ABSTRACT
All crystals except those belonging to the cubic system, are birefringent. One incident ray gives rise to two refracted rays, but these two rays are separated clearly in a small number of crystals only. The ordinary and the extraordinary beams coming out of a calcite crystal are linearly polarized at right angles to each other. The extraordinary beam vibrates in the principal section of the crystal and the ordinary beam in the perpendicular plane. The thickness of the two calcite crystals and the diameter of the circular aperture must be suitably chosen so as to obtain four images, well separated, on the screen. By rotating a polarizer in its own plane about the beam incident on the plate or about the beam coming out of the plate, two systems of sharp rings are in general observed.
