COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 59 



after-images, but he invariably reported that no visual phenomenon of 

 any sort was present to consciousness, after the cessation of the stimulus. 

 We can only conclude that the functioning of the peripheral retina is 

 followed by an after-effect which is tenaciously persistent and is wholly 

 latent in character ; and that this sub-liminal capacity is called into active 

 functioning by subsequent stimulation. 



Just how extensive, spatially, is the domain of influence of the after- 

 effect is not shown by our experiments. That it is less circumscribed 

 than the retinal image which occasions it seems, however, evident. One 

 may infer that its extension is a function of the intensity and duration 

 of the stimulus, and possibly of other factors as well. A few experi- 

 ments devised for the purpose of determining in a general way the 

 extent of its influence gave results which seem to indicate that with a 

 bright stimulus and a long exposure the after-effect may extend over 

 the whole retinal surface. 



2. THE RELATIVE EXTENSION OF THE DIFFERENT RETINAL ZONES. 



These experiments aimed to determine what extent of retinal sur- 

 face is sensitive to a certain red, to a certain yellow, to a certain green, 

 and to a certain blue. The colors here employed as stimuli were those 

 which undergo no change of tone in indirect vision. 



It is evident from the foregoing experiments (p. 53f.) that a 

 retinal zone is not a fixed but a variable quantity ; that its limits may be 

 varied at will by an appropriate variation of the conditions of experi- 

 mentation. It is therefore essential that an investigation which hopes 

 to yield comparative data regarding the variable limits of the retinal 

 zones should provide means for the accurate control of all of the deter- 

 minant factors, and for the variation of but a single factor at a time 

 during the progress of the experimentation. 



A. APPARATUS. 



The apparatus employed in these experiments was identical with 

 that already described (p. 42ff.), but the stimuli employed in the present 

 case were stable colors ; and they were equated both in white-value and 

 in color-value. The first preliminary to the investigation was the estab- 

 lishing of the stable colors. This was accomplished by passing various 

 color stimuli across the retina, and choosing those which underwent no 

 change of tone. This was a comparatively simple problem in the case 

 of the blue and the yellow, but it proved to be exceedingly difficult to 

 find a red and a green which fulfilled the conditions required. It was 



