A Nicol prism is a type of polarizer, an optical device used to produce a polarized beam of light from an unpolarized beam. It is made in such a way that it eliminates one of the rays by Total Internal Reflection i.e, the O-ray is eliminated and only the E-ray is transmitted through the prism. See polarized light. It was the first type of polarizing prism to be invented, in 1828 by William Nicol (1770–1851) of Edinburgh. It consists of a rhombohedral crystal of Iceland spar (a variety of calcite) that has been cut at an angle of 68° with respect to the crystal axis, cut again diagonally, and then rejoined as shown using, as a glue, a layer of transparent Canada balsam.