4O COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



tures of the Young-Helmholtz theory, and assumes that in direct vision 

 every homogeneous stimulus excites all three sorts of visual substance, 

 but excites each in a characteristically unequal degree. Thus red light 

 stimulates the red-sensing substance most intensively, the green-sensing 

 substance less intensively, and the blue-sensing substance least inten- 

 sively ; the curve which represents the intensity of excitation by red light 

 will, therefore, reach its maximum in the red and will slope off grad- 

 ually towards the blue. Green light stimulates the green-sensing sub- 

 stance most intensively, the red-sensing less, and the blue-sensing still 

 less ; in this case the curve has its crest in the green and slopes off to 

 either side, more abruptly toward the blue. Now, Kick conceives that 

 the relation of the intensities of excitation is wholly different upon the 

 peripheral retina. Here the curves tend to flatten down to straight 

 lines ; all stimuli, of whatever color, here excite the different substances 

 in equal degree. Thus it comes about that a red stimulus applied to 

 the peripheral retina has a physiological effect similar to that produced 

 in direct vision by white light. This assumption certainly makes it 

 comprehensible why changes of color must occur in indirect vision, but 

 that it enables one to understand why the phenomena reported by Bull 

 and others should be observed can scarcely be maintained. Then, too, 

 not only is Kick's explanation forced, but his assumption of a modified 

 function of visual substance does violence to the essential principle of 

 the theory which he advocates and defends. 



Hering conceives that each of the visual substances which his 

 theory assumes has a different retinal distribution. The white-black 

 substance is distributed over the whole retina with approximate uni- 

 formity ; while the other visual substances occur in decreasing quantity 

 as we approach the periphery. Their relative distribution is such that 

 any given region is less richly supplied with red-green than with blue- 

 yellow substance. If now it be assumed that in order to produce a con- 

 stant physiological effect, the stimulation must be increased in propor- 

 tion as the retinal supply of visual substance is relatively scant, the 

 explanation of the phenomena of indirect vision becomes evident. The 

 persistence upon the more eccentric regions of the retina, of gray after 

 the specific color of the stimulus has disappeared, is referred to the 

 presence at the periphery of black-white substance and the almost total 

 absence at that region of color substance. The changes of color tone 

 which occur in indirect vision are the result of a preponderance upon 

 the peripheral retina of blue-yellow over red-green substance. Decrease 

 of saturation is due to a relative increase of functioning upon more and 

 more peripheral regions, of black-white substance. The coincidence of 



