Scanning interferometry of sunfish cones I Longitudinal variation in single-cone refractive index

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Abstract
Interferometric measurements of cone inner segment–cone outer segments (CIS–COS’s) isolated from the green sunfish (Lepomis cyanellus) were made with a He–Ne laser focused to a 0.3-μm spot. Multiple measurements were obtained by moving the spot across the CIS–COS’s in a raster pattern; adjacent measurements were spaced approximately 0.2 μm apart. The CIS–COS’s were oriented with their longitudinal (z) axes parallel to one of the raster directions. By assuming that the CIS–COS’s have circular cross sections in the orthogonal (x–y) planes, we calculated from the interferometric measurements average refractive indices as a function of z. In the cone inner segments these average refractive indices rise monotonically with z in the direction along which light travels through the living photoreceptor. The average refractive index was typically between 1.36 and 1.39 at the base of the photoreceptor and between 1.42 and 1.44 at its junction with the outer segment. These findings confirm previous qualitative assessments of density variations evident in electron micrographs [Rowe et al., J. Opt. Soc. Am. A 11, 55 (1994)].