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THE SENSES 



Hering's Theory of Color Vision. This theory is based on the assump- 

 tion that there are chemical substances in the retina, photogenic substances, 

 which are stimulated by the colors of the spectrum. It assumes three photo- 

 genic substances which are called the red-green, the yellow-blue, and the 



FIG. 474. Diagram to Illustrate the Stimulating Effects of the Three Primary Colors. 

 (Young-Helmholtz theory.) i is the red; 2, green, and 3, violet, primary color sensations. 

 The lettering indicates the colors of the spectrum. The diagram indicates by the height 

 of the curve to what extent the several primary sensations of color are excited by vibrations 

 of different wave lengths. (Helmholtz.) 



white-black substances. By the theory, when the red-green substance is 

 stimulated by red or green light, respectively, the former produces destruc- 

 tive or katabolic changes, the latter constructive or anabolic changes in the 

 substance. When red light falls upon the retina, it produces katabolism in 

 the red-green substance, which in turn develops a nerve impulse that arouses 

 rg w y.b 



FIG. 475. Diagram to Illustrate the Reactions of the Three Photogenic Substances, 

 according to Hering's Theory. (Foster.) 



the sensation of red. When green light, on the other hand, stimulates the 

 retina, it produces anabolism of the red-green substance and the sensation of 

 green. The same rule holds with the other two substances. It will be 

 noticed that this theory is based on the complemental colors. 



