182 ANNUAL OF SCIENTIFIC DISCOVERY. 



of painters who constantly produce green by a mixture of blue and 

 yellow, but the contradiction disappears when we examine the manner 

 in which the effects of the mixtures of colored materials are produced. 

 The coloring materials are generally transparent or more or less trans- 

 lucent. If a pencil of light falls upon a fragment of one of these ma- 

 terials it is partially reflected by the surface without color, and another 

 portion of the light penetrates to the interior of the body, is colored 

 by absorption, and returns to the eye reflected by the second surface, 

 carrying what we consider as the proper color of the substance. If 

 we reduce the coloring matter to a powder, we increase evidently the 

 proportion of the light which returns to the eye, after it has been col- 

 ored by absorption, for the light reflected from the first stratum of 

 powder is added to that of the second, the third, &c., and that which is 

 most modified by absorption which comes from the deepest layer. If 

 a mixture is made of two coloring powders, the light which the mix- 

 ture conveys back to the eye, is mostly composed of rays which have 

 traversed the grains of both kinds of the powder, and which therefore 

 has suffered two different absorptions, and yields the tint of those rays 

 which are absorbed in the least proportion by these two substances. 

 When, for example, wemix a yellow powder with a blue, the yellow 

 grains arrest the blue and violet rays, while they diminish but little 

 the green rays ; the blue grains arrest the yellow orange and red, but 

 permit the greater portion of the green rays to pass ; the tint of the 

 mixture .appears therefore green. Again, when a red powder is mix- 

 ed with a blue powder, a different tint results from that which arises 

 from the prismatic blue and red. Cinnabar and ultramarine give by 

 mixture a violet gray in lieu of the red, which results from the combi- 

 nation of the two analogous prismatic tints. In fine, it results from 

 this explanation, that the tint produced from a mixture of two powders 

 is generally more sombre than that of the powders themselves, a fact 

 which accords with experience. 



When we combine the colors reflected by colored bodies, we obtain 

 the same results as with prismatic colors. For example, if we cover 

 the two sectors of a circular disc with different colors, and give it a 

 rapid rotation, we will obtain with chrome yellow and ultramarine, an 

 almost colorless pale gray. We know on the contrary, that the mix- 

 ture of these two colors is daily employed by the painters to make 

 green. The same results are obtained more neatly in the following 

 manner : If we place upon a black table two colored discs, and place 

 vertically between the two, a glass with parallel faces, so as to view 

 one by transmitted and the other by reflected light, by giving to the 

 eye and to the two discs a suitable position, we may render the two 

 images equally or unequally intense, and cause them to be superposed ; 

 in this manner the colors of the two discs may be combined at the bot- 

 tom of the eye, and the result of the combination is always similar to 

 that of the prismatic colors ; thus the yellow of chrome and gamboge 

 combined in this manner with the azure blue of cobalt, gives a perfect 

 white : with artificial ultramarine, a reddish white ; with Prussian blue, 

 a white, slightly greenish. The vermillion gives a rose-tint with blue, 



