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Rasmus Bartholin (/bɑːrˈtoʊlɪn, ˈbɑːrtəlɪn/; Latinized: Erasmus Bartholinus; 13 August 1625 – 4 November 1698) was a Danish scientist, physician and grammarian. As part of his studies, he travelled in Europe for ten years. Professor at the University of Copenhagen, first in Geometry, later in Medicine. He was a younger brother of Thomas Bartholin. He wrote, in Latin, the first grammar of the Danish language, the 1657 De studio lingvæ danicæ.
Rasmus Bartholin is remembered especially for his discovery (1669) of the double refraction of a light ray by Iceland spar (calcite). He published an accurate description of the phenomenon, but since the physical nature of light was poorly understood at the time, he was unable to explain it. It was only after Thomas Young proposed the wave theory of light, c. 1801 that an explanation became possible.