ARE BLACK AND WHITE COLORS? 815 



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 illumina- 

 tion 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 differ- 

 ences 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 pro- 

 duces 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 pro- 

 duce 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-produc- 

 tion. Reference will be made, however, to subjective color-impres- 

 sions 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 wave- 

 length from those of the predominant kind. Or, to put it another 

 way, the rays from a surface having a definite hue may find their rep- 

 resentatives in the solar spectrum not only in the portion correspond- 

 ing 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 



