Further observations on the clear zone eye of Ephestia

Abstract

1. The radius of curvature of the facets of the moth Ephestia, measured by scanning electron microscopy, ranges from 13 to 17 nm, with best value at about 14.5 μm. Values of refractive indices, measured by interference microscopy, are as follows, cornea 1.49 to 1.51 almost homogeneous; centre of cone 1.43, middle zone of cone 1.40, outer zone of cone 1.37. 2. Rays were traced through scale diagrams of cornea and cone by use of these values. Rays that are parallel when they strike the facet pass into the clear zone as a diverging bundle. Ray paths do not fit the relation required by Exner’s theory of superposition images. 3. Measurements of the divergence of eyeshine show that a parallel beam entering the eye does not forma sharply focused image. Instead, eyeshine rays can be traced through the eye in a way which agrees closely with the results of ray tracing through single cones. 4. The inhomogeneity of the cones has the effect of refracting some rays towards the axis of the cone. This effect makes the distribution of light into the clear zone more like a normal distribution about the axis through the cone than would be the case in a simple lens. 5. The directions of rays crossing the clear zone suggest that the theory of summation of scattered light is a reasonable indication of how the dark-adapted eye functions. By this means some sensitivity is gained at the expense of acuity, but the contribution of light passing down the inner and outer light guides remains to be measured in the dark-adapted eye.