A device for producing plane-polarized light (see polarizer). It consists of two pieces of calcite cut with a 68° angle and stuck together with Canada balsam. The extraordinary ray (see double refraction) passes through the prism while the ordinary ray suffers total internal reflection at the interface between the two crystals, as the refractive index of the calcite is 1.66 for the ordinary ray and that of the Canada balsam is 1.53. Modifications of the prism using different shapes and cements are used for special purposes. It was devised in 1828 by William Nicol (1768–1851).