Page:Elementary Text-book of Physics (Anthony, 1897).djvu/487

§ 386] forming a cone, which will become a hollow cylinder of light on emerging from the crystal. This phenomenon is called conical refraction. It was predicted by Hamilton from a mathematical analysis of the wave propagation in such crystals.

If a plate be cut from a biaxial crystal perpendicular to the line bisecting the angle formed by the optic axes, and placed between the polarizer and analyzer in a conical pencil of light, there will be seen a series of colored curves called lemniscates, resembling somewhat a figure 8. The existence of this phenomenon was also predicted and the forms of the curves investigated by mathematical analysis before they were seen.

385. Double Refraction by Isotropic Substances when Strained.—A piece of glass between the crossed polarizer and analyzer, if subjected to forces tending to distort it, will restore the light beyond the analyzer and in some cases produce chromatic effects. Unequal heating produces this result, and a long tube made to vibrate longitudinally shows it when the light crosses it near the node. Pieces cut from plates of unannealed glass exhibit double refraction when examined by polarized light. Indeed, the absence of double refraction is a test of perfect annealing.

386. Effects of Plates of Quartz.—A quartz crystal is uniaxial, and gives an ordinary and an extraordinary ray, but is unlike Iceland spar in that the extraordinary wave front in it is a prolate spheroid and lies within the spherical ordinary wave. The effects due to plates of quartz in polarized light differ very greatly from those due to Iceland spar or selenite. If a plate of quartz cut perpendicularly to the axis be placed in a beam of parallel, homogeneous, plane polarized light at right angles to its path, the light is, in general, restored beyond the analyzer, and is unchanged by the rotation of the quartz through any azimuth. If the analyzer be rotated through a certain angle, depending on the thickness of the quartz plate, the light is extinguished. It is evident that the plane of polarization has simply been rotated through a certain angle. Light of a different wave length would have been rotated through a different angle. A beam of white polarized light, therefore, has the