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.
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 find1 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
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.
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.
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 lobe4 is almost exactly the same as that seen subjectively.
Fig. 2.
12. Entoptic appearance of cone mosaic.—Appearances corresponding to the cone mosaic of the retina may be seen in several ways5 (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.
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.
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.
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.
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 paper8 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.
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.
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