118 



VERTEBRATE PHOTORECEPTORS 



reasons for regarding the limiting membrane of the foveal 

 cUvus as constituting a refractive surface. The internal 

 limiting membrane cannot differ markedly from the rest 

 of the retina in refractive power or there would be a condition 

 detrimental to vision. He regards the general retinal tissue 

 as differing from the vitreous in refractive index. If the 

 retina had a lower refractive index, total reflection in the 

 deeper part of a convexiclivate fovea would render the central 

 portion of such a fovea blind. Going originally on the 



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Fig. 81. Convexiclivate fovea of Saurians to illustrate refraction of light 

 along the foveal clivus and spreading of the image on the foveal photoreceptors, 

 according to Walls. (Figure redrawn from Walls, 1937. Arch. Ophth., v. 18.) 



assumption that the retina has a higher refractive index 

 than the vitreous, Walls produced logical reasons in support 

 of the idea that the foveal clivus refracts the light, thus 

 broadening the retinal image and bringing into play a larger 

 number of visual cells than would otherwise be the case 

 (Figure 81). By reason of this property there thus occurs 

 at the fovea a mechanism for increasing the resolving power 

 over that produced by the afoveate area centralis. Using 

 a theoretical example, employing certain assumptions. Walls 

 figured the refractive index of the retina to be about 1.403 

 as compared with 1.336 for the vitreous. He also stressed 

 the point that even in the concaviclivate fovea of man 

 where the refractive index may not be so high as 1.4, there 

 should still be an expansion of the image, although such a 

 form optically would be inferior to that of reptiles and birds. 



