Refractive index gradients in sunfish cones

Jay Enoch's pioneering work with whole-mounted retinas established that vertebrate photoreceptors behave as dielectric waveguides. Theoretical analyses lead to the expectation that the distribution of refractive index in the photoreceptor inner segment will affect its waveguiding behavior. To determine the distribution of refractive indices in isolated photoreceptors, we examined them with a scanning microinterferometer. Our measurements of cones from green sunfish (Lepomis cyanellus) reveal systematic gradients of refractive index within the inner segments. For example, the refractive index of single cones (averaged over transverse cross-sections) increases from ca. 1.38 to ca. 1.42 along the inner segment longitudinal axis. Such gradients are large enough to affect waveguiding properties. Interferometric measurements and analysis of subcellular variations in refractive index will allow us to address a variety of unresolved issues in photoreceptor optics.