Nearly synonymous with birefringence but may be applied in a restricted sense to transparent (at visible frequencies) media with sufficient birefringence that images seen through them are split. The term originates from the Danish physician Erasmus Bartholinus, who in 1669 wrote about what he observed through the crystalline solid calcite: “objects which are looked at through the crystal do not show, as in the case of other transparent bodies, a single refracted image, but they appear double. ”