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Nicol prism

Nicol prism (nĬk´əl), optical device invented (1828) by William Nicol of Edinburgh. It consists essentially of a crystal of calcite, or Iceland spar, that is cut at an angle into two equal pieces and joined together again with Canada balsam. An ordinary beam of light entering the crystal undergoes double refraction, i.e., is split into two parts, each of which is affected in a different way. One of these parts, the so-called ordinary ray, undergoes total reflection at the Canada-balsam joint and is turned off from its course to pass out at one side of the crystal. The other ray, the extraordinary ray, passes on through the crystal. By means of this device a beam of light can be polarized (see polarization of light) or a beam of polarized light can be subjected to analysis. The principle involved has been applied to the microscope in the illumination of the field.

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nicol prism

nicol prism 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’.

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