PROPERTIES OF THE RETINA. 



363 



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. 

 Red-green 



RETINAL PROCESS. 

 f Disassimilation 

 ' \ Assimilation 

 v , ! / Disassimilation 



Yellow-blue \ Assimilation 



f Disassimilation 



White-black, 



\ Assimilation 



SENSATION. 

 red 

 green 

 yellow 

 blue 

 white 

 black 



It will be observed that the theory gives an independent ob- 

 jective cause for the sensations of white, black, and yellow, and in 



rg 



Fig. 154. Schema to illustrate the Hering theory of color vision. (Foster.) The 

 curves indicate the relative intensities of stimulation of the three color substances by dif- 

 ferent parts of the spectrum. Ordinates above the axis, X-X, indicate catabolic changes 

 (disassimilation), those below anabolic changes (assimilation). Curve a represents the 

 conditions for the black-white substance. It is stimulated by all the rays of the visible 

 spectrum with maximum intensity in the yellow. Curve c represents the red-green sub- 

 stance, the longer wave lengths causing disassimilation (red), the shorter ones assimilation 

 (green). Curve b gives the conditions for the yellow-blue substance. 



this respect satisfies the objections made on this score to the Young- 

 Helmholtz theory. It fits better, also, the facts of partial and total 

 color blindness. In the latter condition one may assume, in terms ol 



* For discussion of color theories see Calkins, "Archiv f. Physiologic/' 

 1902, suppl. volume, p. 244; also Greenwood in Hill's "Further Advances 

 in Physiology/^ p. 378, 1909. 



