PROPERTIES OF THE RETINA. 333 



that each photochemical substance serves simply as a means for the 

 excitation of a nerve fiber, and that the quality of the sensation 

 aroused depends on the ending of this fiber in the brain. The phe- 

 nomenon of negative after-images finds a simple explanation in terms 

 of this theory. If we look fixedly at a green object, for example, 

 the corresponding photochemical substance is chiefly acted upon, and 

 if subsequently the same part of the retina is exposed to white light, 

 the red and violet substances, having been previously less acted 

 upon, now respond in greater proportions to the white light, and 

 the after-image takes a red- violet that is, purple color. Many 

 objections have been raised to the Young- Helmholtz theory. It 

 has been urged, for instance, that we are not conscious that white 

 or yellow sensations are blends or compounded color sensations; 

 we perceive in them none of the supposed component elements as 

 we do in such undoubted mixtures as the blue-greens or the purples. 

 The theory explains poorly or not at all the fact that on the periphery 

 of the retina we are color blind and yet can perceive white or gray, 

 and it breaks down also in the face of the facts of partial and com- 

 plete color blindness. The explanation given for black is also 

 unsatisfactory in that it assumes an active state of consciousness 

 associated with a condition of rest in the visual mechanism. 



77. Hering's Theory of Color Vision. This theory also assumes 

 the existence in the retina of three photochemical substances, but 

 of such a nature as to give us six different qualities of sensation. 

 There is a white-black substance which when acted upon by the 

 visible rays of light undergoes disassimilation and sets up nerve 

 impulses that arouse in the brain the sensation of white. On the 

 other hand, when not acted upon by light this same substance under- 

 goes assimilatory processes that in turn set up nerve impulses which 

 in the brain give us a sensation of black. There are in the retina also 

 a red-green and a yellow-blue substance. The former when acted 

 upon by the longer rays undergoes disassimilation and gives a 

 sensation of red, while the shorter waves cause assimilation and 

 produce a sensation of green. A similar assumption is made for 

 the yellow-blue substance. The essence of the theory may be stated, 

 therefore, in tabular form, as follows *: 



Photochemical Substance. Retinal Process. Sensation. 



P _. f Disassimilation = red 



Keel-green <j Assimilation = green 



,. . . f Disassimilation = yellow 



Yellow-blue { Assimilation - blue 



, Tri ., , . , f Disassimilation = white 



White-black . | Assimi i at ion = black 



* For discussion of color theories see Calkins, "Archiv f. Physiologie," 

 1902, suppl. volume, p. 244. 



