Trilobite eyes and the optics of Des Cartes and Huygens.

The thick lenses in the aggregate eyes of a group of trilobites were double structures designed to eliminate spherical aberration. The shape of the optically correcting interface is in accord with constructions by Des Cartes and Huygens and is dictated by a fundamental law of physics. Trilobites may have evolved such sophisticated eye-lenses to maximise optic neurone response in a dimly lit environment.