Danish physician, mathematician, and physicist who discovered the optical phenomenon of double refraction.

While professor of medicine (1657–98) at the University of Copenhagen, Bartholin observed that images seen through Icelandic feldspar (calcite) were doubled and that, when the crystal was rotated, one image remained stationary while the other rotated with the crystal. Perceiving that light passing through calcite was split into two rays, he called the stationary image the “ordinary beam” and the moving image the “extraordinary beam.” Although Bartholin himself was unable to explain double refraction, it was recognized as a serious contradiction to Isaac Newton's optical theories.