The biconvex lenses of phacopid trilobites are made up of doublet structures that were constructed for an unmistakable purpose: to correct for the large spherical distortion (aberration) of simple thick lenses. I reached this rather surprising conclusion after examining the large amount of evidence on lens morphology that had been accumulated by Euan N. K. Clarkson (1968) during his systematic investigation of trilobite eyes. A preliminary account of this discovery was contained in the first edition of this book (Levi-Setti 1975). Dr. Clarkson and I have since copublished a paper in Nature (Clarkson and Levi-Setti 1975) that describes this evidence and our interpretation of the extraordinary function of such doublet structures.
When we humans construct optical elements, we sometimes cement together two lenses that have different refractive indices, as a means of correcting particular lens defects.
One of the earlier examples of schizochroal eye, carried by the Otdovician dalmanitid ttilobite Pterygometopus brongniarti (Reed). The eye lenses are missing in this specimen, but the framework of alveoli in which they were set has remained intact. (Collected at Girvan, Scotland, on a field trip guided by E. N. K. Clarkson). (x8.3)
Internal latex mold of the eye of Pterygometopus brongniarti (Reed), shown in the preceding plate. The lenses are convex and arranged in vertical dorso-ventral files. (x!8)
A later form of schizochroal eye, that of Odontochile hausmanni (Brongniart), has clearly separated lens elements. The growth mechanism of the visual surface is illustrated by lenses in the process of being released from the generative zone at the base of the eye. (Specimen from the Lower Devonian of Tetin, Bohemia. Loaned by E. N. K. Clarkson.) (x!9)
The schizochroal eye of one of the earliest phacopid ttilobites,
Acernaspis (Eskaspis) sujferta (Lamont). The lenses are closely packed and almost hexagonal in shape. Latex mold from external impression, specimen from the Lower Silurian of the Pentland Hills, Scotland. (Collected on a field trip guided by E. N. K. Clarkson, RLS coll.) (x! 5)
(Barrande) exemplifies later, perfected forms in the evolution of the schizochroal eye. The lens elements are rather widely spaced and prominent above the plane of the sclera. (Specimen from the Silurian of Bohemia. Negative loaned by E. N. K. Clarkson.) (xl8)
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