Page:Elementary Text-book of Physics (Anthony, 1897).djvu/474

460 is continued the first image reappears, while the second grows dim and disappears when the angle between the principal planes is 180°. These changes show that the light which emerges from the first crystal of spar is not ordinary light. Another experiment shows this in a still more striking manner. Let the extraordinary ray be cut off by a screen, and the ordinary ray be received on a plane unsilvered glass at an angle of incidence of 57°. When the plane of incidence coincides with the principal plane of the spar, the light is reflected like ordinary light. If the mirror be now turned about the incident ray as an axis, that is, so turned that, while the angle of incidence remains unchanged, the plane of incidence makes successively all possible angles with the principal plane of the crystal, the reflected light gradually diminishes in brightness, and when the angle between the plane of incidence and the principal plane of the crystal is 90° it fails altogether. If the rotation be continued it gradually returns to its original brightness, which it attains when the angle between the same planes is 180°, and then diminishes until it fails when the angle is 270°. The extraordinary ray presents the same phenomena except that the reflected light is brightest when the angle between the planes is 90° and 270°, and fails when that angle is 0° and 180°. Beams of light after double refraction present different properties on different sides, and are said to be polarized. The explanation must, of course, be found in the character of the vibratory motion.

In the polarized beam it is plain that the vibrations must be transverse; for if the light were the result of longitudinal vibrations, or even of vibrations having a longitudinal component, it could not be completely extinguished for certain azimuths of the second crystal or of the glass reflector. This conclusion is verified by the experiments of Presnel and Arago on the interference of polarized light. The difference between ordinary and polarized light is explained if we assume that, in both, the vibrations of the ether particles take place at right angles to the line of propagation of the wave, and that in ordinary light they occur irregularly in all azimuths about that line, and may be performed in ellipses or