A Special contribution to the birefringence is found in magnetically‐ordered systems. This phenomenon was, until now, mainly used to study magnetic domain structures, ferrimagnetic garnets and magnetic phase transitions. Distinct results were obtained for the transition‐metal difluorides, in which the magnetic contribution to the birefringence is directly proportional to the magnetic contribution to the internal energy. The magnetic birefringence is determined by the spin pair correlations.