SENSE OF SIGHT. 591 



absorbs all the others. So that when the two are mixed, the only 

 color which is not absorbed by the crayon is green, and therefore 

 the line appears green. 



Nor can the colors which are transmitted through transparent 

 bodies be united to produce white light, or the combination which 

 the mixture of colored lights produces on the retina, any more 

 than can the pigments mentioned above, and the reason for this is 

 obvious. If, for instance, sunlight is transmitted through red 

 glass, all the rays which produce other colors than red are absorbed, 

 and the red only being transmitted, gives the red color to the glass. 

 If the same is done with green glass, only the green rays will be 

 transmitted, all the other rays being absorbed. If now white 

 light is transmitted through red glass, only the red will remain, 

 and if this is transmitted through green glass, no light will come 

 through, for green glass will not transmit the red rays, and the 

 green rays have already been absorbed by the red glass. To pro- 

 duce white light or the various combinations of the spectral colors, 

 the rays must fall upon the retina and the colors be mixed phy- 

 siologically, the mixture producing effects which are interpreted 

 by the brain. 



Young-Helmholtz Theory of Color. Inasmuch as this theory 

 was advanced by both Young and Helmholtz, it bears the name 

 of both. It is based on the view that there are in the retina three 

 substances which are stimulated by the three primary colors, re- 

 spectively, of red, green, and violet, and that when all three fall 

 upon the retina in proper proportion the sensation of white is 

 produced ; and when any two of the three stimulate the retina, 

 the effect is to produce some intermediate color, as, for instance, 

 violet and green produce blue ; red and green, yellow and orange ; 

 red and violet, purple. That such substances actually exist, there 

 is no proof; the term " substance " is used for want of a better 

 one. The term " fiber " is used by Helmholtz, and " red fibers," 

 " green fibers," and " violet fibers'" are spoken of. This theory 

 supposes, also, that each of the primary colors stimulates to some 

 extent all the three substances, but one is stimulated so much 

 more than the others that the effect upon the others is not 

 noticed. Especially marked is this differentiation of the red, 

 green, and violet near the fovea centralis, and when the light is 

 not too intense. If the light falls upon the portions of the retina 

 near the ora serrata, or if that which falls on the retina in the 

 neighborhood of the fovea is of very little or of very great inten- 

 sity, all the rays seem to stimulate the three substances alike, for, 

 under such circumstances, the colors of objects are not readily 

 made out. The theory has been advanced that the power to dis- 

 tinguish colors resides in the cones, and that the stimulation of the 

 rods by light gives the sensation of luminosity without color. Von 

 Kries states that the rods are color-blind, their stimulation re- 



