248 THE HUMAN BODY 



The Hering Theory. This theory frankly makes no attempt to 

 accord with the doctrine of specific nerve energies, but seeks 

 rather to explain on a rational basis those visual phenomena 

 which the Young-Helmholtz theory explains poorly or not at all. 

 It is based upon the observation that whereas we recognize certain 

 colors as being combinations of two others, as bluish-green, or 

 reddish-yellow, there are no colors which we recognize as com- 

 binations of complementary colors ; greenish-red or yellowish-blue 

 do not occur. The existence of these mutually exclusive colors 

 suggested to the author of this theory that there might be two 

 opposing processes going on in the retina, one a process of chemical 

 breaking down, or disassimilation; the other a process of building 

 up, or assimilation. 



He therefore postulated three photochemical substances, a 

 white-black substance, a yellow-blue substance, and a red-green 

 substance. He supposed that white light falling on the retina 

 breaks down the white-black substance and gives rise to the 

 sensation of white ; whenever no white light is falling on the retina 

 this substance is building itself up; this gives rise to the sensation 

 of black. Similarly the sensation of red is the result of breaking 

 down the red-green substance, and green of its assimilation. The 

 white sensation resulting from stimulation of complementary colors 

 is explained as due to neutralization of opposing effects. When red 

 and green light come together into the retina the red-green sub- 

 stance is neither broken down nor built up. Both red and green 

 light have a disassimilatory effect on the white-black substance as 

 do rays of all colors, according to the theory. The only effect, 

 therefore, of the complementary colors, is to produce a sensation 

 of white. Contrast is explained as due to the maintenance of a 

 sort of chemical balance in the retina whereby a breaking down of 

 one of the elements in part of it is accompanied by building up of 

 the same element in neighboring areas. So, if the yellow-blue 

 substance is being broken down in part of the retina by yellow 

 light, and built up in adjoining part by blue light, at the border 

 between them each process is heightened by the near presence of 

 the other. 



The theory explains very well, also, the facts of negative after- 

 images, of color blindness, and of the distribution of color vision 

 in the retina. The chief criticism that has been offered against it, 



