Popular Science Monthly/Volume 29/October 1886/Are Black and White Colors?
By HARRY AUSTIN DOTY.
ALTHOUGH there is no general agreement as to whether black and white are or are not colors, it is very commonly held both by scientists and artists that they are not colors. Encyclopædias, dictionaries, and text-books usually class black and white separately from colors, defining the former as the absence of all color and the latter as the sum of all colors. Von Bezold ("Theory of Color," p. 41) says, "An object appears black if, in the light falling upon it, those species of rays are wanting which alone it is capable of reflecting"; and, again, "White and black . . . which, indeed, are not colors at all in the true sense of the word." But, on page 90 of the same work, the heading of paragraph 48 is "White is a mixed color"; and, again, on page 113 it is stated that "white and all the very pale colors which are closely allied to it must be counted among the cold colors." It is not meant to attach much importance to such little inconsistencies in this very excellent work, but simply to indicate an indecision regarding the limitation of the word color. For another instance may be quoted Field, an English artist, who says of black that the artist is bound to regard it as a color; that "it is colorless, but extinguished light"; that "to be perfect it must be neutral with respect to color and destitute of sheen or reflective power in regard to light," and that "there is no perfectly pure and transparent black pigment." And the same author regards white light as colorless. These latter quotations are not made to emphasize their obvious inaccuracies, but to further illustrate the absence of anything like a unanimity of opinion regarding the classification of black and white in the chromatic scale. Many other opinions might be quoted, showing not only an indecision on the particular point herein discussed, but also widely different ideas concerning the nature of black and white.
In endeavoring to answer the question propounded we can do little more than test the propriety of restricting the application of the word color to less than the entire range of visual impressions. It will be necessary first to inquire just what relation black and white have to other retinal impressions.
At the outset it should be noted that we have no retinal standard. An object may convey a color impression which varies in the same individual with the conditions of rest or fatigue of the eye, with the character of the prevailing illumination, and also according to the influence of neighboring bodies which may produce effects of contrast. There is often a temporary or permanent difference in the color-perception of the two eyes of the same person; and among persons there are of course still wider differences, even excluding abnormal eyes from the comparison. It is easy to mix a paint which will be called black by one and gray by another, and with a little less illumination the most sensitive eye might detect no gray whatever in the mixture; and even among a number of pigments, all of which would be classed as undoubtedly black, one may by comparison see differences and be able to select some which are "blacker" than the rest. Crumple a piece of white paper, and it exhibits lights and shades of greatly different degrees, some of the shades perhaps being deep enough to be designated black, and all intermediate shades may exist, but any two persons would not be likely to agree upon exactly at what particular shade should be drawn the line between gray and black. The retinal impression, therefore, under ordinary circumstances is not a reliable guide to the classification of the cause which produces it.
Consider, then, how we get impressions of color from objects. The sun emits waves of light varying in length by infinitesimal gradations between the extreme red and the extreme violet of the solar spectrum. As far as our purpose is concerned, we may disregard the ultra-violet and ultra-red rays, which are without perceptible effect upon the retina. These luminous or visible rays, acting together, produce in the eye the impression of white; separately, the longest waves produce red; those a little shorter, orange, and so on to the shortest, which produce violet. Aubert calculated that there were at least one thousand distinguishable primary color-impressions to be obtained from the solar spectrum. These rays of various lengths falling upon the things about us are partly absorbed, partly reflected, the latter portion producing in the eye sensations of color. Nearly all of our color-sensations are produced by this "selective reflection," and it will be unnecessary here to consider the other causes of color-production. Reference will be made, however, to subjective color-impressions further on. Now it is very rare indeed, or never, that but one kind or length of waves is reflected by a pigment or surface; usually several kinds are present, and even surfaces having apparently a pure color not uncommonly reflect rays differing considerably in wavelength from those of the predominant kind. Or, to put it another way, the rays from a surface having a definite hue may find their representatives in the solar spectrum not only in the portion corresponding to that particular hue, but also in one or more remote parts of the spectrum. For instance, the light from green leaves contains not only, in predominance, green rays, but some red rays and some violet rays, which find their representatives in the middle and each end of the spectrum respectively. It is true, further, that almost every hue in nature or art is made up not only of several kinds of rays, but of all kinds found in the spectrum; that is, some white light is almost always present in that which we receive from illuminated surfaces.
Now, if blackness were the complete absence of light, the question as far as it is concerned would be much simplified; but I shall endeavor to show that black is not a negative impression. All black pigments and materials reflect light, and many of them to an extent which makes the fact readily demonstrable. Compare under a bright illumination half a dozen black things to be found in any home—cloths, book-covers, etc.—and it will be seen on a more or less close examination that they are not identical in appearance. Color-makers have their blacks of various intensities and shades. One of the commonest of blacks, lampblack, in comparison with some others, appears a very obvious gray. These black surfaces and pigments can not all be devoid of reflecting power, as they would then be incapable of making any impression upon the retina, and the differences must therefore be due to the various amounts or kinds of light which they reflect. Moreover, light reflected by black pigments is white light; that is, they reflect all the different kinds of rays in sunlight. Professor Rood ("Text-Book of Color") found that the black pigments used in his experiments reflected from two to six per cent as much white light as white paper (which, itself, reflects about forty per cent of the light falling upon it), the light being the same in kind and quantity as that from white paper under a sufficiently feeble illumination. There are, it is true, small differences in black pigments in power of reflecting the various components of white light. Blue may be slightly in excess of the normal proportion in white light, and so on, but these are so trifling that they do not affect the question before us. A black pigment with no reflecting power seems to be unknown, and is probably an impossibility. And it is by no means certain that absolute darkness should be taken as a standard of blackness, for several reasons. The impossibility of reaching the standard in practice and of making comparisons in perfect darkness would render it valueless. But the most important objection to it is this: after the retina has ceased to be affected by light, there become manifest certain subjective impressions, perhaps caused by circulation of the blood in the retina, which are not at all suggestive of black; in fact, a very black pigment appears to the writer much "blacker" than the darkness of a closet. The influence of contrast, which is of course impossible in perfect darkness, seems to be necessary to the impression of the most intense black. An utter absence of retinal excitement would, of course, be no sensation at all, and would be of no more use as a standard of blackness than is the blind spot of the eye, of which we are unconscious until we find by experiment that it is capable of intercepting a retinal image. It seems legitimate, therefore, that black, which, as far as we know it, is but a feeble white, should be classed with other sensations produced by light.
Inasmuch as black is nothing more than white very greatly reduced in intensity, if we can show that white is entitled to rank as a color, evidently black also should be similarly ranked. But with white the case is somewhat different from that of black, in that we have a recognized standard of white light, viz., the sum of the rays in the solar spectrum. These, as already stated, acting in concert upon the retina, produce the impression which we call white. The fact, however, that white light is composite, affords no reason for placing it without the scale of colors, for as far as the sensation produced by it is concerned it is quite as simple as red or green, and no eye is able to analyze it into its components. On the contrary, the sensation of white is brought into close relation with many colors because, like them, it may be produced by various mixtures of less than all kinds of rays. According to Rood, the following pairs of spectrum colors when combined produce a white which is indistinguishable from complete sunlight: red and green-blue, orange and cyanogen-blue, yellow and ultramarine-blue, greenish yellow and violet, green and purple. Groups of three or more kinds of rays may also produce a white, and these white mixtures seem to differ in no essential respect from such other mixtures as yellow and red, which make orange, or red and violet, which make purple. That all of the solar rays produce together white seems to be simply an accident of the retinal constitution; for it is quite conceivable, and consistent with the color theory of Young and Helmholtz, that an eye might be so constituted that the combined effect of the solar rays might be, for instance, blue, while pairs of colors similar to those mentioned might still produce white; and under these circumstances white would probably be called a color and blue would be the standard. Something of this kind does take place under artificial illumination. By gas or oil light, which are both very yellow compared with sunlight, a piece of paper which in sunlight is white would still be looked upon as white, although we know perfectly well that the light it sends to the eye is yellow in hue. The white of daylight appears blue by gaslight. On the other hand, objects which are yellow in daylight we are apt to believe white by gaslight, as they appear of the same hue as white paper seen under the latter light. These illusions are explained by the fact that the prevalent illumination is always regarded as white, no matter what hue it has referred to the standard of sunlight. White paper reflects equally well all the rays falling upon it, and, artificial light having an excess of yellow rays, the paper is really yellow in that light; a yellow object has the property of reflecting principally yellow light, which it exercises in gaslight the same as in sunlight, and it also is yellow under the former illumination; the white paper and yellow object, therefore, appear of the same hue; but knowing the paper to be white, and through an error of judgment accepting the prevailing illumination as white, the yellow object appears of this color. In view of these facts, it would seem that deductions drawn from the composition of white light are in favor of making it one of the colors. As supporting this view of the matter, might be mentioned Langley's investigations which have shown that the true color of sunlight, before some of its constituents have been filtered out by the atmosphere, is decidedly blue; and that, according to Brücke, ordinary daylight is slightly reddish in tint. It might be claimed as a reason for excluding white from the color series that it has no representative in the solar spectrum, but there is equal reason for excluding purple, unquestionably a color, which has no type in any part of the spectrum, being produced only by a mixture of rays from the red and violet portions of the spectrum. And it has been proved by several observers that all of the spectrum colors when increased in intensity tend toward white, and if made dazzling actually become white. Accepting this fact in a liberal sense, it is plain that white has a representative in every part of the spectrum; and this tendency toward white with increasing illumination being also a property of black, we have a direct argument for the inclusion of the latter with the colors. In conclusion, it may be urged that the adoption of white and black into the chromatic scale is desirable for the sake of simplicity and uniformity in the nomenclature of this subject.