Nicol prism

:  a device used for the production or analysis of polarized light consisting of the two parts of a rhombohedron of clear calcite bisected obliquely at a particular angle and subsequently cemented together with a transparent cement of which the refractive index lies between that of calcite for the ordinary ray which is totally reflected at the cement interface and the maximum refractive index of calcite for the extraordinary ray which is alone transmitted, both rays being plane-polarized at right angles to each other