Schematic section through a Nicol prism. The original calcite rhombohedron is split along a diagonal plane, cemented back together again with Canada balsam, and then ground down to the desired shape. Optical polarimeters employ pairs of Nicol prisms.
A Nicol prism is an optical device for producing a beam of plane polarized light. Two pieces of calcite crystal are cemented together with Canada balsam. Incident light is split into ordinary and extraordinary linearly polarized rays in the prism. The ordinary ray hits the balsam layer obliquely, and is totally internally reflected; the other ray emerges plane polarized for a certain range of incident angles.
The nicol prism was invented by the British physicist William Nicol (1768–1851). It is used in the polarizing microscope, and also in geology to examine thin sections of rock microscopically.