7O COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



that the photo-chemical substrates of the three visual processes represent 

 different degrees of complexity of development, and hence presumably 

 also represent different degrees of tenacity of function. The rapid 

 adaptation and the persistent after-effects of the peripheral retina may 

 be traced to an ad hoc hypothesis of a peculiar degree of chemical sta- 

 bility of partially dissociated molecular groups. The seeming contra- 

 diction between the presence of a bluish tinge in the stable red and the 

 stable green and the absence of that tinge from the specific sensations 

 furnished by the erythrogenic and chlorogenic substances, disappears 

 when we remember the influence of the yellow pigmentation of the 

 macula.* 



(e) The Hering theory assumes the existence of three sorts of 

 visual substance, each of which has its own retinal distribution, and its 

 own specific function. The whole retinal surface is supplied with all 

 three substances, but their distribution is such that any given region 

 contains a richest supply of white-black substance, and a scantest 

 amount of red-green substance. These hypotheses adapt themselves to 

 the coincidence of the zonal limits of the pairs of colors, because the 

 substance which is active in the sensing of a color is also supposed to 

 mediate the sensation of its complementary. The lesser extension of the 

 red-green zone is referred to the relatively circumscribed distribution of 

 the red-green substance. In common with all other theories it must 

 trace the variability of zonal extension to the fact that the variation of 

 a stimulus of constant tone, produces a variation of intensity of physi- 

 ological action. The fading out of the peripheral colors through yellow 

 or blue to gray is accounted for by the assumption that the retina con- 

 tains a richer supply of white-black substance than of yellow-blue, and 

 of yellow-blue than of red-green. The relatively rapid adaptation of 

 the peripheral retina is referred to its relatively scant supply of chro- 

 matic substance a hypothesis which also accounts for the lesser satura- 

 tion of colors seen in indirect vision. The persistent duration of the 

 after-effects of peripheral stimulation is expressed physiologically in 

 the statement that the restoration of chromatic equilibrium is more 

 sluggish upon peripheral than upon central regions. Whether this 

 physiological characteristic can be brought into relation with the rela- 

 tively undeveloped state of the peripheral retina is not clear. 



(3) The third view occupies a mediating position between the first 

 and the second. It has never attracted a following in the literature, nor 



*Mrs. C. Ladd-Franklin. Eine neue Theorie der Lichtempfindung. Zeit- 

 schrift fur Psychologic und Physiologic der Sinnesorgane, IV, 1893, S. 211-241; 

 On Theories of Light Sensation, Mind, N. S., II, 1894, pp. 473-489; Baldwin's 

 Dictionary of Philosophy and Psychology, II, 1902, pp. 792ff. 



