THE CONSTANTS OF COLOR. 



643 



from the vermilion consists, for example, of eighty per cent, of red 

 light from such a region of the spe'ctrum, plus twenty per cent, of 

 white light. If we set the amount of light reflected hy white paper 

 as 100, then a surface painted with " emerald-green " reflects about 

 eight parts of white light; artificial ultramarine, two or three parts ; 

 red lead, seven or eight, etc. Some white light is always present ; 

 its general effect is to soften the color and reduce its action on the 

 eye; when the proportion of white is very large, only a faint reminis- 

 cence of the original hue remains ; we say the tint is greenish-gray, 



Fig. 2. Spectrum crossed by Band op Superimposed White Light. 



bluish-gray, or reddish-gray. The specific effects produced by the 

 mixture of white with colored light will be considered in a future 

 chapter ; it is enough for us at present to have obtained an idea of 

 one of the constants of color, viz., its purity. The same word, it 

 may be observed, is often used by artists in an entirely different 

 sense : they will remark of a painting that it is noticeable for the 

 purity of its color, meaning only that the tints in it have no tendency 

 to look dull or dirty, but not at all implying the absence of white or 

 gray light. 



Next let us suppose that in our study of these matters we have 

 presented to us for examination two colored surfaces, which we find 

 reflect in both oases eight-tenths red light and two-tenths white light. 

 In spite of this the tints may not match, one of them being much 

 brighter than the other, containing, say, twice as much red light and 

 twice as much white light ; having, in other words, twice as great 

 brightness or luminosity. The only mode of causing the tints to 

 match will be to expose the darker-colored surface to a stronger light, 

 or the brighter surface to one that is feebler. It is evident, then, that 

 brightness or luminosity is one of the properties by which we can* 

 define color ; it is our second color-constant. This word luminosity 

 is also often used by artists in an entirely different sense, they calling- 

 color in a painting luminous simply because it recalls to the mind 

 the impression of light, not because it actually reflects much light to 



