726 TEXT-BOOK OF PHYSIOLOGY 



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. 

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

 stance, 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-sensations 

 arise. If a light mainly excites the red- or green- or blue-sensitive substance 

 of a retinal area, we term it red, green, or blue, respectively. But if two of 

 these photo-chemical substances are stimulated simultaneously, quite differ- 

 ent sensations arise. Thus simultaneous 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. 

 Simultaneous stimulation of all three substances of a certain area produces 

 the sensation of white. According to this theory, complementary colors 

 are any two which together excite all 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 underlying 

 assumption that the process of restitution in a nerve-element is capable of 

 exciting a sensation. This theory asserts that there are three visual sub- 

 stances 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 by any other simple or mixed color, 

 produces the sensation of white, while the process of restitution or assimila- 

 tion in this substance produces the sensation of black. Similarly, red light 

 produces dissimilation or decomposition in the red-green substance, and 

 this, again, the sensation of red. Green light, however, favors the process 

 of restitution or assimilation in the red-green substances, and thus gives rise 

 to the sensation of green. In the same way the sensation of yellow has its 

 cfruse in the decomposition of yellow-blue substance induced by yellow light, 

 while the sensation of blue is produced by an assimilative process in the 

 same substance. Simultaneous processes of dissimilation and assimila- 

 tion in the same visual substance antagonize each other, and consequently 

 produce no color-sensation by means of this substance, but only the 

 sensation of white, by reason of decomposition, by both colors, in the 

 white-black substance. Thus, yellow and blue, impinging on the same 

 retinal area, have no effect on the yellow-blue substance, because they are 

 antagonistic in their action on this substance, but produce only the sensation 

 of white, as both yellow and blue decompose the white-black material. 

 Color-blindness is explained by the assumption of the absence of either the 

 red-green or the yellow-blue visual substance in the retina. 



