THE CONSTANTS OF COLOR. 



645 



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 ob- 



/Md&ti 



Fig. 3. Colored Disk with Small Black-and-white 

 Disk. 



Fig. 4. Colored Disk with 

 Small Black-and-White 

 Disk in Rotation. 



server 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 ordei'ly 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 con- 

 veniently 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- 



