LITTLE KNOWN FACTS ABOUT BIREFRINGENT CRYSTAL.

各向异性透明晶体如方解石、石英等的折射率，是其固有的特性，称为永久双折射。If the electric field has the course in the optical axis, one obtains the amazing index ne. This is possible provided that the propagation direction (additional precisely, the route from the k vector) is perpendicular on the optical axis.