Page:Elementary Text-book of Physics (Anthony, 1897).djvu/475

§ 376] circles as well as in straight lines, while in polarized light they occur in one plane. In the ordinary ray in Iceland spar the vibrations are in a plane at right angles to the optic axis. In the extras ordinary ray they are in the plane containing the optic axis and the ray. If we assume that the rigidity of the ether is different in different directions in the crystal, that at right angles to the optic axis it is a minimum and along the optic axis a maximum, and varies between these two directions according to a simple law, all the phenomena of double refraction and polarization in the crystal are accounted for. If a crystal be cut so as to present faces parallel to the optic axis, and if light enter along a normal to one of these faces, the vibrations, which previous to entering the crystal were in all azimuths, are resolved in it in two directions—that of greatest and that of least elasticity, or parallel to and at right angles to the optic axis. The wave made up of vibrations parallel to the optic axis is propagated with the greater velocity. In this case the two wave fronts continue in parallel planes, and upon emergence constitute apparently one beam of light. If the incidence be oblique and in a plane at right angles to the principal plane, the two component vibrations are still parallel to and at right angles to the optic axis, but a refraction occurs which is greater for the ray of which the vibrations are m the direction of least elasticity. If the incidence be oblique and in the principal plane, it is evident that there may be a component vibration at right angles to the optic axis, but the other component, since it must be at right angles to the ray, cannot be parallel to the optic axis, and therefore cannot be in the direction of greatest elasticity in the crystal. The second component is, however, in the direction of greatest elasticity in the plane of vibration, which direction is at right angles to the first component. In general, if a ray of light pass in any direction within the crystal, the line drawn at right angles to that direction and to the optic axis, that is, at right angles to the plane determined by the ray and the optic axis, is in the direction of least elasticity. One of the component vibrations is in that direction. A line drawn at right angles to the ray and in the plane formed by