I recently demonstrated the untenability of the usual explanation of the foveal depression,1 which holds the latter to be simply an excavation of tissue which would interfere with optimal function of the macular center as a region of maximal resolving power. It was suggested that the purpose of the depression, at least where it is deep and convexiclivate, as in Sauropsida, is to expand the image locally by a peripheral refraction of the eikonogenic rays where they strike the internal limiting membrane along the clival surfaces.
At the time, I had only logical grounds for believing the refractive index of the retina to be higher than that of the vitreous—a prerequisite for the postulated magnifying action of the foveal pit—and was unaware that actual measurements of retinal indexes had long since been made by Valentin.2
Valentin admonished against ignoring the disparity of the retinal and vitreous refractive indexes
WALLS GL. POSTSCRIPT ON IMAGE EXPANSION BY THE FOVEAL CLIVUS. Arch Ophthalmol. 1940;23(4):831–832. doi:10.1001/archopht.1940.00860130925009
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