698 



THE SENSES 



as shown in figure 474. When red rays fall upon the retina, they stimulate 

 the red-perceiving elements strongly and the green and violet very feebly. 

 The resulting sensation is that of red. So also is it with green and violet rays. 

 When the retina is stimulated by both red and green rays, the two correspond- 



rv FlG ' 474- Diagram to Illustrate the Stimulating Effects of the Three Primary Colors 

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

 1 he 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 bv vibrations 

 of different wave lengths. (Helmholtz.) 



ing color-perceiving elements are strongly stimulated. The resulting color 

 perception, however, is a combination of the two sensations and corresponds 

 to some region of the spectrum between the red and green, according to the 

 relative intensity of the two stimuli. When all three color-perceiving ele- 



B 



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

 according to Hering's Theory. (Foster.) 



ments are stimulated at the same time, the. theory assumes that white light 

 will be perceived. In a similar manner all the various color sensations are 

 arrived at. 



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



