have put forward a theory of reflection optics as a method by which a superposition image is formed on the retina in macruran decapods, notably the crayfish. A feature of the reflecting superposition eyes investigated so far by Vogt & Land is that the corneal facets have negligible focusing properties, such focusing as exists being achieved by total internal reflection in the proximal part of the crystalline cone. The present work shows for the first time that the cornea of an Australian

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... h, Cherax destructor, which otherwise has all the required anatomical features of such a reflecting superposition eye (Fig. 1 A, B, C ), possesses corneal facet lenses each of which focuses light from a distant object (forming a real inverted image) to a position behind the cornea approximately corresponding to that of the distal end of the rhabdom. Each corneal facet lens consists of two layers, the outer layer having a flat front surface and the inner layer having a convex back surface (see Fig. 3 ). The optical measurements were made on freshly isolated pieces of cornea cleared of all adhering tissue and suspended from a coverslip on a drop of bovine serum albumen solution with a refractive index of either 1-411 or 1-365. Albumen does not affect the refractive index of the cornea (Vogt, 1974), and the two values of albumen refractive index were chosen to encompass the range of probable in vivo refractiveindex values. The back focal distance (B.F.D.) of individual corneal facet lenses (the distance between the back of the lens and the back focal point) was determined by focusing consecutively on the back surface of a lens and on the image of a distant object formed by the lens (object distance 50 mm, effectively infinity for a lens with a front focal length of about 400 /mi). The Zeiss compound microscope used had a calibrated micrometer gauge attached to the stage for accurate measurement of the focal distances, B.F.D. values were corrected for the apparent depth effect (Galbraith, 1955) . The medium in front of the cornea was either Ringer solution or air, the refractive indexes of which closely approximate the natural environment of this crayfish, which lives in and beside ponds. Phiages of a figure ' 7 ' formed by several corneal facet lenses are shown in Fig. 2 .