Lenses of trilobites' eyes were made of calcite (calcium carbonate, CaCO3). Pure forms of calcite are transparent, and some trilobites used crystallographically oriented, clear calcite crystals to form each lens of each eye. Rigid calcite lenses would have been unable to accommodate to a change of focus like the soft lens in a human eye would; in some trilobites, the calcite formed an internal doublet structure, giving superb depth of field and minimal spherical aberration, according to optical principles discovered by French scientist René Descartes and Dutch physicist Christiaan Huygens in the 17th century. A living species with similar lenses is the brittle star "Ophiocoma wendtii".