Colour vision

by Sir William de Wiveleslie Abney

CHAPTER XV.

Examples of colour blindness have been brought to your notice, and various measurements made by persons possessing normal and defective colour vision have been recorded, but no attempt has been made to discuss the two leading rival theories that have been laid before you. Regarding these theories you may expect me to say something, and to avow myself a partisan of one or the other. This last I must decline to do, though it will have been seen by the line that I have taken in these lectures that the Young theory attracts me. There are, however, difficulties in adapting it to explain several facts of colour vision which seem to render it, to say the least, incomplete. For instance, to explain the colours produced by simultaneous contrast, the Young theory has to betake itself into psychological ground. I will show you some excellent examples of contrast colours. We have upon the screen a patch of white reflected light, superposed over a patch of red light. Placing a thin rod in the paths of the two beams, we have two shadows—one illuminated by white and the other by red, and lying between them a mixed light of red and white. The shadow illuminated by the white does not appear white, but a bluish-grey. When the spectrum colour is changed to orange the blue is intensified, whilst when it is green, what should be white appears of an orange-salmon colour. Other colours give the white different hues which I need not describe.

These contrast colours are usually said to be complementary to the spectrum colours employed, though it must be recollected that what a complementary colour should be is determined by the quality of the white light which the two, when mixed, are made to match. But recent measures of my own show that they are not truly complementary in most instances, whatever the white light may be. But whether they are or not does not much matter when the explanation offered by the followers of the Young theory is considered, for it is asserted that such contrast colours have no real existence, but are psychological, or—what this comes to be—simply delusions. If they are not real colours felt by the retina, they have a very good resemblance to them, and the same series of delusions are so persistent and so constant for all normal vision that they can always be measured as having a constant value. I bear in mind the experiment in which the contrast colour, after being produced, is isolated in the eye from the colour producing it and the background, and the continuance of the hue produced by the contrast. This retention may be psychological, but there are no grounds to my mind for saying that its production is due to the same cause, more especially as experiments have been arranged to show that one eye may see a contrast colour, whilst the other may see it of its uncontrasted hue. In this last experiment it can scarcely be conceived that one eye should be subject to delusion, whilst the other was free from it. If, then, we may presume that they are real colours, the Young theory fails to explain them, and the explanation offered by the Hering theory is much more acceptable, as it propounds the idea that the retina has to be considered as a whole, and that if (say) red light is at work at one part its complementary colour (blue-green) must be felt at another. It would be still more acceptable had it happened that the contrast colours were truly complementary, and if the same action was noticeable when the adjacent part of the retina was not also stimulated.

For what I may call the straightforward part of colour vision, dealing with ordinarily bright colours, the Young theory is amply sufficient; but when we come to the feeble luminosities and the colour fields, it is again difficult to adapt to explain the phenomena observed. When we reduce the luminosity of a coloured ray sufficiently we feel the sensation of grey light: no colour is felt. Why is this? On the Hering theory it is capable of the explanation that we have the white sensation left unextinguished, but I fail to see any explanation on the Young theory. When we take colour fields with pure colours (see appendix, [page 208]), we are met with the unexplained difficulty that the colour from a bright spot of light vanishes almost suddenly towards the periphery of the retina, and is replaced by a bright white light, and that the extent of the field depends on the brightness of the colour. This, perhaps, is the most telling observation which can be recorded against the Young theory as it stands at present. It has this support, however, in the sequence of the phenomena observed, viz., when the boundary for the colour which we will suppose to be pure red is being taken (as described at [page 11]), that close to the point where it bursts into pure white, it assumes a pink colour (i.e., a mixture of red and white), whilst, if the red be scarlet, containing according to this theory a little green sensation, it becomes orange before white, showing that the red sensation is dimmed slightly before the green, and so with the other colours. What are called “after images” I have not touched upon so far, nor shall I here, for it is at this point that we step into very debateable ground. The colours perceived in them are, as yet, not capable of being put to the test of physical measurement, and I must leave the psychologist or the physiologist to account for them in their own way.

Viewing the Hering theory from a physical standpoint, and in the light of colour measurement, it appears to be deficient in several respects. To take one point. We have seen that when blue and yellow are mixed together to make white the sum of the luminosities of the two colours separately is equal to the luminosity of the white produced. According to the Hering theory, the yellow colour contains a certain amount of the white-black sensation besides the yellow sensation, as does also the blue colour besides the blue sensation. The theory tells us that when white is produced by the mixture, the blue sensation undoes the work that the yellow sensation has done, and the white sensation is alone left behind. If this be the case, the sum of the separate luminosities cannot be the same as that of the white produced, but should be greater. The theory also has to be strained sometimes to make it fit in with other observed facts. Take, for instance, the case of persons who are called red-blind and green-blind on the Young theory. We are told by the Hering theory that both are red-green-blind—that is, blind to both green and red, and only see blue and yellow—and that the only difference between them is that the former has his spectrum slightly shortened at the red end, the maxima of the yellow-blue sensations being shifted a little further towards the violet end of the spectrum. The natural question to ask is: Why this shift occurs? Surely it is more rational to adopt a theory which does not require such a supposition? If the sensitive matter acted upon by the yellow-blue rays be always of the same chemical composition, the shift cannot occur. It might, perhaps, be allowed that one shift was practicable, but, unfortunately, the shifts must become numerous when the cases of partial colour blindness are to be accounted for, and this would necessitate a constantly varying chemical composition of this matter, and of that acted upon by the red-green rays.

Again, in the extinction of the spectrum, the red and the green sensations in quantities to neutralize one another should be extinguished nearly together, even allowing for what physiologists tell us is the case, that the breaking down, or dissimulation, of cell tissue continues longer than its building up, but we find a large difference between the two. As already indicated, the luminosity curve of the feeble spectrum favours the theory of Hering being that here we only have the white-black sensation, and naturally the persistency curves must be scored in its favour. But the cases of B. C. and M., it seems to me, cannot be explained by the theory without any undue straining or assumptions. If we try and fit the cases of colour blindness due to tobacco scotoma to the theory, we find that in many cases yellow is not recognised, though blue is invariably. If the blue be active, the yellow should also be so.

And here I may remark that it has been assumed that the two classes of colour blindness are due to different causes. A question to ask ourselves is whether all colour blindness may not have been caused originally by disease. In the congenital form, it is true, no disease of the retina is traceable in the eye, and it is usually hereditary, but it does not follow that the want of response of the perceiving apparatus to certain sensations may not have been due to what, for want of a better expression, I may call an hereditary partial paralysis of the perceiving apparatus. If this be so, we have a connecting link between the two classes, and then a perfect theory should explain both classes on the same grounds. The suspicion that the monochromatic vision of P. and Q. might possibly be due to disease before birth, owing to the behaviour of their eyes under certain conditions, would then be explicable. I have no desire to press this view, though it seems to me to be one which is not out of all reason, taking analogies from other defects which are hereditary.

It has been usually accepted that the fields for blue and yellow in the eye are approximately the same, as are those of the green and red, and this has been taken as showing the interdependence between the two pairs according to the Hering theory. It has already been pointed out that the question of extent of fields requires still further investigation beyond that which it has received, and measures made by the method given on [page 208] seem to cast a doubt as to whether this interdependence can be upheld. It will be noticed that the fields do not extend proportionately on the nasal and temporal sides (see also [Fig. 3]). It should also be remarked that the order of extent of field for the different colours does not follow the same order as their disappearance. A point that is sometimes raised in favour of Hering’s theory is the negative image formed after the eye is fatigued by looking at bright red or bright green. The negative images (see [page 30]) are said to be the complementary of these colours. The Young theory tells us that the red or the green sensation suffers fatigue by one or other colour, and that when the eye subsequently rests on a grey surface the other two sensations are chiefly stimulated and cause the complementary colour. It is said that it is easier to produce a negative green image than a negative red image, and the adherents of Hering tell us that this is due to the fact that destructive action is more readily carried out than constructive. In the Young theory, it is held that the green sensation is always mixed with white, whilst the red is fairly pure, and thus, for equal luminosities, the surplus green sensation is much less stimulated than the red, which offers a consistent explanation of this fact. There are several other minor difficulties in the way of accepting Hering’s theory as it stands from a physical point of view, but we need not discuss them now.

The final sensation curves for the spectrum colours on the Young theory are still under consideration, and are not definitely fixed, though the observations made have been very numerous. Recently Helmholtz, in the last edition of his “Physiological Optics,” has calculated, from Kœnig’s observations, that no one of the three sensations is singly stimulated by any colour, even at the extreme ends of the spectrum, and he makes the three fundamental sensations vary considerably from those given in these pages. Every colour he states is considerably mixed with white light. The calculations by which he arrived at this conclusion are of a complicated nature, and I think if he had had besides the colour equations of Kœnig, the luminosities and the extinction measures before him, there might have been a modification of his views, for these last give evidence to the contrary.

There is a possible modification of the Young theory which would account for a good many of the phenomena that are unaccounted for by it in its present form, though it may raise new difficulties in the minds of some. Let us suppose that each of the three sensations were compounded of fundamental light and of colour in fixed and definite proportions, and not in the same proportion in each; and further that the apparatus in the eye which was responsible for each sensation had two functions, one of which was to respond to the fundamental light sensation and the other to the colour. One essential difference between this modification of the Young theory and that of Hering is that, whilst in the latter the white sensation is a sensation distinct from the colour sensations, in the former it is a definite part of them. The fact that the sensation of colour is lost before the sensation of light is one of the greatest significance, and any theory to be accepted must offer a reasonable explanation of it. If the modification suggested be made, it accounts for the existence of this residuum of light equally as well as Hering’s theory, and without its drawback. It is not hard to imagine the apparatus which gives rise to two sensations, on the assumption of different kinds of atomic motion, induced by the ether motion, or at least three kinds are possible. When extinction of colour is made, the ether vibrations would have sufficient energy to induce but one kind of motion; and when all light was extinguished from the same ray, they would not be capable of inducing any sensible motion whatever. In the case of Miss W., who saw all colours as white, it might be that disease had entirely prevented the first kind of motion in all three sensations, and that in P. and Q. the red and green sensations were absent or paralysed in their entirety, whilst the blue sensation was left in full operation. In B. C. the blue and red sensations would be similarly absent, leaving the green sensation unchanged. The coincidence of their persistency and luminosity curves would then indicate that the proportions of fundamental light and colour remained the same throughout. Other examples and considerations seem to indicate that the proportion of colour to fundamental light is greatest in the red sensation, next in the green, and least in the blue. This would explain why with increasing intensities blue appears white sooner than green, and much sooner than red. The proposed modification would also offer the necessary explanation as to the disappearance of colour from the field.

Looking at colour vision from what I may call an evolutionary point of view, the “light-colour” theory commends itself as probable. There are many reasons for thinking that the visual sensation first evolved was that of light, subsequently followed by that of colour. The first evolved colour sensation would appear to have been the blue, and the last the red. The discussion of this hypothesis would carry me beyond my limits, and I must leave it thus baldly expressed for your consideration.

For my own part, whatever theory of colour sensations may prove to be the right one, I lean strongly to the idea that the cause of vision will be found in chemical action, induced by the impact of the different wave-lengths of light falling on sensitive matter. A white substance may absorb all the wave-lengths found in the spectrum, and if it have three sets of molecules, one of which has an atom or atoms vibrating with the same period as the waves of light which show a maximum for one sensation and another for another, and so on, the requirements for the colour sensations are met. It may be that the sensitive part of the retina is like a photographic plate, but with this essential difference—that the sensitive material is constantly changing. A photographic plate receives an impression which is not recognisable by the eye, though it can be shown that a change in the material does take place during the impact of light, by electrical and other means. When the eye receives an impression of light, Dewar has shown that in this case also a current of electricity is generated. Recent published experiments of my own have demonstrated that with a low intensity of light, the chemical change that occurs in a photographic salt is by no means proportionate to that which takes place with a greater intensity. In the eye, too, there is a limit of sensibility to very feeble light. Again, the curves of the stimulation of the colour sensations to the spectrum are closely of the same form as the curves of sensitiveness of the various sensitive salts used by photographers. These are analogies and, of course, must not be pressed too far. There must be such a complexity in the sensitive material in the eye, both chemical and physiological, that it may be that the changes induced by light on the sensitive surface of the retina have to be considered from both aspects. The purely chemical change is naturally that to which a physicist is most prone to incline, and his bias must be discounted, as must also that of the physiologist.