COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 15 



into gray with progressive decrease of saturation. Violet went out 

 through blue to black or gray. Purple-red appears successively violet- 

 ish, blue, and black or gray. 



Woinow is convinced that the areas of the color zones are fixed 

 and invariable ; he finds that their limits change neither with change of 

 area of stimulus nor with change of brightness of background. He 

 does not state what is the area of each zone, nor does he give data from 

 which their relations may be calculated. Woinow concludes that the 

 distribution of all three visual substances becomes more and more scant 

 towards the periphery, until only green-sensing substance is present in 

 the outermost zone. The extreme periphery is therefore not only red- 

 blind but violet-blind as well. 



Bow* discusses the problem without any reference to the previous 

 literature, and evidently under the impression that he is the pioneer 

 investigator in the field. He reports that a blue-green glass held before 

 the eye and regarded indirectly appears blue, while a yellow-green glass 

 appears yellow under similar conditions. Scarlet becomes orange at 

 30 from the visual axis and yellow at 40. Green passes over into 

 yellow, and purple into blue, in indirect vision. On the basis of the 

 different refrangibility of light of different colors, he advances the 

 following remarkable explanation of his results : The retina contains a 

 three-ply layer of sensitive elements. Each stratum is composed of 

 fibers which are sensitive to but a single color, and each is separated 

 from its fellows by intervening strata of non-sensitive tissue. The 

 presence of the intermediate strata is a sine qua non of normal color 

 vision. They are, however, absent from the peripheral retina, and since 

 the sensitive layers are here in contact, they are incapable of function- 

 ing normally, f 



Maxwell! found that when a white surface is observed through a 

 bluish-green filter, the fixation-point appears to be a pinkish spot upon 

 a bluish-green ground. He also reported that a certain mixture of red, 



*R. H. Bow. On the Changes of Apparent Color by Obliquity of Vision. 

 Proceedings of .the Royal Society of Edinburgh, VII, 1871, pp. 155-160. 



t Since we shall not have occasion to refer to this theory again, we may re- 

 mark, in passing, that the following objections have been urged against it: In 

 order that lights of different refrangibility may be brought to a focus exactly upon 

 the layers which are to sense .them, the red-sensing layer must occupy an inner- 

 most and the violet-sensing layer an outermost position; Bow gives them the 

 reverse order. Moreover, the difference in focal distance, between the red and 

 violet rays, in the reduced eye, is given by Helmholtz (Physiol. Optik, S. 131) 

 as .434 mm. Now, the retina in its thickest part is scarcely half thick enough to 

 meet Bow's requirement. Again, the central region is at once the thinnest part 

 of the retina, and the most capable of diverse visual functioning. 



tj. Clerk Maxwell. On Color Vision at Different Points of the Retina. Re- 

 port of the British Association for the Advancement of Science, 1870. Notices 

 and Abstracts, etc., pp. 4of. 



