Popular Science Monthly/Volume 21/May 1882/Color-Blindness and Color-Perception
TO physiologists that part of the function of vision which is concerned in the perception of colors has always been one of great interest, but it was not until the genius of Thomas Young offered them his theory of vision that they had anything like a plausible working hypothesis. This theory, as elaborated and promulgated by Professor Helmholtz, has until very recently been the one most relied upon in explanation of all the phenomena of colored vision. It is, however, a pure hypothesis, since not one of its fundamental principles is a demonstrated or even a demonstrable fact. By a process of deductive reasoning, and most probably with little, if any, experimentation—for it is said that Young prided himself on being independent of the necessity of experiment—the vivid imagination of this original mind seized upon an hypothesis which seemed to satisfy the demands of an acceptable theory, in so far as it accounted for all or nearly all of the observed phenomena. At that time, however, and even when Helmholtz resurrected and revivified the theory, the question of color-blindness had not been investigated to the extent it has within the past ten years, and most physiologists rested content with the belief that at last the true theory of colors had been found,
Such, however, is no longer the case, and there are many who are not almost but quite persuaded that the true theory of vision is one of the questions to be solved by the coming physiologist. This theory of Young-Helmholtz, as it is called, demands three primary or fundamental colors, by the admixture of which all other colors are produced. These colors are supposed, by Helmholtz, to be red, green, and violet. All other colors and shades are made from the proper mixture of two or more of these colors. White is the sensation produced by the proper mingling of all three sensations; black is the absence of sensation. Corresponding to these three primary sensations there are in the retina, or terminal expansion of the optic nerve, three distinct sets of nerves which respond to the wave-lengths of the luminiferous ether which physically represent these colors.
This is all very simple and extremely plausible, but certain phenomena of vision make it necessary to so modify this simplicity as to spoil its beauty and give an elasticity to the theory which can not be gratifying to the student of exact science. It becomes necessary to suppose, for instance, that the nerve-fiber which responds to red is also affected, in a less degree, by the green waves, and in a still less degree by the violet; and the green waves, while principally affecting the green fibers, affect also the red and violet; and the violet waves influence the red and green fibers, though in a much less degree than they do the violet. In this theory gray is but a white of diminished intensity.
Color-blindness is explained in keeping with this theory as follows: Any one or all three of the color-fibers may be wanting, or lacking in functional activity. Consequently there may be red-, green-, or violet-blindness, or there may be total color-blindness. Since, however, it is supposed that each one of the color-fibers is affected (though in a less degree) by both other colors as well as by its own peculiar color, there must be a sensation produced by each color, though it will be of lessened intensity in the case of the lacking color, and that sensation must be other than that of the color belonging to the missing fiber. Under these circumstances, even a saturated primary color would not, when its fiber was missing, appear black, though it would appear darker than to one with normal color-perception. To a red-blind person, a spectral red, for example, while appearing a color much less luminous than is usual, would not be black; and, if a solar spectrum were presented to such a color-blind individual, it need not appear shortened at the red end. If the green fiber is the lacking one, green will not appear as black, but when of a certain shade will appear as gray, and for the following reason: White is the product of the sum of all the sensations which the mind is capable of perceiving through the eye. When the eye is normal, we have it when all three of the fibers are affected in about the same degree, and in the color-blind when the two remaining fibers are thus affected. Any color, therefore, which contains, besides green, a certain proportion of the other colors (red and violet)—as certain shades of what we call green do—will cause, when presented to such a green-blind individual, the sensation of white of diminished intensity. When the solar spectrum is placed before him, there should be a gray or neutral band at the line which divides the two colors which are unmistakably distinguishable; and, in the green-blind, it is nearer the red end of the spectrum than in the red-blind.
When the violet is the lacking fiber, we have phenomena analogous to those where the red fiber is missing, though, of course, there are differences in details.
In accordance with this theory, therefore, there can be no color-blindness, in the strict acceptation of the term, except when all the color-fibers are lacking; because all colors produce an impression of some kind, though it may not be the one experienced by those of normal color-perception. There is, however, a marked confusion of the various colors, and by the special character of this confusion one kind of color-blindness is differentiated from another.
In making an examination for the diagnosis of color-blindness, nomenclature, or the naming of the colors which are presented to the person to be examined, is entirely discarded. It has been found that an individual maybe able to name the several colors correctly, and yet make mistakes when called upon to match them; and, on the other hand, he may not be able to name a single color correctly, and yet make no serious mistakes in "matching." The method of comparison is therefore the only one which should be adopted in making examinations for color-blindness.
The method of Professor Holmgren, which is the simplest and, on the whole, the most convenient, consists in placing on a table before the examinee a large assortment of skeins of colored worsteds. A "sample" skein of light-green is laid to one side, and the individual is told to select from the pile all the skeins which are of the same color—lighter or darker. If he places by the sample a shade of any other color but green, he is color-blind. This examination, however, does not fix the particular color to which he is blind, and, in order to find the color which is lacking in his chromatic scale, a purple or rose-colored skein is laid aside as a sample and the confusions he makes here are supposed to fix the diagnosis. If he matches the purple with blue and violet or one of them, he is red-blind. If, however, he selects the greens and grays, he is green-blind. Violet-blindness (which is very rare) is recognized by a confusion of red, purple, and orange in the test with the purple skein.
Another plan for employing the comparative method is to have two solar spectra, one above the other, the upper of which is movable. A colored band is isolated in the fixed spectrum, and the upper spectrum is moved until what is supposed to be the same color is immediately above it. Or, the isolated band may be matched with a skein of colored wool. Of course, the same mistakes will be made here as in the preceding method.
Another method of examination rests on the phenomena of what are called contrast colors. When a white surface is illuminated simultaneously by a red and a white light—as by two lamps, for example, before one of which a red glass is held—an object, a pencil, for instance, held midway between the two will cast two shadows, one from the red light and another from the white light. To one of normal color-perception, one of these shadows (that cast by the white light) will be red, while the other (that cast by the red light) will be green; to any one blind for either one of these colors, there will be no difference in the color of the shadows. If rings cut from black or gray paper are laid upon red or green paper and the whole is covered with tissue-paper, the rings will have a reddish tinge if the ground is green, and green if the ground is red. If, however, the individual is blind to either of these colors, no such difference will be noted; and, if letters cut from black or gray paper are used instead of rings, they can not be distinguished when laid on the colored ground and covered with the tissue-paper.
Another method is to make letters of certain colors on different colored grounds—shades of red letters, for instance, on a green ground. When these are of the requisite tints, the color-blind person is not able to distinguish them.
There are other methods, but they are all modifications to a greater or less extent of the foregoing, and any one who is color-blind to any considerable degree can be detected by any one, or at least by any two, of the methods indicated.
There is another theory of colors brought forward within the last few years by Professor Hering, of Prague, which is adhered to by many physiologists, and is a vigorous rival of the Young-Helmholtz theory. Professor Hering assumes that there are three chemical visual substances in the retina, which he calls the black-white, the red-green, and the blue-yellow. Light acts upon these substances by what he calls assimilation (A), and dissimilation (D). When light acts in a dissimilating or decomposing manner on the black-white substance, the sensation of white is produced; when there is an assimilation or regeneration of this substance, the sensation is black. Hering is by no means certain which are the A-and which the D-colors, but he is disposed to regard red as the dissimilating color of the red-green substance, and green the assimilating color. Blue, he thinks, causes dissimilation of the blue-yellow substance, while its regeneration is caused by yellow. All colors, he supposes, act in a dissimilating manner on the black-white substance—that is, they produce the sensation of white in addition to their own peculiar color. They act, however, in varying degrees of intensity, yellow acting with the greatest power, the strength of action diminishing toward the two ends of the spectrum. In accordance with this theory, there are, therefore, four fundamental colors instead of three (excluding black and white), namely: red, green, yellow and blue, and they are supposed to be produced as follows: Red is the product of the dissimilation of the red-green substance, green is the result of its assimilation; blue is the result of the dissimilation of the blue-yellow, and yellow of its assimilation. When the A-and D-action on the red-green and blue-yellow substance are equal there is no color sensation, but only the D-action of these colors on the black-white substance, that is white. Simultaneous A-and D-action on the black-white substance, however, is not attended by abolition of sensation, but by the sensation of gray.
It will be seen from this that, in the Hering theory, what were before considered as complementary colors are antagonistic and tend to neutralize each other. It will be remembered that those colors have been called complementary which, when mixed together, would produce white (we speak now of spectral colors). This was explained by the Young-Helmholtz theory on the principle of combination; it is accounted for by the Hering theory on the principle of subtraction. When red and green, for instance, form white on being mixed, the white is not produced by the sum of the sensations of red and green, but the red and green, being antagonistic, neutralize each other, and there only remains the D-action of both colors on the black-white substance—that is, white.
As in the Young-Helmholtz theory, the other colors, aside from the primary, are the results of mixed sensations.
Color-blindness, in accordance with this theory, is of two forms. In one, both color substances are wanting, and there only remains the black-white substance to be acted on by light (achromatopsia). In the other form, one of the two color-substances is lacking and only the two colors of the remaining color-substance are distinguishable (dichromatopsia). If the red-green substance is lacking, there will be red-green blindness or blue-yellow vision; if the blue-yellow substance is the missing one, there will be blue-yellow blindness, or red-green vision.
To satisfactorily account for some of the phenomena of color-blindness, however, it becomes necessary to suppose that, when one color-substance is wanting, the light rays which act specifically on that substance produce an A-or D-action on the remaining color-substance. In red-green blindness, for example, red, yellow, and green act in a dissimilating manner on the remaining blue-yellow substance, giving rise to the sensation of yellow, while blue alone acts in an assimilating manner. The most strongly dissimilating color will be yellow, while the others will be more or less varying in their action. In the case of blue-yellow blindness, red, yellow, and blue are the dissimilating colors and green the assimilating color. It will be readily understood, when we have this state of affairs, that in the dichromatrope, where the A-and D-action of the one remaining color-substance are equal, gray will be the result, because, as we have before remarked, where two colors neutralize each other there still remains the action of both on the black-white substance, which will give rise to the sensation of gray or white of diminished intensity. But the same colors will not appear gray to all color-blind persons, for the reason that the same colors do not act in every case with the same intensity of dissimilation and assimilation. In most individuals it is the purple and the blue-green which give rise to the impression of gray.
A spectrum should, in accordance with this theory, appear in only two colors to the color-blind, and may or may not be shortened according as the dissimilating power of the two remaining colors is intense or very feeble. The only colors, of course, which such a color-blind person can with certainty distinguish are the two belonging to the one remaining color-substance, blue and yellow, for instance, when there is red-green blindness, and red and green when there is blue-yellow blindness. It is not to be understood, however, that such an individual can never correctly distinguish other colors. Most frequently he can, but there is always a liability to confusion, often of the most astonishing character; and, moreover, the distinctions are made, not by the sense of color, but by some other characteristic, different degrees of luminosity, most commonly.
The evidences which the phenomena of color-blindness have brought against the three-fiber theory of Young-Helmholtz are:
1. That the red-blind can not distinguish perfectly the greens and violets, nor the green-blind the reds and violets; yellow and blue being the only colors about which they make no mistakes.
2. Even in a spectrum which is very much shortened the red-blind finds the brightest place, not in the bluish-green, as we should expect, but in the yellow, as in the normal eye.
3. This theory can not satisfactorily explain the extreme shortening of the spectrum, extending, as it sometimes does, into the orange, and even into the yellow.
4. The line of demarcation in the spectrum is sharply at the blue, all to the left almost always appearing of one color, and all to the right of another, there being no lines of division between blue and violet, nor between the red and yellow and the yellow and green.
5. The gray or neutral band is far from being invariably present, and when it is it is often, in the red-blind, in the position it should be in the green-blind, and vice versa (Mauthner).
Against the Hering theory the following objections have been advanced:
1. There is no reason for supposing that red and green and blue and yellow are opposing colors. They are all active in their specific line, and even Hering has not been able to determine which possesses the A-action and which the D-action.
3. The white, which comes from the union of two of Hering's antagonistic colors, is not the result of subtraction, but of addition, as is shown when, with a double spectroscope, a saturated violet being made to cover a yellow, a white is produced which is manifestly more intense than the yellow, while another yellow of the same intensity as the violet added to the yellow does not produce a yellow intenser than the yellow resulting from the first experiment. 4. White is not a direct independent sensation; it is absent in the spectrum where, in red-blindness or violet-blindness, the specific color is absent (Donders). </poem>
From the foregoing, and from a study of the phenomena as presented by a number of color-blind persons, two important facts are forced upon the unbiased observer: 1. That we have not yet arrived at any fixed laws governing the phenomena; that all cases can not be classed as distinctly red, green, or violet blindness, though it seems probable that all might be classed under the heads of red-green and blue-yellow blindness. 2. That neither of the two prominent hypotheses fills the demands of an acceptable theory, inasmuch as both fail to account consistently for all the phenomena.
It seems to us that, in the consideration of the subject of color-blindness hitherto, too much stress has been laid on the part which the retina plays in color-perception. There are three distinct agents at work in the perception of color. The impression is first made on the retina; this is carried thence by means of the optic nerve to the center in the brain which presides over the function of vision, and it is there converted into a sensation. Let any one of these agents become incapacitated, from any cause, for properly carrying on its function, and there must be a perversion or absence of sensation. In certain affections of the retina and optic nerve we have instances of color-blindness from deranged or abolished functional activity of the first two agents, and in some forms of toxic action, particularly alcoholic poisoning, we have in all probability examples of the cerebral form of color-blindness. The supposed color-fibers or color-substances may be in a perfect condition and acted upon in a perfectly normal manner by light, but the optic nerve may be incapacitated by some change in its molecular structure from carrying all of the impressions correctly to the brain-center, and, even should all the separate impressions arrive there, the cerebral center itself may not be in condition to convert them into the proper sensation. The conducting power of the nerve, or the converting power of the cerebral center, may be but slightly deranged or totally deficient for some color or colors, and so the phenomena presented by two cases falling under the same category would be very different; and, when we consider the infinite degrees of incapacity that may exist for all the different colors, we can readily understand the infinite variation in the mistakes of the color-blind, and the impossibility of laying down exact rules for diagnosis.
It is my belief that a large number, perhaps a majority, of the cases of congenital color-blindness have not their seat in the retina at all, but are cerebral in their character. In other words, I believe that in these cases the brain-center of vision has not the power to differentiate the various impressions it receives. This opinion will seem the more plausible when we remember that the sense of sight is a developed or educated one. Though we have received from our ancestors the potentiality of vision, every child that is born must learn to see for itself. Without here entering into a discussion of the question of the development of the color-sense, which has received much attention at the hands of Mr. Gladstone, Magnus, and others, it is safe to assume, with our knowledge of analogous matters, that the differentiation of colors is a power partly inherited and partly developed in the individual; and, moreover, we should expect to find this power, which is undoubtedly cerebral in its character, most strongly developed where the faculty was most used. And so we do find it. Women, who are much more concerned than men in the selection and comparison of colors, are rarely affected with color-blindness; and we all know how much quicker the feminine eye is in detecting slight differences in shades of color than is that of men who are not color-blind. In those cases of color-blindness which, for the sake of distinction, we shall call central, we believe that the brain-center of vision has not been developed to its full or at least to its ordinary power for discriminating between the impressions corresponding to the different colors. The retina may be capable of properly responding to these various impressions, and the optic nerve may carry them as separate impressions to the brain-center; but this has not the power of converting them into individual sensations.
From what has already been said, it is evident that neither of the two at present prominent theories satisfactorily accounts for all the phenomena of color-blindness. Moreover, it seems to me, the true theory of colors when found will be simple; and the laws governing the sense of vision will be found to bear some analogy to those governing the other senses—at least, I do not believe it will be found necessary to invent new processes and new reactions of tissues to agents affecting the economy. The true theory, I believe, will be found to lie in the direction pointed out by the recent researches on the physical reaction of certain simple substances to the undulations of the luminiferous ether. This reaction may be in its restricted sense chemical, purely physical, or chemico-physical; but it will be due to the changes in the molecular structure of simple substances, caused by the action of the either. In other words, the variation in the sensation produced will have its basis, not in complexity of tissue, but in the varying action of the affecting agent.
Without entering into a discussion of the question in detail, I would say that it seems probable that the optic nerve is merely a highly organized nerve of common sensation. In some of the lower forms of animal life light is perceived over the whole cutaneous or external surface, as shown by the action of the animals when exposed to its influence. Furthermore, it is now a generally admitted fact that heat and light are due to vibrations of the same ether, differing only in their wave-lengths. The effect of both heat and light is to produce molecular change. When heat produces a sensation through the cutaneous nerves, it is most probable that it does it by means of a molecular change in the terminal filaments of these nerves which is communicated to the brain-center through the nerves, probably also by a rapidly progressive change in their molecular structure. The nerves of common sensation, however, do not seem to possess the power to differentiate variations in wave-lengths—they take cognizance only of the varying intensity of the vibratory motion; that is to say, they distinguish quantities rather than qualities. It would, however, be doing no violence to known facts to suppose that a high specialization would enable these nerves to carry as distinct impressions the changes wrought by the separate wave-lengths. In fact, it is highly probable that they do so, but the cerebral centers in which they terminate have not been educated to the point of making distinctions between these separate impressions and fixing them as individual sensations.
In framing a theory of color-perception on the basis we have indicated, we would suppose the retina to be a body whose molecular structure is such that it will respond with promptness to all or nearly all the wave-lengths of perceptible light. This molecular change produced in the retina is carried by the optic nerve to the center of vision in the brain, and is there converted into a sensation. This is, to some extent, going back to the original theory of Newton, who, in speaking of the action of light upon the retina, considered that "the rays impinging upon the ends of the optic nerve excite vibrations which run through the optic nerve to the sensorium. Here they are supposed to affect the sense with various colors according to their nature and bigness."
The chief objection to this hypothesis, advanced by Young, was that the frequency of these vibrations must be dependent upon the constitution of the substance of the retina, and it was almost impossible that every sensitive point should have an infinite number of different particles to respond to this infinite number of vibrations. He therefore supposed the number to be limited to three which corresponded to red, green, and violet.
It will be seen that the difference in the different theories of colors lies in the supposed reaction of the retina to light. After the impression has passed beyond the retina, there is no special or important difference m the views as to the final conversion into a sensation The objections to these two hypotheses we have already stated. The acceptance of such an hypothesis as we propose, however, does not involve the necessity of inventing new laws, or of creating new issues, but only applies known laws and analogous reactions of other substances to the elucidation of the phenomena observed. We know that there are membranes which respond with promptness to any number of simple aerial vibrations at the same time, and recent discoveries have shown that there are substances which, when in proper condition thus respond to wave-lengths of light. Silenium, when in a crystalline condition, alters its molecular condition (as manifested by its varying resistance to the passage of the electric current), not only when acted on by light of varying intensity, but also by the different wave-lengths If, then, we suppose the retina to be a substance of this nature but responding more promptly, and in a more delicate manner, than any other known substance to the wave-lengths of light, we have a basis for a theory of vision which is extremely simple in its nature, and founded on known physical laws.
We will not here enter upon a detailed application of this theory to the elucidation of all the phenomena of colored vision, but will simply mention a few points in connection with color-blindness. One general principle may be laid down which will cover all cases of retinal color-blindness as distinguished from cerebral or central, and that is, that in these cases the molecular structure of the retina is so altered as to allow it to respond feebly or not at all to light rays of certain wave-lengths. We know, for example, that silenium must be in a crystalline state—that is, its molecular structure must be in a certain definite condition—before it can respond in such a delicate manner to variation m the intensity of the light-waves; and we know that there are certain wave-lengths of the ether—the ultra-red and the ultra-violet—which call forth no reaction on the part of the retinal substance. It would, therefore, be a highly justifiable supposition that a slight alteration m the molecular structure of the retina would render it incapable of being affected by certain wave-lengths to which it, when in a normal condition, readily responds. This incapability may be partial or complete as regards any particular wave-lengths. In some instances of color-blindness, for example, the spectrum is shortened at the red end even under the most intense illumination, while in others there is a shortening only when the illumination is feeble—becoming of normal length when the intensity of the illumination is increased—showing, in the latter case, that the reaction to the red rays is still present when they are of sufficient intensity.
When we come to cerebral color-blindness, which is, according to my view, the most common, the explanation is still simple. In this instance we have only to suppose the cerebral center of vision incapable of distinguishing between the impressions of wave-lengths which lie relatively near together as regards their vibration numbers. It will be noticed, as an important fact, that there is confusion only of those colors which lie toward the same end of the spectrum. Red and green, for instance, are the colors which are most commonly undistinguishable; blue and yellow less commonly; but no instance is on record in which red and blue, or green and yellow, were constantly confounded. It seems from the examinations thus far made that the color-blind make, as a rule, distinctions between only two classes of color-sensations. A most intelligent color-blind man, whom I recently examined with the spectrum, saw it only as two colors—the line of demarcation being sharply at the blue-green junction, all to the right was blue, all to the left was what he called red. He could distinguish no line of separation between the red, green, and yellow, and the maximum of intensity was at the yellow, as is the case with normal eyes. As Mauthner says, there are no fixed rules which serve us for a diagnosis between red-and green-blindness. The two colors are confused, but how are we to know which is the one correctly perceived? The individual who is found to be green-blind by one method of examination is often found to be red-blind by another, and in some cases to have a shortening of the red end of the spectrum. Moreover, the red-blind can not unerringly pick out the greens, nor the green-blind the reds.
If, as we believe, a large number, perhaps a majority of the cases of congenital color-blindness are cerebral rather than retinal, and due more to a want of education of the color-sense than to any anatomical defect, a plan for the diminution or eradication of color-blindness would be by no means chimerical. The fact that women are less frequently color-blind than men we consider most probably due to the circumstance that their faculty for color is in more active and constant use, and for this reason has become more highly developed, and has been transmitted as a sexual peculiarity from mother to daughter. It seems, therefore, quite reasonable to suppose that if boys could have their color-sense educated to the same extent as girls, and the process were continued through a number of generations, the defect of color-blindness would in course of time disappear, except as a rare anomaly.
- A paper read before the Philosophical Society of Washington, December 18, 1880.