COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 13 



all ; upon a white ground, green had the widest extension, then followed 

 yellow, blue, and red in order. In the face of this conflicting evidence 

 Aubert averaged the values obtained with the different backgrounds, 

 and arrived at the following results: Blue 34, yellow 33, green 

 31, and red 27. From these data he concluded that there is but little 

 difference between the extensions of the various color zones.* 



(6) There is a close analogy between the functioning of the central 

 and peripheral parts of the retina. Colored objects appear colorless in 

 both direct and indirect vision when the visual angle is sufficiently small, 

 when the illumination is sufficiently weak, when the stimulation is 

 sufficiently long-continued, and when certain conditions of contrast 

 between stimulus and background are not fulfilled.! Aubert argues 

 that the only difference between the color sensitivity of different parts 

 of the retina is a difference of degree, the "color sense" of the periphery 

 being less acute. And this lesser sensitivity is to be explained, in part 

 from a different distribution of retinal elements, in part, from their 

 more rapid fatigue. 



Helmholtz** pointed out that the changes of color tone which 

 appear in indirect vision are similar to those which occur in direct vision 

 when the luminosity of the stimulus is progressively increased. Thus 

 red and green become distinctly yellow, blue passes over into gray with- 

 out change of tone, and purple becomes bluish. Helmholtz found that 

 a mixture of red and green-blue which appears colorless in direct vision 

 becomes green-blue at but a slight distance from the visual axis. From 

 this experiment it would seem that the peripheral retina is more sensi- 

 tive to blue and to green than to red. Accordingly Helmholtz assumed 

 that the red-sensing substance is lacking at the periphery, and referred 

 the color phenomena of indirect vision to the red-blindness of the eccen- 

 tric region. 



The view of Helmholtz was confirmed by Schelske,ff who was the 

 first to employ spectral colors in the investigation of the problem. He 

 determined the inner limits of the red-blind zone in his own eye to be 68 

 on the nasal side, 37 on the lower side, 53 in the temporal, and 38 in 

 the upper direction. 



*Graefe's Archiv., 1. c., S. 55. 



tGraefe's Archiv., Ill, 2, S. 61. 



j Oracle's Archiv., Ill, 2, S. 61 ; Grundziige, S. 545. 



Aubert devoted a paper to the phenomena of fatigue and after-images; this 

 will 'be referred to in 'Connection with our own results. 



**Hermann von Helmholtz. Handbuch der physiologischen Optik., Leipzig, 

 1860, S. 300-301, and S. 845. The publication of this volume covered a period 

 extending from 1856 to 1866. The second Lieferung, which contains his first dis- 

 cussion of indirect vision, appeared in 1860. 



ffR. Schelske. Ueber Farbenblindheit des normalen Auges, Graefe's Archiv., 

 IX, 3, 1863, S. 39-62. 



