Erasmus Bartholinus (13.08.1625-4.11.1698) was a Danish physicist and mathematician. He was born in Roskilde. He studied at the University of Leiden and furthered his knowledge in France, Italy and England (in 1654 he received the degree of master). From 1657 he was professor at the University of Copenhagen (he was dean of the faculty of medicine and rector).
In his work “Experiments with crystals of Iceland calcite spar, which show a remarkable and strange refraction” (1669) he described the phenomenon of double refraction in crystals of Iceland spar, which he had discovered. He noticed that a ray of light falling on the surface of Iceland spar is refracted and splits in two, and that there is a direction in the crystal along which this does not happen. Bartholinus's discovery was the beginning of crystal optics. A follower of R. Descartes.