Visibility of photoreceptors in the intact living cane toad eye.

Photoreceptors are invisible under the usual conditions of viewing the fundus; light reflected from other structures obscures them. Individual photoreceptors are shown to be visible in the intact living eye of the cane toad when viewed with intense oblique illumination from one side. The photoreceptors act like optical waveguides in channelling light from the sclera to the observer. If the retinal mosaic were so fine as to oversample the image, the optical system would not be able to resolve a single mosaic element. The fact that adjacent photoreceptors are seen as separate spots of light, using the eye's optical system, indicates that oversampling of the optical image by the retinal mosaic is not occurring.