My book reads "When unpolarized light is incident on nicol prism (made of 2 crystals joined by Canada balsam a type of glue) it divides into 2 rays, both rays are plane polarized and electric field vectors of one of the rays are perpendicular to plane. These ray is Ordinary Ray. And electric field vector of another ray have oscillatios parallel to the plane. These ray is Extra ordinary ray. For these refractive index are 1.658 &1.486 respectively.The refractive index of Canada Balsam is 1.55. so, ordinary ray experiencs TIR at the surface of the glue Canada Balsam and comes out from one side of the prism while extra ordinary ray comes out as plane polarized light"
My question is
"Is it possible for a medium (here calcite) to have two refractive indices?"