634 TEXT-BOOK OF PHYSIOLOGY. 



only to the most sensitive part of the retina. At the more peripheral 

 parts of the retina the colors are seen somewhat differently, as is 

 shown by the following table giving the limits up to which the colors 

 are recognized : 



White. Blue. Red. Green. 



Externally 90 80 65 50 



Internally 60 55 50 40 



Superiorly 45 4 35 3 



Inferiorly 70 60 45 35 



Theories of Color-perception. The theory of v. Helmholtz, 

 originated by Thomas Young (1807), assumes in its latest form the 

 existence in the human retina of three different kinds of end-organs, 

 each of which is loaded with its own photo-chemical substance 

 capable of being decomposed by a certain color, and thus exciting 

 the fiber of the optic nerve. 



In the first group these end-organs are loaded with a red-sensitive 

 substance, which is affected mainly by the red part of the spectrum; 

 the second group has its end-organs provided with a green-sensitive 

 substance, which is mainly excited by the green color; while the third 

 group is provided with a blue-sensitive substance, this latter being 

 mainly affected and decomposed by the blue-violet portion of the 

 spectrum. All these three different end-organs are present in every 

 part of the most sensitive area of the retina, and are connected by 

 separate nerve-fibers with special parts of the brain, in the cells of 

 which each calls up its separate sensation of red or green or blue. 



Out of these three primary color-sensations all other color-sensa- 

 tions arise. If a light mainly excites the red- or green- or blue-sensi- 

 tive substance of a retinal area, we term it red, green, or blue, re- 

 spectively. But if two of these photo-chemical substances are stimu- 

 lated simultaneously, quite different sensations arise. Thus simul- 

 taneous stimulation of the red and green substances gives rise to the 

 sensation of yellow, that of red and blue to the sensation of purple, 

 and that of blue and green to the sensation of blue-green. Simul- 

 taneous stimulation of all three substances of a certain area produces 

 the sensation of white. According to this theory, complementary 

 colors are all those which together excite all the three substances. 

 Color-blindness is explained by this theory, on the assumption that 

 two of the photo- chemical substances have become similar or equal 

 in composition to each other. 



The theory of Hering, brought forward in 1874, has the under- 

 lying assumption that the process of restitution in a nerve- element 

 is capable of exciting a sensation. This theory asserts, that there are 

 three visual substances in the retina a white-black, a red-green, and 

 a yellow-blue visual substance. A destructive process in the white- 

 black substance, such as is induced not only by white light, but also 



