COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 6l 



and the mixture contained exactly equal proportions of each component, 

 since the two windows were of the same width. The process of equating 

 the color-values of these two stimuli consisted in changing the com- 

 binations of gelatines in the windows by adding, subtracting, or sub- 

 stituting until the mixture showed no trace of color. When this result 

 had been attained, when the mixture of the two stimuli in equal propor- 

 tions gave a colorless gray, their color-values could be said to be 

 equivalent. 



The processes of equating the color-values and the white-values 

 were, as a matter of fact, carried on simultaneously and not successively, 

 as we have described them. For it is evident that if one equalization 

 had been completed before the other was undertaken, then any con- 

 siderable change in the composition of either stimulus during the second 

 process of equating would destroy the equation already obtained. The 

 possibility of ever accomplishing the equalizations is realized only by the 

 presence of a great variety of color tones, and of saturations in the 

 gelatines to which one has access. 



B. 'METHOD. 



The method employed in these experiments was analogous to that 

 already described. The observer's retina was adapted to darkness for 

 fifteen minutes before each sitting began. The series opened with the 

 stimulus-lantern at or near the outer end of the quadrant. It was moved 

 in, ten degrees at a time, until the color of the stimulus was perceived. 

 At the next sitting the innermost point at which gray had been reported 

 was the starting point for the more accurate determination of the color 

 limits. The amount of shifting of the lantern after each exposure was 

 now one degree. The series was continued until color was perceived; 

 and the point at which it was first recognized was recorded as the outer- 

 most limit of sensitivity to the stimulus in question. 



C. RESULTS. 



Eight half-meridians of the retina were explored in the manner 

 shown in the preceding paragraph. The results show that the zone of 

 stable red is co-extensive with that of stable green; that the zone of 

 stable yellow is co-extensive with that of stable blue ; that the yellow- 

 blue zone is much more widely extended in all directions that is the red- 

 green zone ; that the nasal side of the retina has the widest extension of 

 color sensitivity, and that there is a wide individual variation in zonal 

 extension. 



