Here's looking at you kid - 300 million years ago

This is the fossil eye of a trilobite; it is about 5 mm in diameter and it last saw light some 300 million years ago.

Trilobites are fossil group of extinct marine arthropods. Superficially, they look rather like modern woodlice but are not at all closely related to them. They first appeared of in the fossil record over 500 million years ago and became extremely diverse and successful, roaming the world's oceans for over 270 million years. They finally disappeared in the mass extinction of life that took place at the end of the Permian about 250 million years ago.

Like modern insects, trilobites had complex, compound eyes. The lenses that make up the eye, and which can be clearly seen in the fossil, are elongate prisms of pure, clear calcite (calcium carbonate). The number of lenses in a single eye varied from species to species, some trilobites had only one, while others had thousands; the eye in the photograph has about 200.