Popular Science Monthly/Volume 19/May 1881/Color-Blindness
WE have become so accustomed to color in all the objects about us, that we may almost be said to take no notice of it. Day after day we look upon the wealth of color in the landscape by which we are surrounded, without hardly ever giving it a thought. Some of us never awake to the perception of the beauty of color in nature; to others the knowledge of this beauty is only opened through the medium of art. A person who has taken little interest in paintings, but who, by some circumstance or other, is at last led to a more attentive study, especially of landscape-painting, will frequently be surprised by the enhanced interest which Nature ever after awakens in him. He finds charms where he never sought them before, and sees beauties to which he had been totally blind. The mystery of color has been unfolded to him, or rather he has been made conscious of his own faculty of perceiving color—a faculty which had, indeed, been always in him, but which had lain dormant.
Even to those, however, who are fully alive to the charm of color, the latter is so much a matter of fact that they take its presence for granted, and accept as a foregone conclusion that it can never be otherwise. The question, How would the world look without color? has never troubled their minds, and, if it were really proposed to them, they would probably meet it with the reply that there was no need of speculating about impossibilities. Yet that which appears to be so impossible is really possible; for there are not only people in existence who do not see, never saw, and never will see color, but we may even create something approximating such a colorless world for ourselves, at least as far as the artificial sphere is concerned in which we move within our houses.
Before me, as I write this, hangs a Chinese painting, executed in all the brilliancy of Oriental coloring—rich vermilion, fine blues of various shades, greens, and other full colors. I light an alcohol-lamp, into the wick of which I have rubbed some common table-salt. I turn down the gas, and, as I now look at the Chinese painting in the dim light of my magic lamp, all its color has disappeared. I know the vermilion, the blues, and the greens are all there, but I can not see them. And yet I see the picture itself quite plainly, with its outlines and its delicate gradations; but it is all black and gray, with only a faint trace of yellow here and there, where a yellow pigment has been employed by the artist. Beside me on my writing-table lies a sample-chart of water-colors; but, however intently I look at it, I can see nothing but spots that seem to have been produced by India-ink in various gradations. I travel round my room, and all the objects appear to me of the same somber hue. The embroidered cushion on the lounge, the carpet on the floor, even the flowers in the vase—they are all black and white, or at the best yellow. The little world that surrounds me is colorless.
Imagine, then, for a moment the whole world deprived of color. How would it look? An enamored poet singing to his adored in the world as we at present know it might, perhaps, prelude his ditty thus:
And hair as golden as the sun's bright rays!"
Translated into the language of a poet of the colorless world, this strain would run about as follows:
And hair as white as freshly fallen snow!"
We, who are accustomed to the charm of color, turn away shudderingly from such a world, in which we would all look like the figures in a steel-engraving, printed in the blackest of ink on the whitest of paper.
And yet, as I have said before, there are people who live in such a world continually, and must continue to live in it to their days' end. Fortunately, however, the instances of people who are totally color-blind—that is to say, who are absolutely incapable of experiencing the sensation of color—are extremely rare; and, to the few people so afflicted, the deprivation is not so great as it would seem to be to us, since, having never known the poetry of color, they do not feel the want of it.
But, although there are only very few people indeed who are totally color-blind, there are, on the other hand, a very large number of persons, especially among the male sex, who are at least partially so; and it is even more difficult to picture to ourselves the world as it is presented to their eyes than to imagine a world entirely destitute of color. Defective color-vision of this kind is most frequently manifested in the inability to see the difference between red and green. A person thus afflicted can detect no difference between the ripe cherry on the tree and the leaves by which it is surrounded, or between the strawberry and the stems and leaves of the plant on which it grows. Even the bright red of some flowers may only present itself to such persons as a lighter shade of the color of the leaves, while yellow and blue are perceived by them quite as distinctly as by persons of normal vision. To them, therefore, the world must bear a resemblance in color to some of the old pottery which is decorated in blue, yellow, and black, on a whitish ground. There are other varieties of defective color-vision, all of which may be generally described as an inability to perceive certain colors, while the perception of certain other colors is normal. The simplest method of picturing to ourselves the world as it is seen by some color-blind persons is to hold up before the eyes a glass vessel with flat, parallel sides, filled with a solution of sulphate of copper. We shall then be pretty much in the same condition as a red-blind person.
The inconveniences which color-blind people must frequently be exposed to are manifest. Numerous stories are told of the most ludicrous mistakes made, especially by red-blind persons: of a tailor, for instance, who mended a black coat with a piece of red cloth; of a hunter who bought red cloth to have made what he supposed would be a green hunting-jacket. The story of the tailor shows how this malady, or, rather, constitutional defect, may do injury to men in their professional capacity. But the consequences that may possibly arise from it are of a far more serious nature when the safety of a large number of human beings is dependent on the color-vision of a single individual. This is the case with railroad operatives, who must be able without fail to tell one signal from another; and, as of late years the conviction has gained ground that color-blindness is far more common than it was formerly supposed to be, the railroad companies are warned more emphatically from year to year by scientific men to see to the eyes of their employees. Some of the European Governments are beginning to turn their attention to this important matter (all the more important because railroad-signals are usually red and green, and red-blindness is the most common form of the failing), and the Swedish Government has lately directed the physicians attached to its state roads to examine all the operatives on these roads, with a view to the detection of the presence of color-blindness. The first fruit of this order is a report by Professor Holmgren, who recently examined the employees of the Upsala-Gefle road, showing that, out of two hundred and sixty-six individuals, eighteen were afflicted with the malady to a degree sufficiently high to incapacitate them entirely for service on the road. The prevalence of the disease varies in different countries, the highest percentage being found in England, where, according to a statement made by Professor von Bezold, in his "Theory of Color," republished in this country in an English translation, one out of every eighteen persons is said to be afflicted with it. Among men, as before remarked, the disease is more common than among women.
The cause of total or partial color-blindness may easily be understood if we accept the hypothesis first brought forward by the English physicist Young, and now subscribed to by the leading scientific observers of all countries. According to Young, all the phenomena of color-vision are due to the (hypothetical) presence of three different kinds of nerve-fibers in the retina—that is to say, in that part of the eye on which the reflected images of the objects of the outer world are projected as upon a screen, and through the agency of which the sensations produced by the impressions so received are transmitted to the brain. One of these sets of nerve-fibers is supposed to respond most readily to red, the other to green, the third to violet, or to a blue which verges closely upon violet. When all these nerve-fibers are absolutely at rest, we see nothing. Improperly speaking, we might say that we then experience the sensation of black, for absolute black really produces no sensation, but is rather the result of the absence of all sensation. On the contrary, when all the nerve-fibers are excited simultaneously and to an equal degree, we experience the sensation of white, provided that the amount of excitation is tolerably great. If the excitation is only feeble, we see what we call gray—gray being simply white of a low degree of luminosity. All other color-sensations are produced by the excitation of groups of nerves variously combined. Thus, whenever the fibers which respond to red and those which respond to green are excited simultaneously, we experience the sensation of yellow; when the two groups which respond respectively to green and to violet are simultaneously excited, we experience the sensation of blue, and so on through the whole scale of colors. Again, when all the nerve-fibers are excited at once, but to an unequal degree, we perceive the result of the mixture of one predominating color with the others. If we suppose the nerves responding to red to be the most violently excited, we shall experience the sensation of red mixed with white, or, in other words, of light red.
It will readily be seen that this hypothesis explains the curious condition of color-blind persons very satisfactorily. In the case of total color-blindness, we need only to assume that the nerve-fibers are in an abnormal condition, so that each set, instead of responding to only one sensation, responds equally to all. The result must necessarily be a total absence of color in the impressions received through the eye. In the case of a red-blind person, the nerves which ought to respond to red may either be paralyzed or they may be wanting altogether, and all other defects in color-vision may be explained upon the same principle.
To a limited extent the inability to tell the difference between certain colors, which is due to partial color-blindness, may be overcome by the use of variously colored glasses; but, after all, no artificial palliative will compensate for the want of a naturally perfect eye.