22 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



Bull* made an important contribution to the literature of the topic. 

 In discussing the relative extension of the color zones he had the insight 

 to recognize that the final solution of this much-disputed problem can 

 never be reached unless two conditions are fulfilled by the investigator : 

 He must employ stimuli whose saturations and brightnesses have been 

 equalized ; and he must choose for stimuli such colors as do not change 

 in tone in peripheral vision. Both of these conditions he claims to meet 

 in his own investigation. His "physiologically pure" colors are a 

 purplish red, a bluish green, a yellow, and a blue, f He found that the 

 limits of the purple-red zone coincide with those of the blue-green zone, 

 while those of the yellow and the blue approximately coincide ; the latter 

 pair of zones is considerably wider than the former pair. Here are 

 his results : 



*Ole Bull. Studien ufoer Licht- und Fanbensinn, Graefe's Archiv., XXVII, i, 

 1881, S. 54-154; Bemerkungen iiber den Farbensinn .unter yerschiedenen physiol- 

 ogischen und pathplogischen Vcrhailtnissen, Graefe's Archiv., XXIX, 3, 1883, S. 

 71-116; Sur la periimetrie au moyen de pigments colores, Annales d' Oculist., CX, 

 1893, pp. 169-181; same title, Ann. d'Ocul., CXI, 1894, pp. 284-285; Perimetrie, 

 iv + 218, Bonn., 1895. 



fThe equating of the four stimuli was accomplished .by two methods. Since 

 his stimuli constituted two pairs of complementary colors, the mixture of each 

 pair in certain proportions gave gray. Now, when gray resulted from the mixture 

 of equal proportions of the purplish red and the bluish green, or of yellow and 

 blue, i. e., when 180 of the one just neutralized 180 of its complementary, the 

 components were taken to be equivalent in saturation. The comparison of relative 

 brightness was facilitated by the interpolation of intermediate 'Color tones ; thus 

 while it is difficult to make a subjective estimate of the relative brightness of a 

 green and a blue stimulus, the matter is simplified when we have to deal with a 

 green and an adjacent bluish green. When these two have been equated, the 

 latter may readily be equated with a green which contains still more blue, etc. 

 This method of paired comparison would finally give us a variety of color tones, 

 all of equal brightness. For the sake of control, the four stimuli were subjected 

 to a final test. All four were mounted upon a uniform gray ground and regarded 

 under such conditions with partially closed eyelids that they appeared color- 

 less. Any error which might have crept into the previous comparison of bright- 

 nesses was revealed by this control. 



JGraefe's Archiv., XXVII, I, plates at end of article; Ann. d'Ocul., CX, p. 177. 



