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
