THE 



POPULAR SCIENCE 

 MONTHLY. 



OCTOBER, 1876. 



THE CONSTANTS OF COLOR 



By Prof. O. N. ROOD, 



OF COLUMBIA COLLEGE. 



rr^HE tints produced by Nature and art are so manifold, often so 

 J- vague and indefinite, so affected by their environment, or by 

 the illumination under which they are seen, that at first it might well 

 appear as though nothing about them were constant ; as though they 

 had no fixed properties which could be used in reducing them to 

 order, and in arranging in a simple but vast series the immense mul- 

 titude of which they consist. 



Let us examine the matter more closely. We have seen that when 

 a single set of waves acts on the eye a color-sensation is produced, 

 which is perfectly well defined, and which can be indicated with pre- 

 cision by referring it to some portion of the spectrum. We have 

 also found that, when waves of light having all possible lengths act 

 on the eye simultaneously, the sensation of white is produced. Let 

 us suppose that by the first method a definite color-sensation is gener- 

 ated, and afterward by the second method the sensation of white is 

 added to it : white light is added to or mixed with colored light. 

 This mixture may be accomplished with an ordinary spectroscope, by 

 removing the scale from the scale-telescope, and replacing it by a 

 vertical slit, as indicated in Fig. 1, which is a view from above. Then, 

 if white lio-ht be allowed to enter this slit, it will be reflected from 

 the surface of the prism into the observing-telescope, and we shall 

 find that the spectrum is crossed by a vertical band of white light. 

 By moving with the hand the scale-telescope, this white band may be 

 made to travel slowly over the whole spectrum, and furnish us with 

 a series of mixtures of white light with the various prismatic tints. 

 (See Fig. 2.) The general effect of this proceeding will be to diminish 

 the action of the colored light ; the resultant light will indeed pre- 



TOL. IX. 41 



