COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



employed. But a given lens extends the limits of different color zones 

 by unequal amounts; it has the greatest influence in the case of blue, 

 less for red, and least for green. He points out that this order is the 

 same as the order of the relative extension of the three zones upon the 

 retina of the naked eye, and expresses his results in the general state- 

 ment that the amount of refractive correction which is required to 

 equalize the extension of the three zones is inversely proportional to 

 their normal extension. 



Albini reports that he has established the existence of a progressive 

 error of refraction for light which falls more and more obliquely upon 

 the eye, and concludes that the lesser color-sensitivity of the peripheral 

 retina is due chiefly to this error of refraction. He also mentions the 

 inferior training of the extra-foveal regions as a subsidiary principle of 

 explanation. He is convinced that his results do not support the view 

 that sensations of color and of brightness are furnished by different 

 sets of retinal apparatus. 



The work of Hess* is in many respects a continuation and a refine- 

 ment of Bull's investigation. Hess employed colored papers and worked 

 with light-adaptation. He found that all colors between yellow and red, 

 and between yellow and green, gradually lose their red or green tints 

 and appear successively yellowish, yellow, and gray, when their images 

 fall upon more and more peripheral parts of the retina; while under 

 similar circumstances all colors lying between blue and green and 

 between blue and red (in the closed spectrum) gradually become blue 

 and finally gray. From these observations it was but natural to expect 

 that certain tones of red, yellow, green, and blue could be found which 

 undergo no change of tone in indirect vision. Hess succeeded in build- 

 ing up these " unveranderliche"^ tones, and it was with these stimuli 

 that the remaining part of his investigation was chiefly concerned. The 

 chromatic absorption of the macula modifies the appearance of colors 

 seen in direct vision. For this reason, Hess confined his experiments 

 to the exploration of the paracentral and peripheral parts of the retina. 



Before proceeding to his more detailed investigation, Hess re- 

 determined the stable colors, by means of spectral light. He found the 

 stable green to be 495 ////, yellow 574.5 /*/*, and blue 471 /*/*. None 

 of the spectral reds proved to be a stable tone ; but from an appropriate 



*Carl He&s. Ueber den Farbensinn bei indirectem Sehen, Graefe's Archiv., 

 XXXV, 4, 1889, S. 1-62. 



f We shall refer to these as " stable " colors, a term which here signifies those 

 color-stimuli which experience no change of tone in passing across the retina. 



