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Zusammenfassung:
The optical properties of fresh corneal lenses ofEphestia kühniella were investigated. The refractive index distribution in slices about 3 μm thick was measured with the interference microscope. An axial and radial variation was found, the radial distribution being symmetric to the lens axis. The lowest values (approx. 1.43) of the refractive index occur in the paraxial proximal region of the cone, while the highest values (approx. 1.54) are found at the distal boundary and the peripheral zone of the lens. This refractive index distribution is equivalent to that of a dispersive lens, whereas the convex corneal surface is a collecting lens of higher power.
This result is confirmed by experiments with the intact lens. The proximal distance between cornea and focal plane is +37.5 μm (Image mediumn = 1.335). This value and the geometrical properties of the lens give an effective lens refractive index of 1.40; this is lower than any value found in the cornea slices. If the convex curvature of the lens is optically compensated, the lens forms virtual erect images of an object.
Measurements of the central diffraction disc showed that the quality of imaging of the corneal lens is limited only by the lens aperture, and not by the refractive index distribution.
