Abstract: General formulae for propagating directions of the polarized light at the inner and external of biaxial crystal are obtained in the case that ray along arbitrary direction is incident upon a crystal of arbitrary making direction. The transition from it to uniaxial crystal, refractive rays lying in the incident plane and cone refraction are discussed. At last, the corresponding numerical results are plotted in curvilineal diagrams for Iceland spar and crystal sulphur, they make the directions of the refractrive rays clear as the crystal rotates. These results provide an important basis for study of new crystal optical components and new measuring methods of crystal optical constants.

Key words: biaxial crystal, inner and external incidences, principal axial and crystal plane coordinate systems, Huygens principle, Fermat's principle