Ray path through a Nicol prism
Nicol prism in brass mount
A Nicol Prism consists of two pieces of calcite glued together. Calcite is birefringent, and thus generates two polarized rays (O-Ray and E-Ray) from unpolarized impinging light. The angle of the glued surfaces causes one of the rays to be internally reflected and lost. The result is that only the plane-polarized E-Ray is emitted from the opposite end.

This type of polarizer was invented by the Scottish physicist William Nicol (1768-1851).
Extinction of polarized light by a Nicol prism (arrow). At alignment (0°) polarized light (from a modern polarizing sheet, background) passes through the prism. When rotated 90° the prism effectively blocks the plane-polarized light.
A polarizing light microscope consists of two polarizers: Below the stage is the Polarizer (Pol), which generates plane-polarized light that illuminates the specimen (Spec). Above the sample is the second polarizer, or Analyzer (Ana). The analyzer is oriented 90 degrees off-axis from the polarizer.

Background light (b) that does not interact with the specimen is effectively blocked by the Analyzer ("Extinction", above). Thus the background is black. This condition is referred to as a polarized darkfield.

Light that interacts with a birefringent specimen (s) is itself plane-polarized, generating two new rays polarized at right angles. One of these two new rays effectively passes through the Analyzer. Thus a crystalline sample appears bright on a dark background.