ing the color under examination can be located by their aid. Afterward, if it is considered desirable, white light can be added to the spectral tint, till it is subdued sufficiently to render exact comparison possible.
Fig. 5.—Facsimile of Rutherford's Drawing of Six-Prism Spectroscope.—{American Journal of Sciences and Arts, 1865.)
The experimental determination of the color-constants is beset with a considerable amount of difficulty, even in the simplest cases, such as cardboards covered with pigments. The best mode of proceeding appears to be to call the luminosity of white cardboard 100, and then to determine photometrically the comparative luminosity of the colored cardboards. The measurement of the amount of white light reflected along with the colored is still more troublesome, and the result likely to be somewhat less exact, while the determination of the tone, or third constant, is moderately easy under favorable circumstances. One of the uses of such determinations is the pro-
