Rasmus Bartholin (Latinized Erasmus Bartholinus; August 13, 1625 - November 4, 1698) was a Danish scientist and physician. As part of his studies, he travelled in Europe for ten years. Professor at Copenhagen University, first in Geometry, later in Medicine. He was a younger brother of Thomas Bartholin.
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. Only after Thomas Young had proposed the wave theory of light, an explanation became possible.
Rasmus Bartholin, Experimenta crystalli islandici disdiaclastici quibus mira & insolita refractio detegitur. Hafnić 1669. English translation: Experiments with the double refracting Iceland crystal which led to the discovery of a marvelous and strange refraction, tr. by Werner Brandt. Westtown, Pa., 1959.