The difference in the index of refraction between the receptor outer segment and the interstitial matrix has been determined to be 0.06 by making use of the waveguide properties of rat rod photoreceptors. This was done first by determining the cutoff wavelength for the transition between the bilobe or annulus modal patterns and the single-lobe <i>H E</i><sub>11</sub> modal pattern, and by measuring the diameter of the transmitting cell. These measurements were made on bleached receptors. Enoch, Scandrett, and Tobey had previously shown that bleaching only slightly alters the outer-segment index of refraction and diameter in the frog. Assuming interspecies transfer, and using Sidman’s prior estimate of bleached rat rod outer-segment index of refraction, then the index of refraction of the interstitial matrix of the albino rat is estimated to be 1.3476. This estimate was made using two different media in contact with the retinal preparation, a miscible mountant tissue culture medium believed to be compatible with the rat retina, and an encapsulating nonmiscible silicone oil which allows only ocular media from the same eye in contact with the outer segments.
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