to the normal-sighted +on being contrasted, have apparently altered very considerably to the +colour-blind. As an example of this, let us take a pure deep yellow, +a bright red, and a bright green. To the normal-sighted the yellow +will be altered very little by comparison with the red or the green, +but a trichromic would say that the colour was green when contrasted +with the red, red when contrasted with the green. This principle of +exaggerated contrast must be borne in mind when examining a candidate. +Thus if a trichromic be doubtful about a yellow, but seems inclined to +call it green, he should be given a pure green to compare with it. In +the same way, in showing the coloured lights, the same colour produced +in a different way should often be shown. Thus an orange-red may be +shown immediately after a pure red. This will not alter the colour +to the normal-sighted, but greatly facilitate the examination of the +colour-blind. + +5. _Many colour-blind match correctly, but name the principal colours +wrongly._ Therefore the test must be a naming test, the examinee being +rejected if he confuse the colours which it is essential he should +distinguish between in his occupation. + +6. _Many colour-blind recognise colours easily when they are close to +them, or the surface is large, but fail to distinguish between them +when they are at a distance or the image on the retina is small._ The +test must be constructed in conformity with these facts. + +7. _The colour-blind are more dependent upon luminosity than the +normal-sighted, and are liable to mistake a change in luminosity for a +change of colour._ The test should have a means of rapidly changing the +luminosity of a colour. + +8. _The colour-blind find special difficulty with faint and dim +colours._ The test should have colours of this kind. + +9. _The colour-blind who have shortening of the red end of the spectrum +cannot see reds reflecting or transmitting only rays corresponding to +the shortened portion._ It is essential that reds of this kind should +form part of the test. + +10. _The colour-blind find more difficulty in comparing colours when +different materials are used, than when the coloured objects are all of +the same nature._ + +11. _Most colour-blind find more difficulty with transmitted than with +reflected light._ + +12. _The colour-blind have a defective memory for colours._ + +13. _Colours may be changed to the normal-sighted whilst leaving +them unchanged to the colour-blind._ When three colours of the +normal-sighted are included in one of the colour-blind, it is obvious +that a change from one colour to another of the three will make no +difference to the colour-blind. Also when the spectrum is shortened, +the addition or rays corresponding to the shortened portion to another +colour will not alter its appearance to the person with the shortened +spectrum. For instance, to a person with shortening of the red end of +the spectrum, a blue will still remain blue, when so many red rays from +the shortened portion have been added to it as to make it appear rose +to the normal-sighted. + +14. _The colour-blind may have a sense of luminosity similar to that of +the normal-sighted._ + +15. _The dichromic distinguish between the colours of the +normal-sighted, which are included in one of theirs by their relative +luminosity and the difference of saturation which is apparent to +them._ A test should therefore have the means of presenting colours of +different saturation in succession. + +16. _Colour-blindness is frequently associated with very high +intelligence and exceptional ability._ + + +V. _The Lantern Test._[11]--1. _Description of apparatus._ The lantern +contains four discs: three carrying seven coloured glasses, and one +with seven modifying glasses. Each disc has a clear aperture. The +other mechanical details are: an electric or oil lamp with projecting +accessories, a diaphragm for diminishing the size of the light +projected, handles for moving the discs and the indicator showing the +colour or modifier in use. The diaphragm is graduated in respect to +three apertures to represent a 5-1/2-inch railway signal bullseye at +600, 800, and 1000 yards respectively when the test is made at 20 ft. +The glasses are as follows:--- + +_Coloured glasses._ + + 1. Red (A and B). + 2. Yellow. + 3. Green. + 4. Signal Green. + 5. Blue. + 6. Purple. + +_Modifying glasses._ + + 7. Ground glass. + 8. Ribbed glass. + 9. Neutral (No. I). + 10. " ( " II). + 11. " ( " III). + 12. " ( " IV). + 13. " ( " V). + +[11] Made by Reiner and Keeler, 9, Vere Street, W.; and Meyrowitz, 1a, +Old Bond Street, W. + +[Illustration: Fig. 3.] + +It will be noticed that three of the discs are similar in every +respect. In some of my lanterns the two reds are put at the end of the +series of colours and numbered Red 1 and Red 2. This makes no important +difference, but the arrangement given here is more convenient. It +should be noted that Red 1 corresponds to Red B and Red 2 to Red A. If +the electric lamp should get broken the projecting apparatus can be +removed and an ordinary kerosene lamp placed behind the aperture. + +[Illustration: Fig. 4.] + +2. _Reasons for special construction._--The lantern has been +constructed conformably with the requirements and facts of +colour-blindness. All the facts I have given have been considered in +constructing the lantern. + +The examiner, on possessing a lantern for the first time, should +carefully test himself with it and ascertain how the different lights +appear to him with different conditions of general illumination. +It is probable that certain improvements may suggest themselves to +him, therefore, I think it will be advisable to deal with certain of +these points, as it will help the examiner in the use of the lantern. +The colours have never been altered, and I certainly should have +altered them if I could have improved the lantern by doing so. I +have never met a single colour-blind person who has not been readily +and easily detected with my lantern, though I have examined many who +have passed other lanterns and in some cases a number of other tests +for colour-blindness. In most cases one turn of the wheel will be +sufficient to make a colour-blind person disclose his defect. + +The examiner may be dissatisfied with the colour of the blue; let us, +therefore, compare an examination of a normal-sighted person with that +of an ordinary dichromic. The normal-sighted person will name every +one of the colours with ease and certainty, with perhaps the exception +of the blue, with which he is in some doubt. Here is the result of an +examination of an ordinary dichromic: he called the yellow, green; the +green, red; the signal green, no colour; the blue, blue; the purple, +green; red A, no colour or light; red B, green. It will be noticed that +the only colour that he has correctly named is the blue. We can try +him again and again, and though he will mistake all the other colours +he will always name the blue correctly. The examiner will have learnt +from this several important facts. He will see that the colour-blind +are really guided by their sensations of colour, and that it is not +simply a matter of guessing. The more an examiner has practical +experience of colour-blindness, the more will he recognise the fact +that the colour-blind are guided by their sensations of colour. He +will notice that the dichromic has readily recognised the blue which +was scarcely apparent to him (the examiner), and therefore cannot have +overlooked as a matter of carelessness colours which are much more +apparent to the normal-sighted. The blue is a valuable colour for other +reasons, for though it is not a colour on which I reject candidates, +anyone miscalling it must be very carefully examined before he is +passed. The trichromic generally call this blue, green. If we wish to +obtain a purer blue, we can do so by combining the blue or the purple +with the signal green. + +Again, the examiner may think that it might be better to have an +apparatus which showed two or three lights instead of only one. I +will therefore give my reasons for adopting only one. This point was +one which occupied my attention for a considerable time, especially +in view of the fact--which, as far as I am aware, I was the first to +discover--that simultaneous contrast is increased to the colour-blind. +I was naturally anxious to turn this fact to account. I found, +however, that I gained nothing by increasing the number of lights, and +that in many cases it was a source of error. A second or third light +could have been easily added to my lantern, but besides unnecessarily +complicating the apparatus it would have served no useful purpose. All +the results which are obtained with simultaneous contrast are obtained +even more effectively with successive contrast. It will be noticed +that when lights are seen in ordinary conditions they are conditions +of successive contrast, and not of simultaneous contrast. An observer +rarely keeps his eyes definitely fixed on one light whilst he names +those adjacent to it, but moves his eyes so that the images of the +respective lights fall successively on his foveas. When more than one +light is employed, all the disadvantages of matching as against naming +are introduced. It will be seen that by presenting one light after +another we are fulfilling all the necessary conditions, only that the +light is moved instead of the eyes, sufficient time being allowed to +elapse to enable a normal-sighted person to readily recognise the true +colour of the next light without being confused by the after-image of +the one he has just seen. Many nervous normal-sighted would name a +yellow light seen between two red lights as green, and it does look +green to them from ordinary physiological conditions. They look first +at one red light, then immediately at the yellow light, then at the +second red light, and then again at the yellow light until they feel +sure that the centre light is a green light, and say so. I have never +met with a normal-sighted person who has miscalled the unmodified light +of my lantern, either red or green. Many humble, nervous normal-sighted +persons are under the impression that they are colour-blind, and yet +would make perfectly efficient officers. Many of these men have been +told by their wives or other persons that they are colour-blind, and, +believing this, try to see colours which are not visible to them. I +have examined many persons of this description, and have noted the +ease and accuracy with which they have gone through the tests for +colour-blindness when they have been assured by me that they were +normal-sighted. On the other hand, it is often very difficult to +convince a self-reliant, colour-blind person that he is colour-blind. +He is on the look-out for the small differences which he notices +between colours, and the fact of having another light for comparison +gives him the desired clue, and, though colour-blind, he passes the +test. + +The material is the best possible, as it will not fade like all dyed +substances, and therefore all records made with one set of apparatus +will be uniform. Again, a coloured light has none of the accessory +qualities which enable the colour-blind to pass through other tests. +Thus many dichromics will call the yellow glass red or green, who would +not think of putting a yellow with a green or red wool, on account of +the difference in luminosity. He will, in the same way, if told to pick +out colours in the Classification Test to match the colour of the light +shown, have to depend upon his colour-perception. This is a useful +method with nervous and undecided candidates. The objection to it is +that it cannot be carried out in the dark or in a dark room. The Test +is not open to any of the objections which may be urged against the +method of simply naming colours, because the character and intensity of +the colour may be changed at will. + +The method is better than that of direct comparison, because the +candidate is forced to use his colour-perception, and has to compare +the colour seen with previous impressions of colour in his mind. By the +use of neutral glasses, etc., I have obviated the fallacy of the method +of naming colours (namely, that these can be distinguished by their +saturation and luminosity), and forced the individual to depend upon +his colour-perception, and not upon some other accessory quality of the +object seen. + +No amount of coaching will enable a colour-blind person to pass this +test, whilst almost any other may be passed in this way. I have tried +on many occasions to coach a man so as to pass my lantern, and without +success. The combinations are so numerous that the only result is to +make the colour-blind man nervous and doubtful and more easily detected +than before. This has occurred with men who could pass other tests with +ease. + +The test also has a quality possessed by no other--namely, that of +enabling the examiner to reject dangerous persons and dangerous persons +only, the lower degrees of colour-blindness being allowed to pass. + +3. _Special directions for conducting the test._--(1) The candidate +should be seated at a distance of twenty feet from the lantern. (2) He +should be asked to name the colour of the light produced by a coloured +glass (1 to 6) alone, or in combination with another coloured glass +or glasses, or with the modifying glasses (7 to 13). (3) A candidate +should be rejected (i) if he call the red, green, or the green, +red, in any circumstances; (ii) if he call the white light, in any +circumstances, red or green, or vice versa; (iii) if he call the red, +green, or white lights, black, in any circumstances. (4) A candidate +who makes mistakes, other than those mentioned above, should be put +through a very searching examination. It is not necessary to have the +room absolutely dark; in fact, I prefer a certain amount of light. The +examiner can, if he wish, make the test at night in the open air. + +The examiner should on no account conduct the examination on any +regular plan, because the candidate, anxious to pass, finds out +from persons who have already passed the order and method of the +examination, and so, though colour-blind, might obtain a certificate. +Any one of the glasses may be shown first, and the candidate required +to name the colour of the light. The following will serve as an example +of the method to be employed in testing a candidate. A red being shown, +the candidate is required to name its colour. Then a blue or green may +be substituted. It is best to use the largest aperture at first and +to show all the colours on one disc. This will give confidence to +the normal-sighted candidate, whilst most of the colour-blind will be +detected. In the case of candidates who appear to be normal-sighted +and yet very nervous, there is no harm in telling them after they have +named all the colours on the disc correctly that this is the case. +No comment should, however, be made on individual answers. Then one +of the neutral, ground, or ribbed glasses should be inserted, not +the slightest intimation being given to the candidate of the nature +of the colour. He should be asked to name or describe the light, and +the answer, if incorrect, together with his other replies, carefully +recorded. The other glasses may then be shown, a combination of the +neutral, ground, ribbed, and coloured glasses being used at irregular +intervals. + +When the candidate has been examined with the largest aperture, +the examiner can go through the same procedure with one of the +smaller apertures. I have found the third aperture the one which is +most generally useful. On account of the great diminution of total +luminosity caused by the diminished area of the light source, the three +smallest apertures can only be used in a dark room. + +If a candidate hesitate about a colour and ultimately name it +correctly, a second and, if necessary, a third glass of the same +colour should be combined with the first. The fact that in one case a +single glass is used, and in another two or three of the same coloured +glass, makes very little difference in the colour of the light to +the normal-sighted. This is not the case with the colour-blind; a +dichromic who has hesitated about a green and then correctly named it +may emphatically call the light red when another green glass is put in +front of the first. + +Care must be taken when the candidate is going to be examined with two +glasses at once, such as one of the neutral, ground, or ribbed glasses, +and a coloured glass, that he does not see the light until both are in +position, or else he may see the colour before it is modified in the +necessary way. + +If the candidate call the standard red, green; or the standard green, +red, in any circumstances--that is, either alone or in combination with +the modifying glasses--he is to be rejected. + +The examiner should ascertain for himself how far the various colours +are visible when modified with the neutral glasses. If the red and +green be not visible with the thickest neutral in the conditions of +luminosity and external lighting which the examiner is employing, he +should use the darkest neutral which allows the colours to be plainly +visible to the normal-sighted. In all cases of doubt the examinee +should be asked to walk towards the lantern and told to say when the +light is visible, and asked to name its colour. The distance at which +the light is visible, and then that at which the colour is visible, +should be noted and compared with the normal. + +Particular attention should be paid to the answers given to the +combination of the thickest neutral glass with the standard red and +green respectively. + +The examiner should utilise the fact that successive contrast is +increased in the colour-blind, as this is an easy method of detecting +the trichromics. The red having been shown, the light should be quickly +changed to yellow or clear, the examiner’s hand being placed over +the aperture if there be any intervening colours. It is necessary +that the yellow should be shown immediately after the red without +any intervening colours being first seen by the candidate. The +normal-sighted do not see any change in the yellow or clear when they +are shown after the red light, but the trichromic call the yellow +light, green. The examinee should then be shown the green light, and +then the yellow or clear, in the same way as mentioned for the red. The +normal-sighted will easily recognise the yellow, but the trichromic +will call it red. This portion of the examination must never be omitted +in any examination in which the candidate is passed. The two divisions +of the test--that is, showing yellow immediately after red and after +green--may be used at different periods of the examination, and, if +there be any doubt, repeated. + +An examiner should, as far as possible, with the exceptions given in +the instructions, avoid all conversation with the candidate, simply +asking, “What colour is this?” and recording the answer without +comment. If an examiner after each answer say, “Quite right,” or some +such expression, the following is likely to occur. The candidate after, +say, six correct answers, makes a mistake; the examiner says, “Are you +sure?” Then the candidate knows at once that he has made a mistake, +and makes a guess, very probably a correct one. When a similar colour +is shown subsequently, he remembers the mistake he made, and gives the +second, and probably the correct answer. + +In addition to being an efficient test, it is a very rapid test, as +many men who have been certified as normal after a lengthy examination +with other tests have at once disclosed their defect by calling the +green light of the lantern red. Many are under the impression that in +an examination with the lantern the dichromics simply guess. This is +entirely wrong. A man who did guess would know that he was incompetent. +I find that men have named the coloured lights in strict accordance +with their colour-perception. A man may, however, guess if examined +by an inexperienced and ignorant examiner, who when the examinee has +made a mistake promptly corrects it in a cross tone. A normal-sighted +person will guess when examined in this way. The examiner must receive +the examinee with a smiling face and courteous manner, and appear +pleased and satisfied with the answers, no matter what they may be. +The candidate is then placed at his ease, and answers according to +his colour-perception. It will be noticed that the lantern detects +those who have a slightly diminished colour-perception, as well as +the dangerous varieties of colour-blindness. The former undoubtedly +are not as efficient as those who have a normal colour-perception, +so that a definite standard will have to be fixed, as in the case +of visual acuity. Further details will be found in my book on +Colour-Blindness.[12] + +[12] _International Scientific Series._ Kegan Paul & Co., 1909. + +_Summary of method of examination._--(1) Show all the colours on one +disc with the largest aperture. (2) Show the reds, greens, and yellow +modified by the neutral glasses. (3) Show all the colours on one disc +with Number 3 aperture. (4) Show red, then immediately afterwards +yellow with largest aperture. Then show green and yellow immediately +afterwards. (5) Test the candidate with the red, green, and yellow with +the smallest aperture. (6) Show the neutrals or ground glass alone. (7) +Show blue made by combining blue or purple with the signal green. (8) +Show a colour, for instance, green, and then combine another glass of +the same colour. (9) Show the red produced by the combination of purple +with red A. (10) Give the combination of red A and signal green. + + +VI. _Other tests for colour-blindness._--I have three other tests for +colour-blindness: the Classification Test, the Pocket Test, and the +Colour-perception Spectrometer. I have also devised an instrument for +estimating the exact amount of red, at different wave-lengths, which is +necessary to neutralise the complementary in different persons. + +1. _The Classification Test._--(_a_) _Description._--This test +consists of 4 test colours and 180 confusion colours; 150 coloured +wools, 10 skeins of silk, 10 small squares of coloured cardboard, and +10 small squares of coloured glass. The whole series of colours is +represented. In addition, there are a large number of colours which +have been chosen by colour-blind persons as matching the test colours. +The test colours are Orange, Violet, Red, and Blue-green, labelled I, +II, III, and IV respectively. The colours are chosen with the view +of presenting as much difficulty as possible to the colour-blind, +and as little as possible to the normal-sighted. The colour-blind +find especial difficulty in matching or naming a colour lying at the +junction of two of their colours. As the normal-sighted often find +difficulty in saying which colour predominates in a blue-green, so do +the tetrachromic with their purple-green, or the trichromic with their +red-green. A colour-blind person may, however, match a colour correctly +which corresponds to the centre of one of his colours. In addition to +choosing those colours for tests which are particularly liable to be +mistaken for other colours by the colour-blind, I have used coloured +materials of different kinds--wools, silks, glass, and cards--so as to +force the colour-blind to judge by colour, and not by saturation or +luminosity. (See Fig. 5.) + +[Illustration: Fig. 5.] + +(_b_) _Method of examination._--The candidate should be given the four +test colours, and, having named each, he should be told to select all +those which are similar in colour to the test colour. He should be told +to pay no attention to the fact of a colour being lighter or darker; as +long as it is the same colour it should be put with the test skein. The +examiner should not go through the test before the candidate first of +all, neither should one candidate be allowed to watch another making +his selection. A shrewd colour-blind person might pass the test if he +had seen a normal-sighted person go through it previously. In order to +show the candidate the difference between a shade and a colour, the +examiner should take one of the wools which is not a test colour--blue, +for instance--and pick out four or five shades of the colour. The wools +should be arranged without the knowledge of the candidate, so that a +yellow or a grey is placed beside a red and the examinee asked to name +its colour. At another period of the examination the yellow should be +placed adjacent to a green, and the examinee again asked to name it. + +The examinee may pick out a certain number of colours correctly, and +then stop, saying that there are no more exactly like the test colour. +This may embarrass the examiner; he should, however, examine any +candidate who has omitted any colours as carefully as if mistakes had +been made. He should ask the candidate to match one of the omitted +colours. + +The examiner will soon find out from experience those colours which are +named and matched wrongly by colour-blind persons; he should ask the +examinee to name some of these colours. + +Any candidate should be rejected who calls an orange or red, green +or brown; black, red or vice versa; or green, either purple, rose, +red, grey, brown, or violet. Similar mistakes in matching necessitate +objection. A candidate who puts purple, rose, or blue with violet, or +yellow-brown with orange is most probably dangerously colour-blind +and should be very carefully examined. There are cases which pass the +Holmgren test with ease that fail in the most conclusive manner with my +Classification Test. They put green with orange, brown and black with +red, and grey with blue-green. This is due to a different selection +both of test colours and confusion colours. Orange is by far the most +important test colour, and its confusion with green by the dichromics +is very conclusive. The three other test colours, violet, red, and +blue-green, represent both ends of the spectrum and the neutral point +in dichromic cases, and practically these colours are those with which +most mistakes are made. This test can only be regarded as supplementary +to the Lantern Test. + +2. _The Pocket Test._--This consists of nineteen cards, on nine of +which are 112 single threads of wool, and 14 pieces of twisted silk, +similar to those in the Classification Test. These are numbered +consecutively, with the exception of the first thread of the first four +cards, and the last thread of the next four cards. The end threads of +the first four cards, I to IV, form the tests; they are Orange, Violet, +Red, and Blue-green. There are also cards on which red, orange, green, +blue, violet and purple, and grey, respectively are to be found. There +are also two special cards marked “Without Red” and two special cards +marked “Without Green.” (See Fig. 6.) + +[Illustration: Fig. 6.] + +Many normal-sighted persons might object to the inclusion of some of +the colours on the orange card, but this card clearly shows the colours +which may be taken as a match. Fine distinctions are not wanted. The +series of colours I have selected and arranged so as to confuse the +colour-blind and force them to be guided by their colour-perception, +whilst the quantity of colour is amply sufficient for the +normal-sighted to pick out the colours with the greatest ease. The +cards should be arranged irregularly on a white cloth in a good light. +The two most important tests colours are the Orange and Violet, Nos. I +and II. The person examined should be asked to point out the shades of +colour similar to No. I (Orange). A piece of paper rolled to a point +should be used for this purpose. If he do this correctly, he probably +possesses normal colour-perception. If, however, he match the test +with reds or pinks, he is more or less colour-blind, at best belonging +to the pentachromic class. If, in addition, he match the Violet test, +No. II, with blue, he at least belongs to the tetrachromic class. The +trichromic, in addition, may match the Blue-green test, No. III, with +brown and grey. The dichromic will match the Orange test, No. I, with +yellow-green and yellow-brown. Similar mistakes will be made to those +described in connection with the Classification Test. The examinee +should be asked to name all the colours on one of the cards. He should +also be asked to point out on which of the cards the four test colours +are to be found, and which contain none of the test colour. + +The examiner should continually change the order of the cards. Most of +the varieties of the colour-blind will be readily detected in this way. + +The special advantages of this test are: (1) The colour-blind can be +ranged definitely in their proper classes. (2) Central scotoma can be +detected with its aid. (3) The series of colours are arranged so as +to confuse the colour-blind, whilst the normal-sighted easily match +the test colours. (4) On account of the introduction of different +materials, the relative luminosity and saturation of colours does not +serve as a guide to the colour-blind. (5) Portability. (6) The wools +and silks are kept clean. (7) An important colour is not likely to be +lost. + +3. _The Colour-Perception Spectrometer._[13]--(_a_) _Description of +apparatus._--This instrument is a spectrometer so arranged as to +make it possible to expose to view in the eyepiece the portion of a +spectrum between any two desired wave-lengths. In the focal plane of +the telescope are two adjustable shutters with vertical edges; the +shutters can be moved into the field from right and left respectively, +each by its own micrometer screw, and to each screw is attached a drum, +the one being on the right and the other on the left of the telescope. +On each of these drums is cut a helical slot in which runs an index, +and the drum is engraved in such a manner that the reading of the index +gives the position in the spectrum of the corresponding shutter in +wave-lengths direct. (See Fig. 9.) Thus it will be seen that if, for +instance, the reading on the left drum-head is 5320 and that on the +right drum-head is 5920, the region of the spectrum from wave-length +5320 to wave-length 5920 is exposed to view in the eyepiece. + +[13] Made by A. Hilger, 75a, Camden Road, London, N.W. + +[Illustration: Fig. 7.] + +[Illustration: Fig. 8.] + +[Illustration: Fig. 9.] + +(_b_) _Directions for using the instrument._--It should be used as +far as possible with a known quality and intensity of light. A small +oil-lamp is quite suitable for the purpose. The observer should +first ascertain the exact position of the termination of the red +end of the spectrum, the left-hand shutter being moved across until +every trace of red just disappears. The position of the pointer on +the left-hand drum is noted, and the wave-length recorded. The left +drum is then moved so that the shutter is more towards the middle of +the spectrum. The right-hand drum is then moved, until the pointer +indicates the wave-length recorded as the termination of the red end +of the spectrum. The observer then moves the left-hand shutter in +and out until he obtains the largest portion of red, which appears +absolutely monochromatic to him, no notice being taken of variations +in brightness, but only in hue. The position of the index on the +left-hand drum is recorded. The left-hand shutter is then moved towards +the violet end of the spectrum, the right-hand shutter being placed at +the position previously occupied by the left-hand shutter. In this way +the whole of the spectrum is traversed until the termination of the +violet end of the spectrum is finally ascertained with the right-hand +shutter. The variation of the size of the patches and the terminations +of the spectrum with different intensities of light can be noted. The +instrument can also be used for ascertaining the exact position and +size of the neutral patch in dichromics, the position of greatest +luminosity, and the size and extent of pure colours. When it is used +to test colour-blindness, the examinee should first be shown some +portion of the interior of the spectrum, and then asked to name the +various colours which he sees. In this way he will have no clue to the +colours which are being shown him. + +_Objections to other tests for colour-blindness._--The tests which +have been proposed for colour-blindness are very numerous, but some +are so defective that it is rare to detect a single colour-blind +person with them. I have, for instance, tested men whom I knew to be +colour-blind with certain lanterns with the result that not a single +one was detected. In these so-called tests all the requirements of +a test and facts of colour-blindness have been neglected. I must, +however, refer to three tests constructed by exceptionally able men, +each with considerable knowledge of the subject. I refer to the tests +of Professor Holmgren, Professor Stilling, and Professor Nagel. + +All these tests can be passed at the first attempt without coaching by +certain dangerously colour-blind persons, chiefly varieties not known +to the inventors, but the chief defect of each is that it is very easy +to coach a colour-blind person to pass it. The surgeon to one of our +largest railway companies told me that when they used Holmgren’s test +they rejected one man in three hundred, but with my lantern twelve in +the same number. All these three tests are much better tests when the +persons to be examined have not seen them before. A colour-blind man +may make only one mistake, say for instance, as in a case I examined +the other day, with Nagel’s test (last edition), he passes the test +perfectly with the exception of one mistake, that of calling a grey on +one card, green. All he has to do is to look for some distinguishing +mark on this card in order to go through the test with the ease and +certainty of a normal-sighted person. It is the same with Stilling’s +letters, he has only to note the letter which he was not able to read +and the appearance of the card. A normal-sighted man or woman would +readily help him. The confusion of green and grey does not appeal +to the average man as a serious defect, especially when he sees his +friend go through the rest of the test perfectly. He says to himself, +“I suppose he sees a tinge of green in that grey.” The same man would +rightly regard it as a most iniquitous proceeding to endeavour to coach +his friend through a test when he had seen him mistake a red for a +green light. + +Holmgren’s test rejects a large number of normal-sighted persons, as +may be seen by the reports of the Board of Trade; about 50 per cent +of those who appeal are found to be normal-sighted and to have been +rejected wrongly. + + + WILLIAM BRENDON AND SON, LTD. + PRINTERS, PLYMOUTH + + +Reiner and Keeler, L^{td.} + + OPTICIANS + ----AND---- + INSTRUMENT MAKERS + + +_MANUFACTURERS OF_ + + The Edridge-Green Colour Perception Lantern + The Edridge-Green Classification Test + The Edridge-Green Pocket Test + and other + Optical and Scientific Instruments + + +THE ABOVE COLOUR TESTS ARE CERTIFIED BY PROF. F. W. EDRIDGE-GREEN + + 9 Vere Street, + Cavendish Square, =London, W.= + Telephone: 447 Mayfair. + + + +*** END OF THE PROJECT GUTENBERG EBOOK 75519 *** diff --git a/75519-h/75519-h.htm b/75519-h/75519-h.htm new file mode 100644 index 0000000..b2e1050 --- /dev/null +++ b/75519-h/75519-h.htm @@ -0,0 +1,3158 @@ +<!DOCTYPE html> +<html lang="en"> +<head> + <meta charset="UTF-8"> + <title> + The Hunterian Lectures | Project Gutenberg + </title> + <link rel="icon" href="images/cover.jpg" type="image/x-cover"> + <style> + +body { + margin-left: 10%; + margin-right: 10%; +} + + +h1,h2,h3 { + text-align: center; + clear: both; +} + +h2.nobreak {page-break-before: avoid;} + +h2, h3 { font-weight: normal;} + +.x-ebookmaker h2 {page-break-before: always;} +.x-ebookmaker h2, h3 {page-break-after: avoid;} + +p { + margin-top: .51em; + text-align: justify; + text-indent: 1em; + margin-bottom: .49em; + line-height: 1.3em; +} + +.ti0 { text-indent: 0em; } + +h1 span.t1 { + display: block; + letter-spacing: 0.1em; 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Hyperlinks have been added to assist the reader navigate the +book.</p> +</div> + + +<div class="tac fs160 ws02">THE HUNTERIAN LECTURES</div> + +<div class="tac fs80 mtb1em">ON</div> + +<div class="tac fs120 mb5em ws02">COLOUR-VISION AND COLOUR-BLINDNESS</div> + +<div class="titlepage"> +<h1><span class="t1">THE</span> + +<span class="t2">HUNTERIAN LECTURES</span> + +<span class="t3">ON</span> + +<span class="t4">COLOUR-VISION AND<br> +COLOUR-BLINDNESS</span></h1> + + +<div class="tp1"><i>Delivered before the Royal College of Surgeons of England<br> +on February 1st and 3rd, 1911</i></div> + + +<div class="tp2">BY</div> + +<div class="tp3">PROFESSOR F. W. EDRIDGE-GREEN</div> + +<div class="tp4"><span class="smcap">M.D.Durh., F.R.C.S.Eng.</span></div> + +<div class="tp4"><span class="lowercase smcap">BEIT MEDICAL RESEARCH FELLOW</span></div> + + +<div class="tp5">KEGAN PAUL, TRENCH, TRÜBNER <i>&</i> <span class="smcap">Co., Ltd.</span></div> +<div class="tp6">43 GERRARD STREET, LONDON, W.</div> +<div class="tp7">1911</div> +</div> + +<hr class="chap x-ebookmaker-drop"> + +<p><span class="pagenum" id="Page_5">5</span></p> + + +<div class="chapter"> +<h2 class="nobreak" id="PREFACE">PREFACE</h2> +</div> + + +<p>As there are many who are interested in the +subject of vision and colour-blindness who are +not acquainted with the structure of the eye, I will +give a few details so that these persons may be +able to consider the problem from the point of +view of these lectures.</p> + +<p>The eye is very similar to a photographic camera, +and an actual image is formed on the back of the +eye just as it is on the plate of the photographic +camera or on the view-finder. The eye possesses +a lens and also an iris which acts as an adjustable +stop and regulates the size of the pupil. The +membrane at the back of the eye upon which the +image is formed is called the retina. The retina +has several layers, but the sensitive layer consists +of two elements called, from their shape, rods and +cones. The problem therefore which has to be +considered is, how is the light which forms the +image on the sensitive layer of the retina transformed +into visual impulses?</p> + +<p>Those who are interested in the subject will +find further details in my book on <i>Colour-Blindness<span class="pagenum" id="Page_6">6</span> +and Colour-Perception</i> in the International Scientific +Series. In that book there are three plates +which show how the colour-blind see colours.</p> + +<p>I have been annoyed to find that unauthorised +persons have made lanterns professing to be mine +but grossly inaccurate. The sole makers are those +mentioned on page 53 in this book.</p> + +<p class="tar mr1em"> +<span class="smcap">F. W. Edridge-Green.</span></p> + +<p><span class="smcap">The Institute of Physiology,<br> +  University College,<br> +    Gower Street, London.</span></p> + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_7">7</span></p> + +<h2 class="nobreak" id="CONTENTS">CONTENTS</h2> +</div> + +<div class="center"> +<table id="toc" class=""> +<tr> +<td class="tac fs120" colspan="3"><div>LECTURE I</div></td> +</tr> +<tr> +<td class="tac fs110 pt05" colspan="3"><div>THE THEORY AND FACTS OF COLOUR-VISION<br>AND COLOUR-BLINDNESS</div></td> +</tr> +<tr> +<td class="tar fs70" colspan="3"><div>PAGE</div></td> +</tr> +<tr> +<td style="text-align: left; text-indent: -2em;" class="fs110" colspan="2"><span class="smcap">The Visual Purple the Essential Factor in Vision</span></td> +<td class="tar"><div><a href="#Page_11">11</a></div></td> +</tr> +<tr> +<td class="tar"><div> 1.</div></td> +<td class="tal">Anatomical Evidence</td> +<td class="tar"><div><a href="#Page_11">11</a></div></td> +</tr> +<tr> +<td class="tar"><div> 2.</div></td> +<td class="tal">Physiological Analogy with other Body Cells</td> +<td class="tar"><div><a href="#Page_12">12</a></div></td> +</tr> +<tr> +<td class="tar"><div> 3.</div></td> +<td class="tal">The Relation between the Foveal and the Extra-Foveal Regions</td> +<td class="tar"><div><a href="#Page_13">13</a></div></td> +</tr> +<tr> +<td class="tar"><div> 4.</div></td> +<td class="tal">The varying Sensibility of the Fovea</td> +<td class="tar"><div><a href="#Page_13">13</a></div></td> +</tr> +<tr> +<td class="tar"><div> 5.</div></td> +<td class="tal">Chemical Analogy</td> +<td class="tar"><div><a href="#Page_14">14</a></div></td> +</tr> +<tr> +<td class="tar"><div> 6.</div></td> +<td class="tal">Disappearance of Lights falling upon Fovea</td> +<td class="tar"><div><a href="#Page_15">15</a></div></td> +</tr> +<tr> +<td class="tar"><div> 7.</div></td> +<td class="tal">Illusion of Moving Light</td> +<td class="tar"><div><a href="#Page_16">16</a></div></td> +</tr> +<tr> +<td class="tar"><div> 8.</div></td> +<td class="tal">Purple After-Image</td> +<td class="tar"><div><a href="#Page_17">17</a></div></td> +</tr> +<tr> +<td class="tar"><div> 9.</div></td> +<td class="tal">Currents seen in the Field of Vision not due to the Circulation</td> +<td class="tar"><div><a href="#Page_17">17</a></div></td> +</tr> +<tr> +<td class="tar"><div>10.</div></td> +<td class="tal">Pressure Figure</td> +<td class="tar"><div><a href="#Page_19">19</a></div></td> +</tr> +<tr> +<td class="tar"><div>11.</div></td> +<td class="tal">Macular Star</td> +<td class="tar"><div><a href="#Page_19">19</a></div></td> +</tr> +<tr> +<td class="tar"><div>12.</div></td> +<td class="tal">Entoptic Appearance of Cone Mosaic</td> +<td class="tar"><div><a href="#Page_20">20</a></div></td> +</tr> +<tr> +<td class="tar"><div>13.</div></td> +<td class="tal">Visual Acuity</td> +<td class="tar"><div><a href="#Page_21">21</a></div></td> +</tr> +<tr> +<td style="text-align: left; text-indent: -2em;" class="fs110" colspan="2"><span class="smcap">The Evolution of the Colour-Sense</span></td> +<td class="tar"><div><a href="#Page_26">26</a></div></td> +</tr> +<tr> +<td style="text-align: left; text-indent: -2em;" class="fs110" colspan="2"><span class="smcap">The Facts of Colour-Blindness</span></td> +<td class="tar"><div><a href="#Page_34">34</a></div></td> +</tr> +<tr> +<td class="tar"><div> 1.</div></td> +<td class="tal">Defects of Light-Perception</td> +<td class="tar"><div><a href="#Page_35">35</a></div></td> +</tr> +<tr> +<td class="tar"><div> 2.</div></td> +<td class="tal">Defects of Colour-Perception</td> +<td class="tar"><div><a href="#Page_38">38</a></div></td> +</tr> +<tr> +<td style="text-align: left; text-indent: -2em;" class="fs110" colspan="2"><span class="smcap">The Two Main Varieties of Colour-Blindness</span>:</td> +<td></td> +</tr> +<tr> +<td class="tar"><div> 1.</div></td> +<td class="tal">Dichromic Vision</td> +<td class="tar"><div><a href="#Page_40">40</a></div></td> +</tr> +<tr> +<td class="tar"><div> 2.</div></td> +<td class="tal">Trichromic Vision</td> +<td class="tar"><div><a href="#Page_42">42</a></div><span class="pagenum" id="Page_8">8</span></td> +</tr> +<tr> +<td class="tac fs120" colspan="3"><div>LECTURE II</div></td> +</tr> +<tr> +<td class="tac fs110 pt05" colspan="3"><div>THE DETECTION OF COLOUR-BLINDNESS FROM A<br>PRACTICAL POINT OF VIEW</div></td> +</tr> +<tr> +<td class="tar"><div> 1.</div></td> +<td class="tal">Object of a Test for Colour-Blindness</td> +<td class="tar"><div><a href="#Page_44">44</a></div></td> +</tr> +<tr> +<td class="tar"><div> 2.</div></td> +<td class="tal">The Requirements of a Test for Colour-Blindness</td> +<td class="tar"><div><a href="#Page_45">45</a></div></td> +</tr> +<tr> +<td class="tar"><div> 3.</div></td> +<td class="tal">Persons to be Excluded</td> +<td class="tar"><div><a href="#Page_47">47</a></div></td> +</tr> +<tr> +<td class="tar"><div> 4.</div></td> +<td class="tal">The Construction of a Test for Colour-Blindness</td> +<td class="tar"><div><a href="#Page_48">48</a></div></td> +</tr> +<tr> +<td class="tar"><div> 5.</div></td> +<td class="tal">The Lantern Test</td> +<td class="tar"><div><a href="#Page_53">53</a></div></td> +</tr> +<tr> +<td class="tar"><div> 6.</div></td> +<td class="tal">Other Tests for Colour-Blindness</td> +<td class="tar"><div><a href="#Page_66">66</a></div></td> +</tr> +</table> +</div> + +<hr class="chap x-ebookmaker-drop"> + +<p><span class="pagenum" id="Page_9">9</span></p> + +<p class="tac fs130">LECTURE I</p> + +<p class="tac ptb05"><i>Delivered on February 1st</i></p> + + +<p><span class="smcap">Gentlemen</span>,—Colour-blindness is not a good name +for the condition to which it is applied, and still +worse is the use of the term red-blindness or green-blindness. +In the majority of cases of colour-blindness +there is no blindness to colours in the +ordinary acceptation of the term; a green, red, or +yellow light produces a very definite sensation of +colour. Those who confuse red and green do so, +not because they see red as green or green as red, +but because both give rise to a similar sensation of +colour. The word light must be used in the sense +of referring to those waves which excite the organ +of vision. Because two stimuli excite a sensation +of light, it does not follow that they are similar. +We cannot, for instance, distinguish by the eye +polarised light from non-polarised light. We have +to distinguish between the physical stimuli by +their physical properties apart from their effect on +the organ of vision. I propose to divide the +subject into two parts, and in this lecture to deal +with the theory and facts of colour-vision and +colour-blindness, and in the second lecture with +the detection of colour-blindness from a practical +point of view.</p> + +<hr class="r15 x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_10">10</span></p> + +<h2 class="nobreak" id="I_The_Theory_and_Facts_of_Colour-Vision">I. <span class="smcap">The Theory and Facts of Colour-Vision +and Colour-Blindness</span></h2> +</div> + + +<p>The following is the theory which I have propounded +in order to explain vision and colour-vision. +A ray of light impinging on the retina +liberates the visual purple from the rods and a +photograph is formed. The rods are concerned +only with the formation and distribution of the +visual purple, not with the conveyance of light-impulses +to the brain. The ends of the cones are +stimulated through the photo-chemical decomposition +of the visual purple by light (very +probably through the electricity which is produced), +and a visual impulse is set up which is +conveyed through the optic-nerve fibres to the +brain. The character of the stimulus differs +according to the wave-length of the light causing +it. In the impulse itself we have the physiological +basis of the sensation of light, and in the quality +of the impulse the physiological basis of the +sensation of colour. The impulse being conveyed +along the optic nerve to the brain, stimulates the +visual centre, causing a sensation of light, and +then passing on to the colour-perceiving centre, +causes a sensation of colour. But though the +impulses vary in character according to the wave-length +of the light causing them, the retino-cerebral +apparatus is not able to discriminate +between the character of adjacent stimuli, not +being sufficiently developed for the purpose. At +most, seven distinct colours are seen, whilst +others see in proportion to the development of<span class="pagenum" id="Page_11">11</span> +their colour-perceiving centres, only six, five, +four, three, or two. This causes colour-blindness, +the person seeing only two or three colours instead +of the normal six, putting colours together as +alike which are seen by the normal-sighted to be +different. In the degree of colour-blindness just +preceding total, only the colours at the extremes +of the spectrum are recognised as different, the remainder +of the spectrum appearing grey. Though +my own opinion is that the ordinary form of congenital +colour-blindness is caused by a defective +development of the portion of the brain which has +the function of the perception of colour, we must +not exclude any portion of the retino-cerebral apparatus, +defect of which would have exactly the +same result. It will be noticed that the theory +really consists of two parts, one concerned with +the retina and the other with the whole retino-cerebral +apparatus. I shall in these lectures use +the word cerebral in this sense. I am not aware +of a single fact which does not support this theory, +and I have used it to predict facts which have +subsequently been rediscovered by others and +now form a part of our common knowledge.</p> + + +<h3>THE VISUAL PURPLE THE ESSENTIAL FACTOR +IN VISION</h3> + +<p>I will now state very briefly the evidence which +supports the view that the visual purple is the +essential factor in the retina which enables it to +transform light into visual impulses.</p> + +<p>1. <i>Anatomical.</i>—In the fovea of the retina only +cones are to be found. Immediately external to<span class="pagenum" id="Page_12">12</span> +this each cone is surrounded by a ring of rods. +The number of rings of rods round each cone +increases as the periphery is reached. The outer +segments of the cones are situated in a space +which is filled with fluid. The external limiting +membrane retains this fluid in its place. I find<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">1</a> +four depressions or canals which lead into the +larger depression of the external fovea. These +canals appear to have smaller branches, and serve +to conduct the visual purple into the part of most +acute vision. The cones which are present in the +fovea have very long outer segments which would +present a greater surface for photo-chemical +stimulation. The visual purple is only to be found +in the rods and not in the cones. I determined to +ascertain whether the visual purple could be seen +between the cones in the fovea. I have examined +under the microscope the retinas of two monkeys +which had been kept previously in a dark room for +forty-eight hours. The yellow spot was the +reddest part of the whole retina, and the visual +purple was seen to be between and not in the +cones.<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">2</a></p> + +<p>2. <i>Physiological analogy with other body cells.</i>—It +is far more probable that the rods should produce +a secretion which would affect other cells +rather than themselves. The liver cells do not +form bile in order to stimulate themselves, and the +internal secretions are produced to affect other +parts of the body. I am not aware of a single +instance in which a cell produces a secretion which<span class="pagenum" id="Page_13">13</span> +has the function of stimulating the cell producing +it. The visual purple is regenerated in the rods by +the pigment cells in connection with them.</p> + +<p>3. <i>The relation between the foveal and the extra-foveal +regions.</i>—As the fovea only contains cones, +if any of the older theories of the relative functions +of the rods and cones were true we should expect +to find qualitative differences between the foveal +and extra-foveal regions. This is not the case, +but as we should expect if the visual purple were +the visual substance, all the phenomena which +have been attributed to the visual purple should +be found in the fovea. Von Tschermak, Hering, +Hess, Garten and others have found the Purkinje +phenomenon, the variation in optical white equations +by a state of light and dark adaptation, the +colourless interval for spectral lights of increasing +intensity, and the varying phases of the after-image +in the fovea only gradually diminished.</p> + +<p>4. <i>The varying sensibility of the fovea.</i>—The +fovea is in some conditions the most sensitive part +of the whole retina, and with other conditions the +least. Helmholtz has recorded some of these facts +and regarded them as quite inexplicable. We have, +however, an easy explanation of the facts on the +assumption that when there is visual purple in the +fovea this is the most sensitive part of the whole +retina, but when there is none there time must +elapse before it can diffuse into the spot, and in +the meantime it is insensitive to light. I have +devised several experiments which show the +visual purple flowing into the foveal region. The +following simple experiment shows this very well.<span class="pagenum" id="Page_14">14</span> +If on awaking in the morning the eyes be directed +to a dull white surface, as for instance the ceiling, +the region of the yellow spot will appear as an +irregular black spot, and light will appear to invade +this spot from without inwards. If the eyes be +now closed and covered with the hands, purple +circles will form round the centre of the field of +vision and gradually contracting reach the centre. +When the circle reaches the centre it breaks up +into a star-shaped figure and becomes much +brighter. It then disappears and is followed by +another contracting circle. Now it will be noticed +that if one eye be opened when the circle has +broken up, a brilliant rose-coloured star much +brighter than any other part will be seen in the +centre of the field of vision. This has the exact +hue of the visual purple. If we wait until the +star has disappeared before opening an eye, the +macular region appears as a black spot as before. +This conclusively shows that the central portion +of the retina is sensitised from the peripheral +portions.</p> + +<p>5. <i>Chemical analogy.</i>—The visual purple gives +a curve which is very similar to that of many other +photo-chemical substances. We know that with +photo-chemical substances the chemical effect is +not proportional to the intensity of the light. That +is, a different curve is obtained with weak light from +that which is formed with light of greater intensity. +It is reasonable, therefore, to suppose that the +visual purple which is formed by the pigment +cells under the influence of a bright light would be +somewhat different in character from that which is<span class="pagenum" id="Page_15">15</span> +formed in darkness. Again, from the chemical +analogy which I have just given, even if the +visual purple were of the same character, we +should not expect similar curves with different +intensities of light. It is probable that both +factors are in operation. This deduction gives an +explanation of the Purkinje phenomenon, or the +fact that when the eyes are adapted to darkness +the point of greatest luminosity is shifted more +towards the violet end of the spectrum. Some +dichromics who have shortening of the red end of +the spectrum have a luminosity curve which is very +similar to that of a normal-sighted person with +a spectrum of lesser intensity. We have only to +assume in these cases either that the receiving +nervous apparatus is less responsive, or that the +visual purple formed at one intensity of light is +similar to that formed at a lower intensity by a +normal-sighted person. We also have an explanation +of other conditions, such as erythropsia, or +red vision, white objects appearing more or less +red. If we suppose that the eye has remained +in a state of light adaptation, the visual purple +produced being more sensitive to the red rays, +objects appear of this colour. As we should +expect, erythropsia is frequently associated with +hemeralopia, or difficulty in seeing in the twilight, +the eyes being adapted to light and not to darkness. +In green vision the eyes have probably +remained in a condition of more or less adaptation +to darkness, and are therefore more sensitive +to the green rays.</p> + +<p>6. <i>Disappearance of lights falling upon fovea.</i>—If<span class="pagenum" id="Page_16">16</span> +the cones are not sensitive to light, a ray of +light falling upon the fovea alone and not upon +the adjacent portion of the retina containing rods +should produce no sensation of light, provided that +there is not already any visual purple in the fovea. +It has been known to astronomers for a long time +that if a small star in a dark portion of the sky be +steadfastly looked at, it will disappear from view, +whilst other stars seen by indirect vision remain +conspicuously visible. The following simple experiment +shows the same thing. If a piece of black +velvet about three feet square on a door have a pin +put in the centre, and the source of light be behind +the observer, the pin will be brightly illuminated; +and on looking at it (the observer not being too +close) and keeping the eye quite still, the pin will +disappear, the visual substance diffused into the +fovea centralis being used up and not renewed. +When viewed by indirect vision it is impossible to +make it disappear in this way. When I have +taken great care to have very dark surroundings +and have used only one eye, I have made moderately +bright lights disappear in this manner. +These facts have been attributed to a defective +sensibility of the fovea for feeble light. The +important point, however, that the light is at first +most clearly seen by the fovea and only subsequently +fades, has been overlooked. If these facts +were due to a defective sensibility of the fovea, the +star or light would not be visible at first.</p> + +<p>7. <i>Illusion of moving light.</i>—If a small light be +looked at fixedly in a dark room, it will appear +to move until it comes apparently so close that<span class="pagenum" id="Page_17">17</span> +it could be grasped. The reason of this is that +the eye moves so that the light falls upon a more +peripheral part of the retina. I find that the +movement takes place as if some photo-chemical +substance acted under the influence of gravity. +For instance, when standing the light appears to +travel upwards; resting the head on one side, it +appears to travel in the opposite direction. The +light appears as if we were looking straight at it, +and the eye, which is covered up, remains directed +straight at the object. When the second eye is +opened two images of the light are seen, and the +image which is seen with the periphery of the +first eye rapidly coalesces with that seen directly +by the second eye.</p> + +<p>8. <i>Purple after-image.</i>—A positive after-image +of a purple (rose) colour can be obtained after +white light or any spectral colour. It will be +noticed that when there is little light during the +subsequent observation the colour of an after-image +inclines to blue or green, when there is +more light towards purple or red.</p> + +<figure class="figcenter illowe25_625" id="i_18"> + <img class="w100" src="images/i_18.jpg" alt=""> + <figcaption><p>Fig. 1.</p></figcaption> +</figure> + +<p>9. <i>Currents seen in the field of vision not due to +the circulation.</i>—It occurred to me that if there +were canals in the retina which promoted the easy +flow of the visual purple into the fovea, we ought +to obtain evidence of the currents flowing along +these channels entoptically. I found that this +was the case, and that the currents could be seen +in numerous ways.<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">3</a> If one eye be partially +covered with an opaque disc whilst both eyes +are directed forwards in a not too brightly illuminated<span class="pagenum" id="Page_18">18</span> +room, and special attention be paid to the +covered eye, an appearance of whirling currents +will be seen with this eye (see Fig. 1). These +currents appear to be directed towards the centre, +and have a very similar appearance to a whirlpool +which is fed by four main branches. These<span class="pagenum" id="Page_19">19</span> +again are fed by smaller branches which continually +change their paths. On closing both +eyes all the portion in which the whirling currents +are seen appears as dull purple. These +currents cannot be due to vessels, because we +know that the centre of the retina corresponding +to the point where the greatest movement is seen +is free from vessels. The appearance is also very +different from that of the movement of blood in +vessels. The currents can also be seen in the +light, in the dark, through yellow-green glass, and +with intermittent light. The main branches form +a star-shaped figure with four rays. The currents +carry the visual quality, colour, and brightness +from whence they come into an after-image. +They also tend to move an after-image towards +the centre. The currents behave as if they ran +in definite channels, but could also overrun, on +any further stimulus, the banks of the channels. +For instance, a thin, bright line with a little more +light appears as a broad band, and the central +star figure will enlarge into a rhomboid, oval or +disc. Movements of the eyes affect the broad +currents in the outer part of the field of vision.</p> + +<p>10. <i>Pressure figure.</i>—Pressure on the front of +the eye causes the star-shaped figure to be seen, +and this changes into a rhomboid with a little +more pressure.</p> + +<p>11. <i>Macular star.</i>—It occurred to me that we +ought to obtain evidence of the canals in the +retina in cases where the outflow from the retina +is obstructed, as by tumour. I find this is the +case; the star-shaped figure given by Sir Victor<span class="pagenum" id="Page_20">20</span> +Horsley in his paper on tumour of the frontal lobe<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">4</a> +is almost exactly the same as that seen subjectively.</p> + +<figure class="figcenter illowe27_5" id="i_20"> + <img class="w100" src="images/i_20.jpg" alt=""> + <figcaption><p>Fig. 2.</p></figcaption> +</figure> + +<p>12. <i>Entoptic appearance of cone mosaic.</i>—Appearances +corresponding to the cone mosaic of +the retina may be seen in several ways<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">5</a> (see +Fig. 2). The appearance seen corresponds to the<span class="pagenum" id="Page_21">21</span> +cone distribution of the retina as viewed from its +outer side, the portions occupied by rods appearing +as dark spaces.</p> + +<p>13. <i>Visual acuity.</i>—Visual acuity is most acute +with the fovea, and diminishes from within outwards. +It corresponds very fairly with the cone +distribution of the retina. On the other hand, +there is not one single fact which points to the +rods as being light-sentient organs. This is well +recognised by those best qualified to judge.<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">6</a> I +could give many more facts in support of the +view that the visual purple is the visual substance, +and I have not yet had brought to my notice +any fact which is not readily explicable on that +hypothesis. There may be other photo-chemical +substances in the retina, but there is not the +slightest evidence that such is the case. I regard +the visual purple as the sole visual substance. +We could, of course, split the visual purple into +innumerable simpler photo-chemical substances, +each of which has its own absorption curve, +having its maximum in some particular part of +the spectrum. It is difficult to say at present +exactly how the visual purple acts as a stimulus +transformer, but this is because so many plausible +hypotheses immediately occur to us. It is very +probable that light acting upon the visual purple +is, according to its wave-length, absorbed by particular +atoms or molecules, the amplitude of their +vibrations being increased. These vibrations may +cause corresponding vibrations in certain discs of +the outer segments of the cones, which seem<span class="pagenum" id="Page_22">22</span> +especially constructed to take up vibrations. We +know that when light falls on the retina it causes +an electric current. We know how the telephone +is able through electricity to convey waves of +sound, and something similar may be present +in the eye, the apparatus being especially constructed +for vibrations of small wave-length. +The current of electricity set up by light may +cause the sensation of light, and the vibrations of +the atoms or molecules the sensation of colour.</p> + +<p>In all vital processes there is a condition of +katabolism or chemical change in the protoplasm, +and an anabolic or building-up process, in which +the protoplasm is restored to its normal state. +We have therefore to consider two definite processes +in the visual purple—namely, a breaking +down of the visual purple photo-chemically by +light and its restoration by the pigment cells and +rods. Under ordinary conditions of light, and +during the whole of the daytime, the visual purple +is continually being bleached and reformed. It +is obvious, therefore, that when the eye has been +kept in the dark and is then exposed to light, an +observation taken immediately will not be comparable +with one taken a few seconds afterwards, +because in the first observation we have only to +consider the katabolic change; whilst in the +second observation the anabolic change has to be +considered as well, as the visual purple has to be +reformed for subsequent seeing. There appears +to be very little evidence in ordinary circumstances +of this anabolic process; for instance, if we +fatigue the eye with sodium light in a dark room,<span class="pagenum" id="Page_23">23</span> +and then immediately examine a spectrum, we +find that though all the yellow has disappeared +there is no increase in the blue; in fact, the blue +seems rather diminished than otherwise. Again, +there is not the slightest diminution in either the +red or green, showing conclusively that yellow +cannot be a compound sensation made up by a +combination of red and green. We must therefore +explain in another way the apparent trichromatism +of normal colour-vision, which is so +well known to every photographer, especially +those who are concerned with colour photography. +If my theory of the evolution of the +colour-sense be the correct one, and we have +cases of colour-blindness corresponding to every +degree of the evolutionary process, we have an +explanation of the facts. In past ages all saw +the rainbow made up of only three colours—red, +green, and violet. When a new colour (yellow) +appeared between the red and green, it is obvious +that a mixture of red and green would give rise, +not to red-green, but to the colour which had replaced +it—namely, yellow. The retina, therefore, +corresponds to a layer of photo-chemical +liquid in which there are innumerable wires each +connected with a galvanometer. When light falls +upon a portion of this fluid the needle of the +galvanometer corresponding to the nearest wire +is deflected. The wires correspond to the separate +fibres of the optic nerve, and the galvanometers +to the visual centres of the brain.</p> + +<p>Cases of colour-blindness may be divided into +two classes, which are quite separate and distinct<span class="pagenum" id="Page_24">24</span> +from each other, though both may be present in +the same person. In the first class there is light +as well as colour loss. In the second class the +perception of light is the same as the normal-sighted, +but there is a defect in the perception +of colour. In the first class certain rays are either +not perceived at all or very imperfectly. Both +these classes are represented by analogous conditions +in the perception of sounds. The first +class of the colour-blind is represented by those +who are unable to hear very high or very low +notes. The second class of the colour-blind is +represented by those who possess what is commonly +called a defective musical ear. Colour-blind +individuals belonging to this class can be +arranged in a series. At one end of this series +are the normal-sighted, and at the other end the +totally colour-blind. The colours appear at the +points of greatest difference, and I have classified +the colour-blind in accordance with the number +of colours which they see in the spectrum. The +normal-sighted may be designated hexachromic; +those who see five colours, pentachromic; those +who see four, tetrachromic; those who see three, +trichromic; those who see two, dichromic; and +those who see none, totally colour-blind. There +are many degrees included in the dichromic class. +There may or may not be a neutral band, and this +is widest in those cases approaching most nearly +to total colour-blindness. I have recorded a case +of a patient who was colour-blind with one eye.<span class="pagenum" id="Page_25">25</span><a id="FNanchor_7" href="#Footnote_7" class="fnanchor">7</a> +It is an interesting fact that for form vision the +colour-blind eye was much the better of the two, +and he could recognise fine lines in the spectrum +with this eye which were not visible to the other. +He saw the two ends of the spectrum tinged with +colour and the remainder grey. It will be noticed +that his colour sensations were limited to the extreme +red and the extreme violet—namely, those +colours which present the greatest physical contrast +to each other. Neither the red nor the +violet appeared of the nature of a primary colour, +but gave the impression that they were largely +diluted with grey. A theory of colour-vision +must account for a case of this kind, and also for +the other varieties and degrees of colour-blindness. +The trichromic are a very important class, and +any theory must account for the fact that they +see yellow as red-green, and blue as violet-green. +As we should theoretically expect, when there is +shortening of the spectrum the centres of the +colours are moved towards the unshortened side.</p> + +<p>I will now show on the screen some representations +of pictures painted by colour-blind persons. +The upper picture is the copy, and the one below +is the one which has been painted by the colour-blind +artist. He has been given a selection of +colours on plates, and from them has selected the +one which he thought appropriate in each case. +It will be noticed that the mistakes made are +characteristic of the colour-perception of the +person painting them. Whenever I show these +pictures, I am asked why it is that these characteristic +mistakes should be made, and that<span class="pagenum" id="Page_26">26</span> +the true colour of the object is not used instead? +This undoubtedly would be the case if the artist +were allowed to match the colours by directly +comparing them. But he is not able to do this; +he looks at the copy and decides upon the colour +of an object, and then looks for the colour with +which to paint it.</p> + +<p>A man rarely uses a hue which he does not see +as a definite colour, and therefore it has been +quite possible for me to pick out those who are +more or less colour-blind in the exhibitors of the +picture gallery. For instance, if a trichromic have +to paint a yellow object he will decide, after looking +at it, whether it be a red or green in his estimation, +and represent it accordingly. He will be +greatly influenced by the nature of colours in its +immediate proximity, because simultaneous contrast +is increased in the colour-blind. Thus he +will certainly represent a yellow which is adjacent +to a red as green, and a yellow which is adjacent +to a green as red.</p> + + +<h3>THE EVOLUTION OF THE COLOUR-SENSE</h3> + +<p>There can be no doubt that an evolution of the +colour-sense has taken place: the only point is +how and when did this occur. It is obvious that +in those low forms of animal life in which the most +rudimentary sense of sight exists there can be +no sense of colour. The animal which can only +perceive light and shade can only discriminate in +a rough way between varying intensities of the +stimulus. It is obvious, therefore, that the sense of +light must have been developed first and then the<span class="pagenum" id="Page_27">27</span> +sense of colour. The sense of sight must have been +first developed for those waves which produce their +maximum effect upon the sensitive protoplasm. +The next process of development would be for the +protoplasm to become sensitive to the waves above +and below those which produced the primary +stimulus. In the physical stimulus which produces +the sensation of light there are two factors +to be considered, the length of the wave and its +amplitude: the greater the amplitude within +certain limits the greater the intensity of the +sensation. The wave-length of the physical +stimulus is the physical basis of the sensation of +colour. How did the sensations of colour first +arise? Let us suppose that the physiological +effect of the physical stimulus differed according +to the wave-length of the physical stimulus.</p> + +<p>Let us consider that the eye has reached a stage +in which it has become sensitive to a fair range of +the spectral rays; that is to say, evolution has +proceeded to the extent of making the protoplasm +sensitive to rays of light considerably above and +below those which first caused a sensation of +light. We now have an eye which is sensitive +to the greater part of the rays which form the +visible spectrum. It is, however, an eye which is +devoid of the sense of colour; no matter from +what part of the spectrum the rays be taken the +only difference which will be appreciated will be +one of intensity. I however mentioned that in the +physical stimulus there were two variables, wave-length +and amplitude of the wave. Let us now +suppose that a fresh power of discrimination was<span class="pagenum" id="Page_28">28</span> +added to the eye and that it became able to discriminate +between different wave-lengths of light. +What would be the most probable commencement +of development of the sense of colour? Undoubtedly +to my mind the differentiation of +physical stimuli which were physically most +different. That is to say, the eye would first +discriminate between the rays which are physically +most different in the visible spectrum, the +red and the violet, that is presuming the eye had +become sensitive to this range. It is probable +that it had not, and there has been a steady +evolution as to the extent of the spectrum perceived +as well as to colour. We have examples of +this in those cases of defective light-perception +in which there is shortening of the red or violet +end of the spectrum.</p> + +<p>Let us now work out the evolution of the colour-sense +on the assumption that the rays which are +physically most different, namely, red and violet, +were those which were first differentiated. We +know that the various rays differ in their effects on +various substances; the red rays are more powerful +in their heating effects, whilst the violet rays are +more active actinally, as is well known by the +readiness with which they act upon a photographic +plate, which is scarcely affected by red light. We +should now have an individual who would see the +spectrum nearly all a uniform grey of different +degrees of luminosity, but with a tinge of red at +one end and a tinge of violet at the other. There +is a great deal of evidence to show that this is how +the colour-sense was first developed. For instance,<span class="pagenum" id="Page_29">29</span> +in the degree of colour-blindness just preceding +total the spectrum is seen in this way. I have also +examined a woman who became totally colour-blind, +apparently through disease of the ear. I +examined her when she had recovered a certain +amount of colour sensation; her sensations were +confined to the extreme red and violet. As the +colour sense developed it was not necessary that +the rays should differ so much in refrangibility +before a difference was seen, and so the red and +violet gradually invaded the grey or neutral band, +until at a certain point they met in the centre of +the spectrum. Such cases are called dichromics.</p> + +<p>The next stage of evolution of the colour-sense +is when the colour-perceiving centre is sufficiently +developed to distinguish three main colours in the +spectrum. The third colour, green, appears in the +centre of the spectrum, that is, at the third point +of the greatest physiological difference. In +accordance with the prediction of the theory, I +found a considerable number of persons who saw +the spectrum in this way, about 1·5 per cent of +men. The trichromic see three main colours in +the spectrum—red, green, and violet. They usually +describe the spectrum as consisting of red, red-green, +green, green-violet, and violet. They do +not see yellow and blue as distinct colours, and are +therefore in continual difficulty over them. There +are very few of the tests in general use which can +detect them, especially if names be not used. +They will usually pass a matching test with ease. +An examination with the spectrum shows that +their colour-perception is less than the normal in<span class="pagenum" id="Page_30">30</span> +every part, though the curve has the same general +shape. The three trichromics described in my +recent paper<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">8</a> on “The Relation of Light-Perception +to Colour-Perception” each saw ten +consecutive monochromatic patches in the spectrum +instead of the eighteen or nineteen seen by +those who see six colours in the spectrum. It is +easy to show that the trichromic are dangerously +colour-blind. They will mark out with my colour-perception +spectrometer a patch containing +greenish yellow, yellow, and orange-yellow, and +declare that it is absolutely monochromatic. +When tested with coloured lights they find great +difficulty with yellow and blue. Yellow is continually +called red or green.</p> + +<p>There are several other degrees of colour +perception, and it may be well to say a word or +two about them, though I class all above the +trichromic with the normal-sighted for practical +purposes, as they are not dangerously colour-blind, +and can always, in ordinary circumstances, distinguish +signal lights correctly. In the next stage +of evolution four colours are seen in the spectrum, +and the fourth colour appears at the fourth point +of greatest physiological difference, namely, at +the orange-yellow of the hexachromic or six-colour +people. These persons I have designated “tetrachromic,” +because they see four distinct colours in +the spectrum, that is, red, yellow, green, and +violet. They do not see blue as a definite colour, +and are continually classing blues with greens; +they usually prefer to call blue, purplish green.<span class="pagenum" id="Page_31">31</span> +In the next stage of evolution there appeared +those who see five colours in the spectrum—red, +yellow, green, blue, and violet, blue being now +recognised as a definite colour. These are the +pentachromic group. These people pass all the +tests in general use with ease. They, however, +have a definitely diminished colour-perception +compared with the normal, or those who see six +colours in the spectrum. They mark out in the +spectrum only fifteen monochromatic patches +instead of eighteen. They cannot see orange as a +definite colour; for instance, they can never tell +whether a strontium light, which is red, or a +calcium light, which is orange, is being shown +them.</p> + +<p>In the next stage of evolution orange is recognised +as a definite colour, and thus we get the +hexachromic or normal group, and, as we should +theoretically expect, the yellow of the pentachromic +is now split up into two colours—orange +and yellow. The last stage of evolution which +we appear to have reached are those who see +seven colours in the spectrum, and the additional +one is called indigo. These constitute the heptachromic +group, and this seventh colour appears at +the exact point at which it should appear, according +to my theory, namely, between the blue and +violet. Persons belonging to this class have a +marvellous colour-perception and memory for +colours. They will indicate a certain shade of +colour in the spectrum, and then next day will be +able to put the pointer at precisely the same point, +a feat which is quite impossible to the ordinary<span class="pagenum" id="Page_32">32</span> +normal-sighted person. They see a greater +number of monochromatic patches in the spectrum +than the hexachromic, but the curve has the same +form. The marking out of the heptachromic does +not appear correct to those who see six colours; +for instance, the blue appears to invade the green, +and the indigo does not appear a definite colour +at all. If, however, we bisect the blue of the +seven-colour man, and then bisect his indigo, on +joining the centres we get the blue of the six-colour +man, showing most definitely that the blue +has been split up into two fresh colours. It will be +noticed that there is room for much further evolution, +and we could go on splitting up the spectrum +indefinitely if only we had the power to distinguish +these finer differences, but as a matter of +fact I have never met with a normal-eyed man +who could see more than twenty-nine monochromatic +patches in the spectrum, and there are +really millions, though by monochromatic patches +I do not mean twenty-nine separate colours. Not +only are all the details of the process of the evolution +of the colour-sense supported by all the facts +that we can obtain from literature and museums, +but the theory accounts for facts which were +previously inexplicable. The distinction between +light-sensation and colour-sensation is explained, +and all facts of colour-mixing, complementary +colours, and simultaneous contrast. We can +understand how, as in many cases which have been +recorded, a man may lose his colour-perception +and still have an unaltered sense of luminosity +and visual acuity.</p> + +<p><span class="pagenum" id="Page_33">33</span></p> + +<p>The explanation of complementary colours is a +fundamental part of the theory. It is obvious +that the two colours of the dichromic are only +recognised as different because they are seen in +contrast to each other, and that when they are +mixed they neutralise each other. It is the same +with the other colour-sensations, when they are +developed they replace the colours occupying +their positions. Therefore green which replaces +the grey of the dichromic should be, and is, +complementary to the other two colour-sensations, +red and violet combined. In the same way when +the yellow sensation replaces the red-green of the +trichromic it should be possible to compound it of +both. Also, when the green sensation is in a +feeble state of development it will not have the +value that it has at a subsequent stage, and, +therefore, yellow will be a much redder colour to +those persons than the normal, and in a colour +match of red and green forming yellow, more +green will be required.</p> + +<p>Simultaneous contrast is also explained. When +two colours are contrasted each appears to be a +colour higher or lower, as the case may be, in the +spectrum scale; that is to say, the close comparison +exaggerates the difference. As the colour-blind +have fewer colours, simultaneous contrast +should be greater with them, and this I have +found to be the case.</p> + +<p>There may be some relation between the monochromatic +patches and the discs in the outer segments +of the cones. These are about sixteen in +number in the guinea-pig. As in photography,<span class="pagenum" id="Page_34">34</span> +the intensity of the light is a very important factor +in vision. With colours of moderate intensity, +the periphery of the retina is found to be colour-blind, +but this apparent colour-blindness disappears +when more intense lights are used. A +person may have shortening of the spectrum with +light of moderate intensity, but when the light is +increased be able to recognise the spectrum to its +normal limit. The change in steepness of gradation, +according to the intensity of the light, is +well known to photographers. The Purkinje +effect, a change in maximum sensitiveness of the +eye according to the intensity of the light, is, in +my opinion, a photo-chemical effect. I find that +the Purkinje effect is found for small portions of +the retina if a black object has been situated in the +corresponding part of the field of vision. The +yellow pigment which is found in the yellow spot +probably acts like the yellow screen in photography, +which, by absorbing the blue and violet +rays of the atmosphere, renders visible that which +would otherwise be invisible. This is further +borne out by the fact that hunters in India are +able to hunt later in the day than usual by using +spectacles glazed with golden yellow glass.</p> + + +<h3>THE FACTS OF COLOUR-BLINDNESS</h3> + +<p>When we consider the path along which a +visual impulse has to pass, and that each cell +has probably some special function in connection +with that impulse, it is not surprising that we +meet with a large number of different defects of +colour-perception and light-perception. Defects<span class="pagenum" id="Page_35">35</span> +of light-perception are quite distinct from defects +of colour-perception.</p> + +<p>1. <i>Defects of light-perception.</i>—The person +having the defect is placed in a similar position +to a normal-sighted person with those particular +rays removed or reduced to the same intensity. +Defects of light-perception may be caused by +absorption or by some defect in the visual purple +or cerebro-retinal apparatus. The chief defect of +light-perception which is found is shortening of +the red or the violet end of the spectrum. Let +us consider the influence of a shortened spectrum +upon colour-vision. The first evident fact is that +bodies reflecting only light, the rays of which +occupy the missing portion of the spectrum, +appear black.</p> + +<p>Nearly all colours are compound; that is to +say, the coloured body reflects other rays than +those of the colour seen. Thus a blue-green glass +may transmit the green, blue, and violet rays of +the spectrum. Let us suppose that we have a +substance reflecting the green, blue, and three-quarters +of the violet, the colour of the body to +a normal person being green. Then if we had +another substance which reflected the whole of +the violet, it would appear blue. But with a +person who could not perceive the terminal fourth +of the violet the colour would look exactly the +same as the green one, and as he could not distinguish +between the two he would be in continual +difficulty with blues and greens. All +coloured objects reflecting rays occupying the +missing portion appear darker than they do to<span class="pagenum" id="Page_36">36</span> +the normal-sighted, and are always matched with +darker colours belonging to a point more internal. +Thus a dichromic with a shortened red +end of the spectrum matches a red with a darker +green.</p> + +<p>It will be noticed that a shortened spectrum, +especially if one end only be affected, may interfere +very little with the general appreciation of +shade. If, for instance, we take a case in which +the red end of the spectrum is shortened, so that +only three-quarters of the red of the normal-sighted +is seen, then all bodies which equally reflect +or transmit these rays can be correctly compared, +because a similar portion of light has been +removed from each. It is only when one colour +reflects or transmits the rays occupying the +shortened portion, and the other does not, that +there is any definite interference with the appreciation +of shade. Again, if neither colour reflects +or transmits rays occupying the shortened portion +of the spectrum, there will obviously be no interference +with the appreciation of shade.</p> + +<p>A very common mistake due to shortening of +the red end of the spectrum is the confusion of +pink and blue. If a person with considerable +shortening of the red end of the spectrum is +shown a pink which is made up of a mixture of +red and violet, the red consisting of rays occupying +the missing portion of the spectrum, only +the violet is visible to him, and so the pink appears +a violet without a trace of red. This pink is therefore +matched with a violet or blue very much +darker than itself.</p> + +<p><span class="pagenum" id="Page_37">37</span></p> + +<p>Mistakes which are due to shortening of the +spectrum may be remedied if we subtract the +rays occupying the missing portion from the +colour of confusion. For instance, if we take a +blue and a pink which have been put together as +identical by a person with a shortened red end +of the spectrum, and look at them through a glass +which is opaque to the red, but transparent to +the remaining rays of the spectrum, both will +appear alike in hue and shade. A person with +considerable shortening of the red end of the +spectrum will look at a red light (which is so +dazzlingly bright to a normal-sighted person as +to make his eyes ache after looking at it closely +for a few seconds), at a distance of a few inches, +and remark that there is nothing visible, and that +the whole is absolutely black. It is obvious that +the light must consist only of rays occupying the +missing portion of the spectrum. The same remarks +which I have made for a shortened spectrum +apply to cases in which there is defect of +light-perception through absorption or any other +cause. The person having the defect is placed in +a similar position to a normal-sighted person +with those particular rays removed or reduced to +the same intensity.</p> + +<p>Another effect of shortening of the spectrum +when it is sufficient to interfere with the difference-perception +which appears to be inherent in the +central nervous system, is that the colours appear +to be moved in the direction of the unshortened +portion. For instance, we find the neutral point +of the dichromic, with shortening of the red end<span class="pagenum" id="Page_38">38</span> +of the spectrum, in most cases further towards the +violet end of the spectrum in comparison with a +case in which the spectrum is of normal length. +In the same way a trichromic with a shortened +red end of the spectrum has the junction of the +red and green nearer the violet end than in a case +where there is no shortening.</p> + +<p>The point that I specially wish to emphasise is +that, though every case in which there is defective +light-perception can be explained by a defective +sensibility to light of certain wave-length, +not a single case of the very large number of +persons that I have examined can be explained on +the older theories; that is, the defect of light-perception +cannot be explained on the assumption that +there is a defect in a light-perceiving substance +which is sensitive to rays of light from a considerable +range of the spectrum. A large number of cases +in which there is shortening of the red end +of the spectrum escape detection when only the +green test is used, as is usual according to Holmgren’s +instructions.</p> + +<p>2. <i>Defects of colour-perception.</i>—The colour-blind +have a diminished hue-perception and see +a less number of colours than the normal. All +the symptoms of colour-blindness are such as we +should expect from want of development of the +retino-cerebral apparatus for the perception for +colour. This is evident even in the slighter cases +which show a diminished colour-perception compared +with the normal. We find that the colour-blind +are much more dependent on the luminosity +of the colour than the normal-sighted; they require<span class="pagenum" id="Page_39">39</span> +a stronger stimulus; they fatigue more +easily with colours than the normal-sighted; they +have a more marked simultaneous contrast; the +visual angle subtended by the coloured object +requires to be larger, and they have a very bad +memory for colours. The diminution of colour-perception +with a diminished visual angle evidently +depends upon several causes. It is very +marked when there is diminished light-perception +for those rays which are imperfectly seen. It is +also dependent upon certain retinal conditions, +as in scotoma and allied conditions. There are +colour-blind persons, however, who are able to +recognise colours under as small a visual angle +as the normal-sighted, and I have examined +one dichromic (said to be red-blind by a physicist) +who recognised red easily through the thickest +neutral glass of my lantern, and who had no +difficulty with this colour at a distance.</p> + +<p>Apart from any other defect of light or colour-perception, +every case with which I have met has +fallen naturally into one of the classes I have +given; that is to say, every person is either +heptachromic, hexachromic, pentachromic, tetrachromic, +trichromic, dichromic, or totally colour-blind. +When I first gave this classification of +colour-blindness, the facts that I discovered were +so at variance with those generally stated that it +was very difficult for those who were not well acquainted +with the subject to compare the two +sets. The general knowledge of the subject has, +however, steadily increased, and the facts which +I had so great a difficulty in getting recognised<span class="pagenum" id="Page_40">40</span> +now form part of our common knowledge. It +would be well, therefore, to describe the two main +varieties of colour-blindness which are of chief +practical importance, and to show the relation +which they bear to the writings of other persons. +These two main varieties have dichromic and +trichromic vision.</p> + +<p>1. <i>Dichromic vision.</i>—The cases which come +under this head form the class of the ordinary +red-green blind. It is under this head that nearly +every one of the recorded cases may be classed. +Vision, as far as colour is concerned, is dichromic, +the neutral point being situated in the green of +the normal-sighted at about λ 500. All the +colours on the red side tend to be confused with +each other; therefore red, orange, yellow and +half of the green are seen as one colour, the remainder +of the green, blue and violet as the +other. The luminosity curve in uncomplicated +cases is similar to the normal. There may be +shortening of the spectrum at either the red or +the violet end of a varying degree. All degrees +of shortening of the red end of the spectrum may +be found. Dichromics with normal luminosity +curves are those which were formerly designated +green-blind; but this designation is not in accordance +with the facts, because there is no +defect of light-perception in the green, and the +so-called diagnostic mistakes, as, for instance, +putting a bright green with a dark red, are not +made. Cases of so-called red-blind are dichromics +with shortening of the red end of the spectrum. +I have shown that the defective perception of<span class="pagenum" id="Page_41">41</span> +the red end of the spectrum will not account for +the dichromic vision which is found in these +cases. We may also meet with shortening of the +spectrum with otherwise normal colour-perception. +We also meet with dichromic cases forming +a series from almost total colour-blindness to +those bordering on the trichromic. Any theory +must account for the fact that there are varying +degrees of colour-blindness in dichromic vision, +and why there is a large neutral band corresponding +to the colours of the centre of the spectrum +in some cases, and in others the neutral band is +so small that the dichromic cannot mark it out. +The two colours seen by the dichromic are red and +violet, though where no distinction is seen between +yellow and red, and blue and violet, the brighter +colour will often be selected; that is why so many +dichromics say that they see yellow and blue in +the spectrum. Those who have had practical +experience of colour-blindness will know, however, +that many dichromics make no mistakes +with the red test. The following will give the +normal-sighted the best idea of colour-blindness, +and it is how the dichromic see the spectrum, +and explains why they are able to distinguish +between colours. Let him regard the dichromic +as a man who has two colours—red and violet +and white. The purest red is at the red end of +the spectrum; this becomes less and less saturated +as the violet is approached until the neutral +or white point is reached; then violet comes +into the white, and this increases in saturation +to the termination of the violet. The ordinary<span class="pagenum" id="Page_42">42</span> +dichromic therefore sees green as a much whiter +and less saturated colour than red.</p> + +<p>2. <i>Trichromic vision.</i>—These persons see three +distinct colours in the spectrum—red, green and +violet. They describe the region intermediate +between red and green; that is to say, the orange +and the yellow as red-green, and blue as violet-green. +It will be seen, therefore, that their chief +difficulty is distinguishing yellows and blues. A +yellow, for instance, which is situated next to a +green will be called red, and the same yellow +when adjacent to a red will be called green. +There are various degrees of trichromic vision, +varying from those who are little better than +dichromic to those who are tetrachromic. The +trichromic rarely find any difficulty with their +three main colours—red, green and violet.</p> + +<p>These cases have been described under the +name of anomalous trichromatics. This name is +one which has been given to those persons who +in making a match between a yellow corresponding +to the sodium flame and a mixture of thallium-green +and lithium-red make a mixture which is +different to that of the normal.<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">9</a> A man who +puts too much green in the mixture is called a +green anomaly; whilst a man who puts too much +red in the mixture is called a red anomaly. The +red anomaly is only a trichromic with shortening +of the red end of the spectrum, and this may be +as extensive as in any case of dichromic vision. +I have, however, described trichromic cases which<span class="pagenum" id="Page_43">43</span> +had all the symptoms attributed to the anomalous +trichromatics, but they were not anomalous +trichromatics, as they made an absolutely normal +match.<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">10</a></p> + + +<hr class="chap x-ebookmaker-drop"> + +<div class="chapter"> +<p><span class="pagenum" id="Page_44">44</span></p> +<h2 class="nobreak tac fs130" id="LECTURE_II">LECTURE II</h2> +</div> + +<p class="tac ptb05"><i>Delivered on February 3rd</i></p> + +<h2><span class="smcap">The Detection of Colour-Blindness from +a Practical Point of View</span></h2> + + +<p>I. <i>Object of a test for colour-blindness.</i>—Tests +for colour-blindness are of two kinds; namely, +those which are used for the purpose of ascertaining +the special phenomena of colour-blindness, +and those which are employed when the inquiry +is made for some practical purpose. As with +visual acuity, it is necessary to fix an arbitrary +standard. As we do not wish to exclude a greater +number than is absolutely necessary, the object of +the test should be to exclude dangerous persons +and dangerous persons only. These persons may +have other duties to perform which do not require +them to possess a perfect colour-sense. I should, +however, like to see those persons who are specially +qualified for a certain position, occupy it, for +instance, men who have to keep a look-out on our +most important ships being selected because of +their accurate colour-vision and visual acuity. +I do not mean that I would select only those men +and reject the others, but that I should like to see +a second object of a test, namely, to select those +who are specially efficient so that the Captain +might know on whom to rely in conditions of +exceptional difficulty.</p> + +<p><span class="pagenum" id="Page_45">45</span></p> + + +<p>II. <i>The requirements of a test for colour-blindness.</i>—A +test for colour-blindness, when it is to be +employed for some definite and specific purpose, +as, for instance, excluding dangerous persons from +certain callings, should be such as to show +definitely that the persons rejected are dangerous. +It is very useful to demonstrate to the men and +their fellows that a rejected candidate is dangerous. +The colleagues of a rejected candidate would +refuse to risk their lives with a man who before +their eyes called a red light, green. I was expressing +these views when a superintendent of a +railway company, who is using my lamp, told me +that he had adopted this method with great +satisfaction to himself and to the men. A man, +for instance, who has been working twenty years +on the railway has been rejected for colour-blindness. +He has complained bitterly to the superintendent, +at the same time declaring emphatically +that he is normal-sighted. The superintendent +has replied, “You know red?”—“Yes.” “You +know green?”—“Yes.” “You will therefore +agree that if you call green, red; or red, green, +you ought to be rejected. Bring two or three of +the other men in with you and I will test you.” +The man has readily agreed to this. The superintendent +has then tested him by asking him to +name various coloured objects in the room, and +knowing by experience exactly the coloured +objects which are miscalled by the colour-blind +readily exposes his defect. It is noteworthy that +on some occasions a colour-blind man has been +tested by another person in the same room without<span class="pagenum" id="Page_46">46</span> +making any of the mistakes which he subsequently +made, because none but coloured objects +which he could readily recognise were shown to +him. This is an example of the necessity of a +practical knowledge of colour-blindness in an +examiner. On account of the arrangement of +signals by sea and land, it is necessary that +persons employed in the marine and railway +services should be able to recognise and distinguish +between the standard red, green, and +white lights in all conditions in which they are +likely to be placed. An engine-driver or sailor +has to name a coloured light when he sees it, not +to match it. He has to say to himself, “This is +a red light, therefore there is danger”; and this +is practically the same as if he made the observation +out loud. Therefore, from the very commencement +we have colour-names introduced, +and it is impossible to exclude them. The engine-driver +is told that red is a “danger” signal, +green a “caution” signal, and white an “all +right” signal. Therefore, it is absolutely +necessary that he should know the meaning of +these colour-names. A test should be such as to +make it impossible for the examinee to be coached +through it. This is one of the most important +requirements of a test for colour-blindness and +one that is rarely fulfilled. Nearly every one of +the tests in general use fail on this account.</p> + +<p>A test should be one which can be carried out +as rapidly as is possible with absolute efficiency; +of two equally efficient tests the one which takes +the least time must be selected. A test, therefore,<span class="pagenum" id="Page_47">47</span> +should have no unnecessary details which though +of theoretical interest are not concerned with the +object in hand. The test should be made as easy +and as little complicated for the examiner as +possible.</p> + + +<p>III. <i>Persons to be excluded.</i>—We wish to exclude +all those individuals who are included in the +following three classes: (1) Those who see three +or less colours in the spectrum. (2) Those who, +whilst being able to perceive a greater number of +colours than three, have the red end of the +spectrum shortened to a degree incompatible with +their recognition of a red light at a distance of two +miles. (3) Those who are unable to distinguish +between the red, green, and white lights at the +normal distance through defect or insensitiveness +of the cerebro-retinal apparatus when the image +on the retina is diminished in size.</p> + +<p>I will now explain why these three classes of +persons should be excluded. The first class +includes the trichromic, the dichromic, and the +totally colour-blind, in accordance with the +facts previously stated. The trichromic never, +in ordinary circumstances, mistake green for red, +but confuse yellow with green or red. Colour is a +feeble quality of objects to them, and nervousness +or excitement may reduce them to the condition +of the dichromic. The dichromic are liable to +mistake a green light for red, and vice versa. It +is very important that persons belonging to the +second class should be excluded, and yet none of +the ordinarily used tests detect them. The rays<span class="pagenum" id="Page_48">48</span> +of red at the extreme left of the spectrum are the +most penetrating, as may be seen by looking at a +light or the sun on a foggy day, or through +several thicknesses of neutral glass. It is chiefly +by these rays that we recognise a red light at a +distance; and it is therefore of great importance +that a sailor or engine-driver should be able to +perceive them. The third class contains persons +who are able to distinguish colours easily when +they are close to, but fail to distinguish them at a +distance, owing to the nerve-fibres supplying the +central portion of the retina being impaired. As +a light at a distance occupies the central portion +of the visual field, it is essential that the corresponding +portion of the retina should be normal. +There are cases of central scotoma for colours with +perfect form-vision; these would, therefore, not be +detected by a test for visual acuity. This class +also includes those who without having a scotoma +are unable to distinguish between colours at the +normal distance when the image on the retina is +diminished in size.</p> + + +<p>IV. <i>The construction of a test for colour-blindness.</i>—In +the construction of a test for colour-blindness, +the facts of colour-blindness must be utilised so +that the object and requirements of the test are +fulfilled. The following facts are of practical +importance.</p> + +<p>1. <i>Most colour-blind make mistakes with certain +colours, but are correct with regard to others.</i> This +may be illustrated in the following way. Let us +take an ordinary dichromic, and, having given<span class="pagenum" id="Page_49">49</span> +him the set of wools belonging to the Classification +Test, ask him to pick out all the reds. On examining +the pile of wools selected as reds, it will be +found that the majority are red, but in addition +there will be some browns and yellow-greens. +If he be then told to pick out the whole of the +greens the greater number of those selected will be +green, but there will be also greys, browns, and +reds. In each case, it will be seen that the +majority of wools are of the desired colour.</p> + +<p>If another dichromic be examined in the same +way it will be found that, though he may not make +exactly the same mistakes, he will in all probability +pick out the same greens to put with the +reds, and the same reds to put with the greens. +The same result will be obtained if the colour-blind +persons be asked to name a large number of +colours. They will in most cases name the colour +correctly. It will be noticed that the greens which +were put with the reds when classifying the +colours, will be called red in naming them. It is +evident that the same idea has guided the colour-blind +in each case. This shows that, though a +person may be red-green blind, he is not absolutely +red-green blind in the sense of being totally +unable to distinguish between the two colours. +The fact that they are actually judging by colour +may be demonstrated by giving them coloured +materials of different kinds, or by asking them to +name a large number of coloured objects.</p> + +<p>It will be seen that if we take a dichromic and +ask him to name a number of red and green wools, +in the majority of instances he will name them<span class="pagenum" id="Page_50">50</span> +correctly. But as, almost invariably, the same +wools are chosen, for all practical purposes the +same result would be obtained by asking a person +to name a few of these wools. What more decided +and brighter greens could we have than Nos. 76 +and 94 of my Pocket Test? yet these are two of +the greens which are called reds by the dichromic. +We should have accomplished as much by asking +a colour-blind person to name Nos. 76 and 94 as if +we had asked him to name a large number of +greens. The colours in a test should, therefore, be +those which the colour-blind are particularly +liable to miscall. At the same time, their nature +should be unmistakable to the normal-sighted.</p> + +<p>2. <i>The colour-blind name colours in accordance +with their colour-perception, and thus show definitely +to which class they belong.</i> I have not come +across a man who has guessed correctly when +examined with my test. A man who did guess +would know that he was incompetent. As the +colour-blind are often not aware of their defect +they answer as they see, only guessing when they +feel uncertain as to the nature of the colour shown. +There is probably more misapprehension on this +point than on any other in the practical testing of +colour-blindness.</p> + +<p>3. <i>Colours may be changed to the colour-blind, +whilst leaving them unaltered to the normal-sighted.</i></p> + +<p>4. <i>The phenomena of simultaneous and successive +contrast are much more marked for the colour-blind +than for the normal-sighted.</i> Two colours, which +have not changed in the slightest degree to the +normal-sighted on being contrasted, have apparently<span class="pagenum" id="Page_51">51</span> +altered very considerably to the colour-blind. +As an example of this, let us take a pure deep +yellow, a bright red, and a bright green. To the +normal-sighted the yellow will be altered very +little by comparison with the red or the green, but +a trichromic would say that the colour was green +when contrasted with the red, red when contrasted +with the green. This principle of exaggerated +contrast must be borne in mind when examining a +candidate. Thus if a trichromic be doubtful about +a yellow, but seems inclined to call it green, he +should be given a pure green to compare with it. +In the same way, in showing the coloured lights, +the same colour produced in a different way +should often be shown. Thus an orange-red may +be shown immediately after a pure red. This will +not alter the colour to the normal-sighted, but +greatly facilitate the examination of the colour-blind.</p> + +<p>5. <i>Many colour-blind match correctly, but name +the principal colours wrongly.</i> Therefore the test +must be a naming test, the examinee being +rejected if he confuse the colours which it is +essential he should distinguish between in his +occupation.</p> + +<p>6. <i>Many colour-blind recognise colours easily +when they are close to them, or the surface is large, +but fail to distinguish between them when they are at +a distance or the image on the retina is small.</i> The +test must be constructed in conformity with these +facts.</p> + +<p>7. <i>The colour-blind are more dependent upon +luminosity than the normal-sighted, and are liable to<span class="pagenum" id="Page_52">52</span> +mistake a change in luminosity for a change of +colour.</i> The test should have a means of rapidly +changing the luminosity of a colour.</p> + +<p>8. <i>The colour-blind find special difficulty with +faint and dim colours.</i> The test should have +colours of this kind.</p> + +<p>9. <i>The colour-blind who have shortening of the +red end of the spectrum cannot see reds reflecting or +transmitting only rays corresponding to the shortened +portion.</i> It is essential that reds of this kind +should form part of the test.</p> + +<p>10. <i>The colour-blind find more difficulty in +comparing colours when different materials are used, +than when the coloured objects are all of the same +nature.</i></p> + +<p>11. <i>Most colour-blind find more difficulty with +transmitted than with reflected light.</i></p> + +<p>12. <i>The colour-blind have a defective memory for +colours.</i></p> + +<p>13. <i>Colours may be changed to the normal-sighted +whilst leaving them unchanged to the colour-blind.</i> +When three colours of the normal-sighted are +included in one of the colour-blind, it is obvious +that a change from one colour to another of the +three will make no difference to the colour-blind. +Also when the spectrum is shortened, the addition +or rays corresponding to the shortened portion to +another colour will not alter its appearance to the +person with the shortened spectrum. For instance, +to a person with shortening of the red end of the +spectrum, a blue will still remain blue, when so +many red rays from the shortened portion have +been added to it as to make it appear rose to the +normal-sighted.</p> + +<p><span class="pagenum" id="Page_53">53</span></p> + +<p>14. <i>The colour-blind may have a sense of luminosity +similar to that of the normal-sighted.</i></p> + +<p>15. <i>The dichromic distinguish between the colours +of the normal-sighted, which are included in +one of theirs by their relative luminosity and the +difference of saturation which is apparent to them.</i> +A test should therefore have the means of presenting +colours of different saturation in succession.</p> + +<p>16. <i>Colour-blindness is frequently associated with +very high intelligence and exceptional ability.</i></p> + + +<p>V. <i>The Lantern Test.</i><a id="FNanchor_11" href="#Footnote_11" class="fnanchor">11</a>—1. <i>Description of apparatus.</i> +The lantern contains four discs: three carrying +seven coloured glasses, and one with seven modifying +glasses. Each disc has a clear aperture. The +other mechanical details are: an electric or oil +lamp with projecting accessories, a diaphragm for +diminishing the size of the light projected, handles +for moving the discs and the indicator showing the +colour or modifier in use. The diaphragm is +graduated in respect to three apertures to represent +a <span class="nowrap">5 <span class="fraction"><span class="fnum">1</span><span class="bar">/</span><span class="fden">2</span></span></span>‑inch railway signal bullseye at 600, 800, +and 1000 yards respectively when the test is made +at 20 ft. The glasses are as follows:—-</p> + +<div class="center"> +<table> +<tr> +<td class="tac" colspan="2"><div> <i>Coloured glasses.</i>  </div></td> +<td class="tac pl2" colspan="3"><div><i>Modifying glasses.</i></div></td> +</tr> +<tr> +<td class="tar pr05"><div>1.</div></td> +<td class="tal">Red (A and B).</td> +<td class="tar pl2 pr05"><div> 7.</div></td> +<td class="tal" colspan="2">Ground glass.</td> +</tr> +<tr> +<td class="tar pr05"><div>2.</div></td> +<td class="tal">Yellow.</td> +<td class="tar pl2 pr05"><div> 8.</div></td> +<td class="tal" colspan="2">Ribbed glass.</td> +</tr> +<tr> +<td class="tar pr05"><div>3.</div></td> +<td class="tal">Green.</td> +<td class="tar pl2 pr05"><div> 9.</div></td> +<td class="tal">Neutral </td> +<td class="tal">(No. I).</td> +</tr> +<tr> +<td class="tar pr05"><div>4.</div></td> +<td class="tal">Signal Green.</td> +<td class="tar pl2 pr05"><div>10.</div></td> +<td class="tal">  "</td> +<td class="tal">( "  II).</td> +</tr> +<tr> +<td class="tar pr05"><div>5.</div></td> +<td class="tal">Blue.</td> +<td class="tar pl2 pr05"><div>11.</div></td> +<td class="tal">  "</td> +<td class="tal">( " III).</td> +</tr> +<tr> +<td class="tar pr05"><div>6.</div></td> +<td class="tal">Purple.</td> +<td class="tar pl2 pr05"><div>12.</div></td> +<td class="tal">  "</td> +<td class="tal">( " IV).</td> +</tr> +<tr> +<td class="tar"></td> +<td class="tal"></td> +<td class="tar pl2 pr05"><div>13.</div></td> +<td class="tal">  "</td> +<td class="tal">( "  V).</td> +</tr> +</table> +</div> + +<p><span class="pagenum" id="Page_54">54</span></p> + +<figure class="figcenter illowe25_625" id="i_54"> + <img class="w100" src="images/i_54.jpg" alt=""> + <figcaption><p>Fig. 3.</p></figcaption> +</figure> + +<p>It will be noticed that three of the discs are +similar in every respect. In some of my lanterns +the two reds are put at the end of the series of +colours and numbered Red 1 and Red 2. This +makes no important difference, but the arrangement +given here is more convenient. It should be +noted that Red 1 corresponds to Red B and +Red 2 to Red A. If the electric lamp should +get broken the projecting apparatus can be<span class="pagenum" id="Page_55">55</span> +removed and an ordinary kerosene lamp placed +behind the aperture.</p> + +<figure class="figcenter illowe23" id="i_55"> + <img class="w100" src="images/i_55.jpg" alt=""> + <figcaption><p>Fig. 4.</p></figcaption> +</figure> + +<p>2. <i>Reasons for special construction.</i>—The lantern +has been constructed conformably with the requirements<span class="pagenum" id="Page_56">56</span> +and facts of colour-blindness. All the +facts I have given have been considered in +constructing the lantern.</p> + +<p>The examiner, on possessing a lantern for the +first time, should carefully test himself with it and +ascertain how the different lights appear to him +with different conditions of general illumination. +It is probable that certain improvements may +suggest themselves to him, therefore, I think it +will be advisable to deal with certain of these +points, as it will help the examiner in the use of the +lantern. The colours have never been altered, and +I certainly should have altered them if I could +have improved the lantern by doing so. I have +never met a single colour-blind person who has +not been readily and easily detected with my +lantern, though I have examined many who have +passed other lanterns and in some cases a number +of other tests for colour-blindness. In most cases +one turn of the wheel will be sufficient to make a +colour-blind person disclose his defect.</p> + +<p>The examiner may be dissatisfied with the +colour of the blue; let us, therefore, compare an +examination of a normal-sighted person with that +of an ordinary dichromic. The normal-sighted +person will name every one of the colours with ease +and certainty, with perhaps the exception of the +blue, with which he is in some doubt. Here is the +result of an examination of an ordinary dichromic: +he called the yellow, green; the green, red; the +signal green, no colour; the blue, blue; the +purple, green; red A, no colour or light; red B, +green. It will be noticed that the only colour<span class="pagenum" id="Page_57">57</span> +that he has correctly named is the blue. We can +try him again and again, and though he will mistake +all the other colours he will always name +the blue correctly. The examiner will have learnt +from this several important facts. He will see +that the colour-blind are really guided by their +sensations of colour, and that it is not simply a +matter of guessing. The more an examiner has +practical experience of colour-blindness, the more +will he recognise the fact that the colour-blind +are guided by their sensations of colour. He will +notice that the dichromic has readily recognised +the blue which was scarcely apparent to him (the +examiner), and therefore cannot have overlooked +as a matter of carelessness colours which are much +more apparent to the normal-sighted. The blue is +a valuable colour for other reasons, for though it is +not a colour on which I reject candidates, anyone +miscalling it must be very carefully examined +before he is passed. The trichromic generally +call this blue, green. If we wish to obtain a +purer blue, we can do so by combining the blue +or the purple with the signal green.</p> + +<p>Again, the examiner may think that it might +be better to have an apparatus which showed +two or three lights instead of only one. I will +therefore give my reasons for adopting only one. +This point was one which occupied my attention +for a considerable time, especially in view of the +fact—which, as far as I am aware, I was the first +to discover—that simultaneous contrast is increased +to the colour-blind. I was naturally +anxious to turn this fact to account. I found,<span class="pagenum" id="Page_58">58</span> +however, that I gained nothing by increasing the +number of lights, and that in many cases it was +a source of error. A second or third light could +have been easily added to my lantern, but besides +unnecessarily complicating the apparatus +it would have served no useful purpose. All the +results which are obtained with simultaneous +contrast are obtained even more effectively with +successive contrast. It will be noticed that when +lights are seen in ordinary conditions they are +conditions of successive contrast, and not of +simultaneous contrast. An observer rarely keeps +his eyes definitely fixed on one light whilst he +names those adjacent to it, but moves his eyes +so that the images of the respective lights fall +successively on his foveas. When more than one +light is employed, all the disadvantages of matching +as against naming are introduced. It will be +seen that by presenting one light after another +we are fulfilling all the necessary conditions, only +that the light is moved instead of the eyes, sufficient +time being allowed to elapse to enable a +normal-sighted person to readily recognise the +true colour of the next light without being confused +by the after-image of the one he has just +seen. Many nervous normal-sighted would name +a yellow light seen between two red lights as +green, and it does look green to them from ordinary +physiological conditions. They look first at +one red light, then immediately at the yellow +light, then at the second red light, and then again +at the yellow light until they feel sure that the +centre light is a green light, and say so. I have<span class="pagenum" id="Page_59">59</span> +never met with a normal-sighted person who has +miscalled the unmodified light of my lantern, either +red or green. Many humble, nervous normal-sighted +persons are under the impression that they are +colour-blind, and yet would make perfectly efficient +officers. Many of these men have been told +by their wives or other persons that they are +colour-blind, and, believing this, try to see colours +which are not visible to them. I have examined +many persons of this description, and have noted +the ease and accuracy with which they have gone +through the tests for colour-blindness when they +have been assured by me that they were normal-sighted. +On the other hand, it is often very +difficult to convince a self-reliant, colour-blind +person that he is colour-blind. He is on the look-out +for the small differences which he notices +between colours, and the fact of having another +light for comparison gives him the desired clue, +and, though colour-blind, he passes the test.</p> + +<p>The material is the best possible, as it will not +fade like all dyed substances, and therefore all +records made with one set of apparatus will be +uniform. Again, a coloured light has none of the +accessory qualities which enable the colour-blind +to pass through other tests. Thus many dichromics +will call the yellow glass red or green, +who would not think of putting a yellow with a +green or red wool, on account of the difference in +luminosity. He will, in the same way, if told to +pick out colours in the Classification Test to +match the colour of the light shown, have to +depend upon his colour-perception. This is a<span class="pagenum" id="Page_60">60</span> +useful method with nervous and undecided candidates. +The objection to it is that it cannot be +carried out in the dark or in a dark room. The +Test is not open to any of the objections which +may be urged against the method of simply +naming colours, because the character and intensity +of the colour may be changed at will.</p> + +<p>The method is better than that of direct comparison, +because the candidate is forced to use +his colour-perception, and has to compare the +colour seen with previous impressions of colour +in his mind. By the use of neutral glasses, etc., +I have obviated the fallacy of the method of +naming colours (namely, that these can be distinguished +by their saturation and luminosity), +and forced the individual to depend upon his +colour-perception, and not upon some other accessory +quality of the object seen.</p> + +<p>No amount of coaching will enable a colour-blind +person to pass this test, whilst almost any +other may be passed in this way. I have tried +on many occasions to coach a man so as to pass +my lantern, and without success. The combinations +are so numerous that the only result is +to make the colour-blind man nervous and doubtful +and more easily detected than before. This +has occurred with men who could pass other tests +with ease.</p> + +<p>The test also has a quality possessed by no +other—namely, that of enabling the examiner to +reject dangerous persons and dangerous persons +only, the lower degrees of colour-blindness being +allowed to pass.</p> + +<p><span class="pagenum" id="Page_61">61</span></p> + +<p>3. <i>Special directions for conducting the test.</i>—(1) +The candidate should be seated at a distance +of twenty feet from the lantern. (2) He should +be asked to name the colour of the light produced +by a coloured glass (1 to 6) alone, or in combination +with another coloured glass or glasses, +or with the modifying glasses (7 to 13). (3) A +candidate should be rejected (i) if he call the red, +green, or the green, red, in any circumstances; +(ii) if he call the white light, in any circumstances, +red or green, or vice versa; (iii) if he call the red, +green, or white lights, black, in any circumstances. +(4) A candidate who makes mistakes, +other than those mentioned above, should be put +through a very searching examination. It is not +necessary to have the room absolutely dark; in +fact, I prefer a certain amount of light. The +examiner can, if he wish, make the test at night +in the open air.</p> + +<p>The examiner should on no account conduct +the examination on any regular plan, because the +candidate, anxious to pass, finds out from persons +who have already passed the order and method +of the examination, and so, though colour-blind, +might obtain a certificate. Any one of the glasses +may be shown first, and the candidate required +to name the colour of the light. The following +will serve as an example of the method to be employed +in testing a candidate. A red being +shown, the candidate is required to name its +colour. Then a blue or green may be substituted. +It is best to use the largest aperture at first and +to show all the colours on one disc. This will<span class="pagenum" id="Page_62">62</span> +give confidence to the normal-sighted candidate, +whilst most of the colour-blind will be detected. +In the case of candidates who appear to be normal-sighted +and yet very nervous, there is no harm +in telling them after they have named all the +colours on the disc correctly that this is the case. +No comment should, however, be made on individual +answers. Then one of the neutral, +ground, or ribbed glasses should be inserted, not +the slightest intimation being given to the candidate +of the nature of the colour. He should be +asked to name or describe the light, and the +answer, if incorrect, together with his other replies, +carefully recorded. The other glasses may +then be shown, a combination of the neutral, +ground, ribbed, and coloured glasses being used +at irregular intervals.</p> + +<p>When the candidate has been examined with +the largest aperture, the examiner can go through +the same procedure with one of the smaller apertures. +I have found the third aperture the one +which is most generally useful. On account of +the great diminution of total luminosity caused +by the diminished area of the light source, the +three smallest apertures can only be used in a +dark room.</p> + +<p>If a candidate hesitate about a colour and +ultimately name it correctly, a second and, if +necessary, a third glass of the same colour should +be combined with the first. The fact that in one +case a single glass is used, and in another two or +three of the same coloured glass, makes very little +difference in the colour of the light to the normal-sighted.<span class="pagenum" id="Page_63">63</span> +This is not the case with the colour-blind; +a dichromic who has hesitated about a +green and then correctly named it may emphatically +call the light red when another green glass +is put in front of the first.</p> + +<p>Care must be taken when the candidate is going +to be examined with two glasses at once, such as +one of the neutral, ground, or ribbed glasses, and +a coloured glass, that he does not see the light +until both are in position, or else he may see the +colour before it is modified in the necessary way.</p> + +<p>If the candidate call the standard red, green; +or the standard green, red, in any circumstances—that +is, either alone or in combination with the +modifying glasses—he is to be rejected.</p> + +<p>The examiner should ascertain for himself how +far the various colours are visible when modified +with the neutral glasses. If the red and green +be not visible with the thickest neutral in the +conditions of luminosity and external lighting +which the examiner is employing, he should use +the darkest neutral which allows the colours to +be plainly visible to the normal-sighted. In all +cases of doubt the examinee should be asked to +walk towards the lantern and told to say when +the light is visible, and asked to name its colour. +The distance at which the light is visible, and +then that at which the colour is visible, should +be noted and compared with the normal.</p> + +<p>Particular attention should be paid to the +answers given to the combination of the thickest +neutral glass with the standard red and green +respectively.</p> + +<p><span class="pagenum" id="Page_64">64</span></p> + +<p>The examiner should utilise the fact that successive +contrast is increased in the colour-blind, +as this is an easy method of detecting the trichromics. +The red having been shown, the light +should be quickly changed to yellow or clear, +the examiner’s hand being placed over the aperture +if there be any intervening colours. It is +necessary that the yellow should be shown immediately +after the red without any intervening +colours being first seen by the candidate. The +normal-sighted do not see any change in the +yellow or clear when they are shown after the red +light, but the trichromic call the yellow light, +green. The examinee should then be shown the +green light, and then the yellow or clear, in the +same way as mentioned for the red. The normal-sighted +will easily recognise the yellow, but the +trichromic will call it red. This portion of the +examination must never be omitted in any examination +in which the candidate is passed. The +two divisions of the test—that is, showing yellow +immediately after red and after green—may be +used at different periods of the examination, and, +if there be any doubt, repeated.</p> + +<p>An examiner should, as far as possible, with +the exceptions given in the instructions, avoid all +conversation with the candidate, simply asking, +“What colour is this?” and recording the answer +without comment. If an examiner after each +answer say, “Quite right,” or some such expression, +the following is likely to occur. The +candidate after, say, six correct answers, makes a +mistake; the examiner says, “Are you sure?”<span class="pagenum" id="Page_65">65</span> +Then the candidate knows at once that he has +made a mistake, and makes a guess, very probably +a correct one. When a similar colour is +shown subsequently, he remembers the mistake +he made, and gives the second, and probably the +correct answer.</p> + +<p>In addition to being an efficient test, it is a very +rapid test, as many men who have been certified +as normal after a lengthy examination with other +tests have at once disclosed their defect by calling +the green light of the lantern red. Many are +under the impression that in an examination with +the lantern the dichromics simply guess. This +is entirely wrong. A man who did guess would +know that he was incompetent. I find that men +have named the coloured lights in strict accordance +with their colour-perception. A man may, +however, guess if examined by an inexperienced +and ignorant examiner, who when the examinee +has made a mistake promptly corrects it in a +cross tone. A normal-sighted person will guess +when examined in this way. The examiner must +receive the examinee with a smiling face and +courteous manner, and appear pleased and satisfied +with the answers, no matter what they may +be. The candidate is then placed at his ease, and +answers according to his colour-perception. It +will be noticed that the lantern detects those who +have a slightly diminished colour-perception, as +well as the dangerous varieties of colour-blindness. +The former undoubtedly are not as efficient +as those who have a normal colour-perception, so +that a definite standard will have to be fixed, as<span class="pagenum" id="Page_66">66</span> +in the case of visual acuity. Further details will +be found in my book on Colour-Blindness.<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">12</a></p> + +<p><i>Summary of method of examination.</i>—(1) Show +all the colours on one disc with the largest aperture. +(2) Show the reds, greens, and yellow modified +by the neutral glasses. (3) Show all the +colours on one disc with Number 3 aperture. (4) +Show red, then immediately afterwards yellow +with largest aperture. Then show green and +yellow immediately afterwards. (5) Test the +candidate with the red, green, and yellow with +the smallest aperture. (6) Show the neutrals or +ground glass alone. (7) Show blue made by combining +blue or purple with the signal green. (8) +Show a colour, for instance, green, and then combine +another glass of the same colour. (9) Show +the red produced by the combination of purple +with red A. (10) Give the combination of red A +and signal green.</p> + + +<p>VI. <i>Other tests for colour-blindness.</i>—I have +three other tests for colour-blindness: the Classification +Test, the Pocket Test, and the Colour-perception +Spectrometer. I have also devised an +instrument for estimating the exact amount of +red, at different wave-lengths, which is necessary +to neutralise the complementary in different +persons.</p> + +<p>1. <i>The Classification Test.</i>—(<i>a</i>) <i>Description.</i>—This +test consists of 4 test colours and 180 confusion +colours; 150 coloured wools, 10 skeins of +silk, 10 small squares of coloured cardboard, and 10<span class="pagenum" id="Page_67">67</span> +small squares of coloured glass. The whole series +of colours is represented. In addition, there are +a large number of colours which have been chosen +by colour-blind persons as matching the test +colours. The test colours are Orange, Violet, Red, +and Blue-green, labelled I, II, III, and IV respectively. +The colours are chosen with the view +of presenting as much difficulty as possible to +the colour-blind, and as little as possible to the +normal-sighted. The colour-blind find especial +difficulty in matching or naming a colour lying +at the junction of two of their colours. As the +normal-sighted often find difficulty in saying +which colour predominates in a blue-green, so do +the tetrachromic with their purple-green, or the +trichromic with their red-green. A colour-blind +person may, however, match a colour correctly +which corresponds to the centre of one of his +colours. In addition to choosing those colours +for tests which are particularly liable to be mistaken +for other colours by the colour-blind, I have +used coloured materials of different kinds—wools, +silks, glass, and cards—so as to force the colour-blind +to judge by colour, and not by saturation +or luminosity. (See Fig. 5.)</p> + +<figure class="figcenter illowe28_125" id="i_68"> + <img class="w100" src="images/i_68.jpg" alt=""> + <figcaption><p>Fig. 5.</p></figcaption> +</figure> + +<p>(<i>b</i>) <i>Method of examination.</i>—The candidate +should be given the four test colours, and, having +named each, he should be told to select all those +which are similar in colour to the test colour. +He should be told to pay no attention to the fact +of a colour being lighter or darker; as long as it +is the same colour it should be put with the test +skein. The examiner should not go through the<span class="pagenum" id="Page_68">68</span> +test before the candidate first of all, neither +should one candidate be allowed to watch another +making his selection. A shrewd colour-blind +person might pass the test if he had seen a normal-sighted +person go through it previously. In order +to show the candidate the difference between a +shade and a colour, the examiner should take +one of the wools which is not a test colour—blue, +for instance—and pick out four or five shades of +the colour. The wools should be arranged without +the knowledge of the candidate, so that a +yellow or a grey is placed beside a red and the +examinee asked to name its colour. At another +period of the examination the yellow should be +placed adjacent to a green, and the examinee +again asked to name it.</p> + +<p><span class="pagenum" id="Page_69">69</span></p> + +<p>The examinee may pick out a certain number +of colours correctly, and then stop, saying that +there are no more exactly like the test colour. +This may embarrass the examiner; he should, +however, examine any candidate who has omitted +any colours as carefully as if mistakes had been +made. He should ask the candidate to match +one of the omitted colours.</p> + +<p>The examiner will soon find out from experience +those colours which are named and matched +wrongly by colour-blind persons; he should ask +the examinee to name some of these colours.</p> + +<p>Any candidate should be rejected who calls an +orange or red, green or brown; black, red or vice +versa; or green, either purple, rose, red, grey, +brown, or violet. Similar mistakes in matching +necessitate objection. A candidate who puts +purple, rose, or blue with violet, or yellow-brown +with orange is most probably dangerously colour-blind +and should be very carefully examined. +There are cases which pass the Holmgren test +with ease that fail in the most conclusive manner +with my Classification Test. They put green with +orange, brown and black with red, and grey with +blue-green. This is due to a different selection +both of test colours and confusion colours. Orange +is by far the most important test colour, and its +confusion with green by the dichromics is very +conclusive. The three other test colours, violet, +red, and blue-green, represent both ends of the +spectrum and the neutral point in dichromic cases, +and practically these colours are those with which +most mistakes are made. This test can only be +regarded as supplementary to the Lantern Test.</p> + +<p><span class="pagenum" id="Page_70">70</span></p> + +<p>2. <i>The Pocket Test.</i>—This consists of nineteen +cards, on nine of which are 112 single threads of +wool, and 14 pieces of twisted silk, similar to those +in the Classification Test. These are numbered +consecutively, with the exception of the first +thread of the first four cards, and the last thread +of the next four cards. The end threads of the first +four cards, I to IV, form the tests; they are +Orange, Violet, Red, and Blue-green. There are also +cards on which red, orange, green, blue, violet and +purple, and grey, respectively are to be found. +There are also two special cards marked “Without +Red” and two special cards marked “Without +Green.” (See Fig. 6.)</p> + +<figure class="figcenter illowe27_125" id="i_70"> + <img class="w100" src="images/i_70.jpg" alt=""> + <figcaption class="caption"><p>Fig. 6.</p></figcaption> +</figure> + +<p>Many normal-sighted persons might object to the<span class="pagenum" id="Page_71">71</span> +inclusion of some of the colours on the orange card, +but this card clearly shows the colours which may +be taken as a match. Fine distinctions are not +wanted. The series of colours I have selected and +arranged so as to confuse the colour-blind and +force them to be guided by their colour-perception, +whilst the quantity of colour is amply sufficient +for the normal-sighted to pick out the colours +with the greatest ease. The cards should be +arranged irregularly on a white cloth in a good +light. The two most important tests colours are +the Orange and Violet, Nos. I and II. The person +examined should be asked to point out the shades +of colour similar to No. I (Orange). A piece of +paper rolled to a point should be used for this +purpose. If he do this correctly, he probably +possesses normal colour-perception. If, however, +he match the test with reds or pinks, he is more or +less colour-blind, at best belonging to the pentachromic +class. If, in addition, he match the +Violet test, No. II, with blue, he at least belongs +to the tetrachromic class. The trichromic, in +addition, may match the Blue-green test, No. III, +with brown and grey. The dichromic will match +the Orange test, No. I, with yellow-green and +yellow-brown. Similar mistakes will be made to +those described in connection with the Classification +Test. The examinee should be asked to name +all the colours on one of the cards. He should also +be asked to point out on which of the cards the +four test colours are to be found, and which +contain none of the test colour.</p> + +<p>The examiner should continually change the<span class="pagenum" id="Page_72">72</span> +order of the cards. Most of the varieties of the +colour-blind will be readily detected in this way.</p> + +<p>The special advantages of this test are: (1) The +colour-blind can be ranged definitely in their +proper classes. (2) Central scotoma can be +detected with its aid. (3) The series of colours are +arranged so as to confuse the colour-blind, whilst +the normal-sighted easily match the test colours. +(4) On account of the introduction of different +materials, the relative luminosity and saturation +of colours does not serve as a guide to the colour-blind. +(5) Portability. (6) The wools and silks are +kept clean. (7) An important colour is not likely +to be lost.</p> + +<p>3. <i>The Colour-Perception Spectrometer.</i><a id="FNanchor_13" href="#Footnote_13" class="fnanchor">13</a>—(<i>a</i>) +<i>Description of apparatus.</i>—This instrument is a +spectrometer so arranged as to make it possible to +expose to view in the eyepiece the portion of a +spectrum between any two desired wave-lengths. +In the focal plane of the telescope are two adjustable +shutters with vertical edges; the shutters can +be moved into the field from right and left respectively, +each by its own micrometer screw, and to +each screw is attached a drum, the one being on +the right and the other on the left of the telescope. +On each of these drums is cut a helical slot in which +runs an index, and the drum is engraved in such a +manner that the reading of the index gives the +position in the spectrum of the corresponding +shutter in wave-lengths direct. (See Fig. 9.) Thus +it will be seen that if, for instance, the reading on<span class="pagenum" id="Page_73">73</span> +the left drum-head is 5320 and that on the right +drum-head is 5920, the region of the spectrum from +wave-length 5320 to wave-length 5920 is exposed +to view in the eyepiece.</p> + +<figure class="figcenter illowe22_875" id="i_73-1"> + <img class="w100" src="images/i_73-1.jpg" alt=""> + <figcaption class="caption"><p>Fig. 7.</p></figcaption> +</figure> + +<div class="figcontainer"> +<div class="figsub"> +<figure class="figcenter illowe10_375" id="i_73-2"> + <img class="w100" src="images/i_73-2.jpg" alt=""> + <figcaption class="caption"><p>Fig. 8.</p></figcaption> +</figure> +</div> +<div class="figsub"> +<figure class="figcenter illowe14_375" id="i_73-3"> + <img class="w100" src="images/i_73-3.jpg" alt=""> + <figcaption class="caption"><p>Fig. 9.</p></figcaption> +</figure> +</div> +</div> + +<p><span class="pagenum" id="Page_74">74</span></p> + +<p>(<i>b</i>) <i>Directions for using the instrument.</i>—It +should be used as far as possible with a known +quality and intensity of light. A small oil-lamp +is quite suitable for the purpose. The observer +should first ascertain the exact position of the +termination of the red end of the spectrum, the +left-hand shutter being moved across until every +trace of red just disappears. The position of the +pointer on the left-hand drum is noted, and the +wave-length recorded. The left drum is then moved +so that the shutter is more towards the middle of +the spectrum. The right-hand drum is then +moved, until the pointer indicates the wave-length +recorded as the termination of the red end +of the spectrum. The observer then moves the +left-hand shutter in and out until he obtains the +largest portion of red, which appears absolutely +monochromatic to him, no notice being taken of +variations in brightness, but only in hue. The +position of the index on the left-hand drum is +recorded. The left-hand shutter is then moved +towards the violet end of the spectrum, the right-hand +shutter being placed at the position previously +occupied by the left-hand shutter. In this +way the whole of the spectrum is traversed until +the termination of the violet end of the spectrum +is finally ascertained with the right-hand shutter. +The variation of the size of the patches and the +terminations of the spectrum with different intensities +of light can be noted. The instrument can +also be used for ascertaining the exact position +and size of the neutral patch in dichromics, the +position of greatest luminosity, and the size and<span class="pagenum" id="Page_75">75</span> +extent of pure colours. When it is used to test +colour-blindness, the examinee should first be +shown some portion of the interior of the spectrum, +and then asked to name the various colours which +he sees. In this way he will have no clue to the +colours which are being shown him.</p> + +<p><i>Objections to other tests for colour-blindness.</i>—The +tests which have been proposed for colour-blindness +are very numerous, but some are so +defective that it is rare to detect a single colour-blind +person with them. I have, for instance, +tested men whom I knew to be colour-blind with +certain lanterns with the result that not a single +one was detected. In these so-called tests all the +requirements of a test and facts of colour-blindness +have been neglected. I must, however, refer to +three tests constructed by exceptionally able men, +each with considerable knowledge of the subject. +I refer to the tests of Professor Holmgren, Professor +Stilling, and Professor Nagel.</p> + +<p>All these tests can be passed at the first attempt +without coaching by certain dangerously colour-blind +persons, chiefly varieties not known to the +inventors, but the chief defect of each is that it is +very easy to coach a colour-blind person to pass it. +The surgeon to one of our largest railway companies +told me that when they used Holmgren’s +test they rejected one man in three hundred, but +with my lantern twelve in the same number. All +these three tests are much better tests when the +persons to be examined have not seen them before. +A colour-blind man may make only one mistake, +say for instance, as in a case I examined the other<span class="pagenum" id="Page_76">76</span> +day, with Nagel’s test (last edition), he passes the +test perfectly with the exception of one mistake, +that of calling a grey on one card, green. All he +has to do is to look for some distinguishing mark +on this card in order to go through the test with +the ease and certainty of a normal-sighted person. +It is the same with Stilling’s letters, he has only to +note the letter which he was not able to read and +the appearance of the card. A normal-sighted +man or woman would readily help him. The +confusion of green and grey does not appeal to the +average man as a serious defect, especially when +he sees his friend go through the rest of the test +perfectly. He says to himself, “I suppose he sees a +tinge of green in that grey.” The same man would +rightly regard it as a most iniquitous proceeding +to endeavour to coach his friend through a test +when he had seen him mistake a red for a green +light.</p> + +<p>Holmgren’s test rejects a large number of +normal-sighted persons, as may be seen by the +reports of the Board of Trade; about 50 per cent +of those who appeal are found to be normal-sighted +and to have been rejected wrongly.</p> + +<hr class="r15 x-ebookmaker-drop"> + +<p class="tac fs60">WILLIAM BRENDON AND SON, LTD.<br> +PRINTERS, PLYMOUTH</p> + + +<p class="tac mt6em mb2em ti0"><span class="fs240">Reiner</span> <span class="fs130">and</span> <span class="fs240">Keeler</span>, <span class="fs240">L</span><span class="fs130"><sup>td.</sup></span> + +<p class="tac fs240 ls01em ti0">OPTICIANS</p> +<p class="tac mtb-05 ti0">——————AND——————</p> +<p class="tac fs200 ti0">INSTRUMENT MAKERS</p> + +<p class="ml5em u ti0"><i>MANUFACTURERS OF</i></p> + +<div class="container"> +<div class="content"> +<p class="tal fs140 ti0">The Edridge-Green<br>Colour Perception Lantern</p> +<p class="tal fs140 ti0">The Edridge-Green<br>Classification Test</p> +<p class="tal fs140 ti0">The Edridge-Green<br>Pocket Test</p> +<p>and other</p> +<p class="tal fs140 ti0">Optical and<br>Scientific Instruments</p> +</div> +</div> + +<p class="tac">THE ABOVE COLOUR TESTS ARE CERTIFIED<br> +BY PROF. F. W. EDRIDGE-GREEN</p> + +<p class="ml5em mt3em ti0"><span class="fs140">9 Vere Street,<br> +Cavendish Square,</span> <span class="fs180"><b>London, W.</b></span></p> + +<p class="tac fs80">Telephone: 447 Mayfair.</p> + + + +<div class="footnotes"><h3>FOOTNOTES:</h3> + +<div class="footnote"> + +<p><a id="Footnote_1" href="#FNanchor_1" class="label">1</a> +<i>Journal of Physiology</i>, vol. xli, p. 274.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_2" href="#FNanchor_2" class="label">2</a> +<i>Transactions of the Ophthalmological Society</i>, 1902, p. 300.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_3" href="#FNanchor_3" class="label">3</a> +<i>Journal of Physiology</i>, vol. xli, p. 269.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_4" href="#FNanchor_4" class="label">4</a> +<i>British Medical Journal</i>, 1910, p. 556.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_5" href="#FNanchor_5" class="label">5</a> +<i>Journal of Physiology</i>, vol. xli, p. 226.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_6" href="#FNanchor_6" class="label">6</a> +<i>Nagel. Physiol. des Menschen</i>, vol. iii, p. 107.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_7" href="#FNanchor_7" class="label">7</a> +“Colour Blindness and Colour Perception,” <i>International Scientific +Series</i>, p. 196.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_8" href="#FNanchor_8" class="label">8</a> +<i>Proceedings of the Royal Society</i>, vol. B 82, 1910, p. 458.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_9" href="#FNanchor_9" class="label">9</a> +<i>Proceedings of the Royal Society</i>, vol. B 76, 1905.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_10" href="#FNanchor_10" class="label">10</a> +<i>Transactions of the Ophthalmological Society</i>, 1907, p. 255. <i>Proceedings +of the Royal Society</i>, vol. B 82, 1910.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_11" href="#FNanchor_11" class="label">11</a> +Made by Reiner and Keeler, 9, Vere Street, W.; and Meyrowitz, +1a, Old Bond Street, W.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_12" href="#FNanchor_12" class="label">12</a> +<i>International Scientific Series.</i> Kegan Paul & Co., 1909.</p> + +</div> + +<div class="footnote"> + +<p><a id="Footnote_13" href="#FNanchor_13" class="label">13</a> +Made by A. Hilger, 75a, Camden Road, London, N.W.</p> + +</div> +</div> + +<div style='text-align:center'>*** END OF THE PROJECT GUTENBERG EBOOK 75519 ***</div> +</body> +</html> + diff --git a/75519-h/images/cover.jpg b/75519-h/images/cover.jpg Binary files differnew file mode 100644 index 0000000..17884da --- /dev/null +++ b/75519-h/images/cover.jpg diff --git a/75519-h/images/i_18.jpg b/75519-h/images/i_18.jpg Binary files differnew file mode 100644 index 0000000..2b4697c --- /dev/null +++ b/75519-h/images/i_18.jpg diff --git a/75519-h/images/i_20.jpg b/75519-h/images/i_20.jpg Binary files differnew file mode 100644 index 0000000..a1fbfd6 --- /dev/null +++ b/75519-h/images/i_20.jpg diff --git a/75519-h/images/i_54.jpg b/75519-h/images/i_54.jpg Binary files differnew file mode 100644 index 0000000..e8ab104 --- /dev/null +++ b/75519-h/images/i_54.jpg diff --git a/75519-h/images/i_55.jpg b/75519-h/images/i_55.jpg Binary files differnew file mode 100644 index 0000000..b53a8c2 --- /dev/null +++ b/75519-h/images/i_55.jpg diff --git a/75519-h/images/i_68.jpg b/75519-h/images/i_68.jpg Binary files differnew file mode 100644 index 0000000..b025b9e --- /dev/null +++ b/75519-h/images/i_68.jpg diff --git a/75519-h/images/i_70.jpg b/75519-h/images/i_70.jpg Binary files differnew file mode 100644 index 0000000..5d77b7d --- /dev/null +++ b/75519-h/images/i_70.jpg diff --git a/75519-h/images/i_73-1.jpg b/75519-h/images/i_73-1.jpg Binary files differnew file mode 100644 index 0000000..e4a95ea --- /dev/null +++ b/75519-h/images/i_73-1.jpg diff --git a/75519-h/images/i_73-2.jpg b/75519-h/images/i_73-2.jpg Binary files differnew file mode 100644 index 0000000..39f57e7 --- /dev/null +++ b/75519-h/images/i_73-2.jpg diff --git a/75519-h/images/i_73-3.jpg b/75519-h/images/i_73-3.jpg Binary files differnew file mode 100644 index 0000000..4c8793f --- /dev/null +++ b/75519-h/images/i_73-3.jpg diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. 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