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A spherical lens like a marble does not produce a clear image, because the rays reaching its rounded surface must travel different distances, and are thus refracted at different angles. The trilobite eye overcomes this problem by means of a special arrangement. As already mentioned, the trilobite eye lens is made of calcite, the same material, as the shell covering its body. Raw calcite crystals are transparent, allowing light to pass through. Each calcite lens in the trilobite eye is biconvex in shape, that is, convex on both sides.
At the bottom of the lenses, Levi-Setti and Clarkson encountered an extraordinary state of affairs. Every lens actually consisted of two units. The upper lens unit was made of calcite, and the bottom was chitin-based. Descartes and Huygens’ mathematical designs and the actual intermediate surface where the two units came together in the trilobite eye lens were in remarkable accord with one another. Magnesium atoms were arranged along the length of this surface in the lens, in sufficient quantities to overcome the spherical aberration. Thanks to these atoms, every light beam curved to the left was balanced by one to the right, thus enabling light rays initially refracted at different angles to focus on the same spot. (See diagram opposite.)
Levi-Setti expressed his amazement at the situation:
In fact, this optical doublet is a device so typically associated with human invention that its discovery in trilobites comes as something of a shock. The realization that trilobites developed and used such devices half a billion years ago makes the shock even greater. And a final discovery—that the refracting interface between the two lens elements in a trilobite’s eye was designed in accordance with optical constructions worked out by Descartes and Huygens in the mid-seventeenth century—borders on sheer science fiction .117