ABE BLACK AND WHITE COLORS? 817 



are unconscious until we find by experiment that it is capable of in- 

 tercepting 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 re- 

 duced 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 pro- 

 duced 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 be- 

 cause, like them, it may be produced by various mixtures of less than 

 all kinds of rays. According to Rood, the following pairs of spec- 

 trum colors when combined produce a white which is indistinguish- 

 able from complete sunlight : red and green-blue, orange and cyano- 

 gen-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 essen- 

 tial 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 prob- 

 ably 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 be- 

 lieve 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 yel- 

 low light, which it exercises in gaslight the same as in sunlight, and 

 vol. xxix. 52 



