A Nicol prism was an early type of birefringent polarizer, that consists of a crystal of calcite which has been split and rejoined with Canada balsam. The crystal is cut such that the o - and e -rays are in orthogonal linear polarization states. Total internal reflection of the o -ray occurs at the balsam interface, since it experiences a larger refractive index in calcite than in the balsam, and the ray is deflected to the side of the crystal. The e -ray, which sees a smaller refractive index in the calcite, is transmitted through the interface without deflection. Nicol prisms produce a very high purity of polarized light, and were extensively used in microscopy, though in modern use they have been mostly replaced with alternatives such as the Glan–Thompson prism, Glan–Foucault prism, and Glan–Taylor prism. These prisms are not true polarizing beamsplitters since only the transmitted beam is fully polarized.