9 o THE POPULAR SCIENCE MONTHLY. 



In accordance with this theory, there are, therefore, four fundamental 

 colors instead of three (excluding black and white), namely : red, 

 green, yellow and blue, and they are supposed to be produced as fol- 

 lows : Red is the product of the dissimilation of the red-green sub- 

 stance, green is the result of its assimilation ; blue is the result of the 

 dissimilation of the blue-yellow, and yellow of its assimilation. , When 

 the A- and D-action on the red-green and blue-vellow substance are 

 equal there is no color sensation, but only the D-action of these colors 

 on the black- white substance, that is white. Simultaneous A- and D- 

 action on the black- white substance, however, is not attended by aboli- 

 tion of sensation, but by the sensation of gray. 



It will be seen from this that, in the Hering theory, what were be- 

 fore considered as complementary colors are antagonistic and tend to 

 neutralize each other. It will be remembered that those colors have 

 been called complementary which, when mixed together, would pro- 

 duce white (we speak now of spectral colors). This was explained by 

 the Young-Helmholtz theory on the principle of combination ; it is 

 accounted for by the Hering theory on the principle of subtraction. 

 When red and green, for instance, form white on being mixed, the 

 white is not produced by the sum of the sensations of red and green, 

 but the red and green, being antagonistic, neutralize each other, and 

 there only remains the D-action of both colors on the black-white sub- 

 stance that is, white. 



As in the Young-Helmholtz theory, the other colors, aside from the 

 primary, are the results of mixed sensations. 



Color-blindness, in accordance with this theory, is of two forms. 

 In one, both color substances are wanting, and there only remains the 

 black-white substance to be acted on by light (achromatopsia). In 

 the other form, one of the two color-substances is lacking and only 

 the two colors of the remaining color-substance are distinguishable 

 (dichromatopsia). If the red-green substance is lacking, there will be 

 red-green blindness or blue-yellow vision ; if the blue-yellow substance 

 is the missing one, there will be blue-yellow blindness, or red-green 

 vision. 



To satisfactorily account for some of the phenomena of color-blind- 

 ness, however, it becomes necessary to suppose that, when one color- 

 substance is wanting, the light rays which act specifically on that sub- 

 stance produce an A- or D-action on the remaining color-substance. In 

 red-green blindness, for example, red, yellow, and green act in a dis- 

 similating manner on the remaining blue-yellow substance, giving rise 

 to the sensation of yellow, while blue alone acts in an assimilating man- 

 ner. The most strongly dissimilating color will be yellow, while the 

 others will be more or less varying in their action. In the case of 

 blue-yellow blindness, red, yellow, and blue are the dissimilating col- 

 ors and green the assimilating color. It will be readily understood, 

 when we have this state of affairs, that in the dichromatrope, where 



