6O COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



easy to determine wherein any given combination of gelatines was de- 

 fective, but to add or subtract the sheets which would just neutralize the 

 objectionable tone of yellow or of blue required an infinity of patience. 



The desired result was, however, finally obtained. A spectroscopic 

 examination showed that the successful combinations of gelatine trans- 

 mitted spectral bands bounded by the following wave-lengths : 



Yellow 551 ujuto 587 JIM- 



Green 483 to 500 



Blue 448 to 474 



The red stimulus transmitted no part of the visible spectrum. 



The equating of the white-values was accomplished by comparing 

 the brightness of the gray sensation aroused by each upon the peri- 

 pheral retina. A preliminary comparison showed the blue stimulus to 

 have the least white-value of the four. It was therefore determined to 

 reduce the white-value of each of the others to equality with that of the 

 blue. A small motor was attached to the stimulus-lantern, and an 

 episcotister was mounted upon it. Then the two lanterns were de- 

 tached from the perimeter and fastened, the one close above the other, 

 in a frame. It had been determined previously that the two lanterns 

 emitted light of equal brightness. The combination of gelatines which 

 was to furnish the stable blue stimulus was now placed in the front of 

 the fixation-lantern (the small fixation aperture had been replaced by 

 an aperture 15 mm. in diameter), while one of the other combinations 

 was inserted behind the episcotister in the adjacent stimulus-lantern. 

 A point was fixated by the observer, and the frame containing the two 

 lanterns was moved out from his visual axis until both colors appeared 

 gray. Then the setting of the episcotister was shifted until the two gray 

 lights seemed to possess equal brightness. This experiment was re- 

 peated several times, and determinations were made in both an upward 

 and a downward direction to insure accuracy of results ; similar deter- 

 minations were made for the other two stimuli. The average of each 

 set of determinations was taken and recorded for future use. 



The color-values were equated by means of a color disc. This 

 disc was made to fit the motor attached to the lantern. Two windows, 

 of exactly the same width, were cut in the disc at points opposite the 

 circular aperture in the front of the lantern. Then the combination of 

 gelatines which gave the stable red was fastened across one of these 

 windows, while the complementary combination which gave the stable 

 green covered the other window. The rotation of the disc before the 

 aperture in the lantern gave a mixture of the red and the green stimuli ; 



