Refractive index within the lens of a goldfish eye determined from the paths of thin laser beams

The paths of 15 micron diameter laser beams traversing goldfish eye lenses were photographed. Measurements of these photographs gave experimental data for the distance of the exit point of each ray from the lens axis as a function of the corresponding entrance distance. A number of mathematical models with distinct distributions of refractive index within the lens were analysed by tracing rays to simulate the experimental data. The only distributions for which the simulated and experimental data were in agreement have a refractive index N which varies continuously with distance r from the lens center in a manner consistent with that originally proposed by Matthiessen: N2 = a - br2. Estimates for the central (1.55-1.57) and surface (1.35-1.38) refractive indices of the goldfish eye lens are derived from the preferred model, but these differ from those previously given by Matthiessen for other species. The optical performance of the lens models is also compared by third-order analyses.