It turned out that the lenses in trilobites’ compound eyes were made of mineral. Each lens was a single crystal of calcite—the same stuff limestone, chalk, and marble are made of. The problem is that calcite strongly birefracts light: a ray passing through the crystal splits in two. Trilobites got around this by orienting the crystal in each lens so that its optical axis ran strictly along the line of sight, and there is no birefringence in that direction.
The second problem is spherical aberration. In a simple convex lens, rays passing through the edge converge somewhere other than the central rays, and the image gets blurred. A living eye can at least somewhat compensate for this through the shape and non-uniformity of the lens, but a rigid crystal cannot.
In one group of trilobites—the phacopids (Ordovician–Devonian)—the lenses were large and separate, and each consisted of two layers of calcite with different magnesium content: a convex upper part and an intralensar bowl” underneath (see picture). The boundary between the layers matches the profiles of aplanatic lenses described by Descartes and Huygens back in the 17th century. This shape cancels out spherical aberration—the rays passing through the edge of the lens converge at the same point as the central ones. There is even a hypothesis that these lenses were bifocal—the center and the periphery focused at different distances, so a fixed crystal could work for both near and far vision at the same time.
It’s pretty cool that evolution arrived at this solution roughly 400 million years before Descartes and Huygens, while neither of them lived to see the day when people figured out trilobite ophthalmology.
