590 THE NERVOUS SYSTEM. 



absorbed, the color of the object will be the sum of the colors 

 that are transmitted. 



We are now prepared to discuss some of the tacts which have 

 been established in connection with the sensation of color. The 

 union of the spectral colors to produce white light may be demon- 

 strated by cutting out disks of colored paper (Fig. 365) and 

 attaching them to a whirling machine (Fig. 366), or complemen- 

 tary colors may be combined in the same way with the same 

 result ; or, again, by using different colors various combinations 

 may be made. This may be called a physiologic mixture of colors, 

 and is thus explained : When the retina is exposed to a color, this 

 produces a certain effect which remains even after the color which 

 produced it has been removed ; if, before the sensation caused by 

 this color ha^ disappeared, the retina is exposed to another color, 

 the second color is superimposed upon the first, and if the two are 

 complementary, the effect is that of white light, as when these 



FIG. 365. Disks of colored paper. FIG. 366. Whirling machine. 



colors are combined by a prism. If the revolving disks contain 

 all the spectral colors in due proportion, and are revolved rapidly 

 enough, so that all the colors produce their effect on the retina 

 before the sensation produced by any one has faded away, the 

 effect of white light is produced, as when these colors are united 

 by a prism. 



Mixing of Pigments. The effects just described as being pro- 

 duced by the physiologic mixture of colors cannot be produced by 

 the mechanical mixture of pigments. Although blue and yellow 

 when mixed upon the retina produce white, yet when blue and 

 yellow pigments are mechanically mixed the resulting color is 

 green and not white. If a broad line is drawn on a blackboard 

 with a yellow crayon, and over this is drawn another line with a 

 blue crayon, the resulting color will be green. This effect is ex- 

 plained in the following manner : The yellow crayon reflects not 

 only yellow light, but also green light, and absorbs all the other 

 colors. The blue crayon reflects not only blue, but also green, and 



