Rasmus Bartholin (/bɑːrˈtoʊlɪn, ˈbɑːrtəlɪn/; Latinized: Erasmus Bartholinus; 13 August 1625 – 4 November 1698) was a Danish physician and grammarian.

[2] 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).

[3] 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.

[4] It was only after Thomas Young proposed the wave theory of light, c. 1801 that an explanation became possible.