12 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



meridian until its color had wholly disappeared. Eight meridians were 

 explored by means of each stimulus. His results may be summarized 

 as follows : 



(1) The brightness of the background has a most pronounced 

 influence upon the extension both of the color sensitivity and of the 

 brightness sensitivity. Red upon the white ground becomes colorless at 

 1 6 ; upon the black ground at 30. Blue upon white becomes colorless 

 at 13 ; blue upon black at 37. Beyond the limits of the color zones, all 

 colors appear white when the ground is black, black when the ground 

 is white. 



(2) The extension of the color zone increases with increase of 

 area of stimulus. The former increase does not keep pace with the 

 latter, but progresses much more slowly, as appears from the following 

 table. The area of the stimulus-object is expressed in square milli- 

 meters ; the width of the color zone is given in degrees. The squares 

 were exposed at a distance of 20 cm. from the eye. 



(3) The color sensitivity decreases at very different rates upon 

 different retinal meridians. It diminishes much more slowly, or extends 

 much farther out, on the nasal side of the retina. 



(4) The transitions of color tone are as follows: Red passes 

 through reddish-yellow and yellowish-gray to gray ; green becomes yel- 

 lowish, while yellow and blue undergo no change of tone, but decrease 

 in saturation and finally appear gray. 



(5) The relative extension of the different color zones can not be 

 determined with any degree of accuracy. Since the width of the color 

 zone is a function of the luminosity of the stimulus, the color-stimuli 

 employed in the determination of comparative retinal limits must all be 

 equated in brightness. In the opinion of Aubert, the comparison of the 

 relative brightness of stimuli of different colors is attended by such 

 difficulties as to render its accurate accomplishment impossible. In his 

 own experiments, with non-equated stimuli upon a black background, 

 blue had the widest extension, red and yellow less, and green least of 



