A prism invented by William Nicol in 1828 that is made of calcite, the end faces of which are ground to an angle of 68° between one vertical side and the end surface. The prism is cut diagonally in half, the cut being made at right angles to the shorter diagonal of the end face. The parts are then cemented together. Unpolarized light striking an end face is polarized to vibrate in two directions. The extraordinary ray passes through the crystal, and the ordinary ray is reflected to one side at the calcite-balsam interface and is lost.