from the vermilion consists, for example, of eighty per cent, of red light from such a region of the spectrum, plus twenty per cent, of white light. If we set the amount of light reflected by 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 reminiscence of the original hue remains; we say the tint is greenish-gray,
Fig. 2.—Spectrum crossed by Band of 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
