Two pieces of optically clear calcite, cemented by Canada balsam into the shape of a prism. Light entering the base of the prism is doubly refracted, and when both rays reach the Canada-balsam cement one ray is reflected away from the prism while the other ray continues through the prism. Thus the light emerging from the prism is plane polarized. These prisms were invented by William Nicol. Early polarizing microscopes were fitted with nicol prisms for the analyser and polarizer, but modern microscopes are fitted with Polaroid instead, and, although the term ‘crossed nicols’ is still in common use, this has been largely replaced by the term ‘crossed polars’.