be altered; but the black-and-white will blend into a gray. This gray can be altered in its brightness till it seems about as luminous as the red. If we find, for example, that with the disk three-quarters black and one-quarter white an equality appears to be established, we conclude that the luminosity of our red surface is twenty-five per cent, of that of white paper. This is of course based on the supposition that the black paper reflects no light; it actually reflects from two to five per cent., the reflecting power of white paper being put at 100. The results thus obtained are always inexact, and the same observer
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| Fig. 3.—Colored Disk with Small Black-and-white Disk. | Fig. 4.—Colored Disk with Small Black-and-White Disk in Rotation. |
will often obtain different results on different days, though those of a single day may agree pretty well among themselves. In the appendix to this chapter, a peculiar photometer will be described, which has been contrived by the author for the purpose of comparing more accurately together the relative luminosity of different colored surfaces, or that of colored and white surfaces.
But to resume our search for color-constants. We may meet with two portions of colored light, having the same degree of purity and the same apparent brightness, which nevertheless appear to the eye totally different; one may excite the sensation of blue, the other that of red; we say the tones are entirely different. The tone of the color is, then, our third and last constant, or, as the physicist would say, the degree of refrangibility, or the wave-length of the light. It has in a previous chapter been shown that the spectrum offers all possible tones except the purples, well arranged in an orderly series; and the purples themselves can be produced with some trouble, by causing the blue or violet of the spectrum to mingle in certain proportions with the red. Rutherford's automatic six-prism spectroscope can very conveniently be employed for the determination of the tone. (See Fig. 5.) A peculiar eye-piece is to be used, which isolates a little slice of the spectrum in its upper half, as indicated in Fig. 6. In the lower half of the field the fixed lines are seen, and the tone selected as match-
