His mentor at the University was Rasmus Bartholin, who published his discovery of the double refraction of a light ray by Iceland spar (calcite) in 1668, while Rømer was living in his home.
O seu mentor na Universidade era Rasmus Bartholin quen publicou o seu descubrimento da dobre refracción dun raio de luz polo espato de Islandia (calcita) en 1668, cando Rømer estaba vivindo na súa casa.