These crystal eyes are unique to trilobites, with the compound eyes of modern invertebrates being made of chitin, an organic substance. Due to their unusual composition, trilobite eyes were completely rigid and could not be adjusted to focus; instead, the trilobite corrected its focus with an internal eye mechanism which not only solved any potential problems caused by the mineral lens, but also gave the trilobite such good vision, that it could keep both close and distant objects in focus at the same time.
As if that wasn´t bizarre enough, some trilobites had really weird looking eyes; a few had their eyes at the end of long projections, just like the Stalk Eyed Fly, while others had overhanging “eyeshades” on top of the eye, protecting it from bright sunlight. Being made of calcite, trilobite eyes fossilized easily, and therefore we probably know more about trilobite eyes and vision than about those of any other prehistoric creature.
Just in case people would like more info about the critters. http://