6 4 6 



THE POPULAR SCIENCE MONTHLY. 



ing the color under examination can be located by their aid. After- 

 ward, 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. Fac-Simile of Rutherford's Drawing of Sis-Prism Spectroscope. {American Jour- 

 nal 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 pro- 

 ceeding 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- 



