Page:Elementary Text-book of Physics (Anthony, 1897).djvu/489

§ 388] the plane of polarization in one direction, and some in the opposite. Rotation which is related to the direction of the light as the directions of rotation and propulsion in a right-handed screw is said to be right-handed, and that in the opposite direction is left-handed.

Reusch has reproduced all the effects of quartz plates by superposing thin films of mica, each film being turned so that its principal plane makes an angle of 45° or 60°, always in the same direction, with that of the film below. If a plane polarized wave enter such a combination, an analysis of the resolution of the vibration as it passes from film to film will show that the result is equivalent to that of two contrary circular vibrations, one of which is propagated less rapidly than the other. This helps to establish Fresnel's theory of the rotational effects of quartz.

387. Rotation of the Plane of Polarization by Liquids.—Many liquids rotate the plane of polarization, but to a less amount than quartz. A solution of sugar produces a rotation varying with the strength of the solution, and instruments called saccharimeters are made for determining the strength of sugar solutions from their effect in rotating the plane of polarization. In these instruments the effect is often measured by interposing a wedge-shaped piece of quartz, and moving it until a thickness is found which exactly compensates the rotation produced by the solution.

388. Electromagnetic Rotation.—Faraday discovered that when polarized light passes through certain substances in a magnetic field the plane of polarization is rotated through a certain angle. The experiment succeeds best with a very dense glass consisting of borate of lead, so placed that the light may traverse it along the lines of magnetic force, in the field produced by a powerful electromagnet. The amount of rotation is proportional to the difference of magnetic potential between the two ends of the glass. The direction of rotation, as was shown by Verdet, is generally right-handed in diamagnetic media, and left-handed in paramagnetic media. It also depends upon the direction of the lines of force, and is therefore reversed by reversing the current in the electromagnet. It follows, also, that if the light, after traversing the glass with the lines of