force, be reflected back through the glass against the lines of force, the rotation will be doubled. It is important to note that this is the reverse of the efiect produced by quartz, solutions of sugar, etc., which rotate the plane of polarization in consequence of their own molecular state. When light of which the plane of polarization has been rotated by passage through such substances is reflected back upon itself, the rotation produced during the first passage is exactly reversed during the return, and the returning light is found to be polarized in the same plane as at first.
In the magnetic field the effect is as though the medium which conveys the light were rotating around an axis parallel to the lines of force, and carrying with it the plane of vibration. Evidently the plane of vibration would be turned through a certain angle during the passage of the light through the body, and would be turned still further in the same direction if the light were to return.
When we remember that iron becomes magnetic by the effect of currents of electricity flowing in conductors around it, and that Ampere conceived that a permanent magnet consists of molecules surrounded by electric currents, all in the same direction, it is easy to imagine that the magnetic field is a region where the ether is actuated by vortical motions, all in the same direction, and in planes at right angles to the lines of magnetic force. Such a motion would account for the rotational effects of the magnetic field upon polarized light.
Not only glass but most liquids and gases exhibit rotational effects when placed in a powerful magnetic field; and Kerr has shown that when light is reflected from the polished pole of an electromagnet, its primitive plane of polarization is rotated when the current is passed, in one direction for a north pole, and in the opposite direction for a south pole.
389. Maxwell's Electromagnetic Theory of Light.—In Maxwell's treatment of electricity and magnetism he assumed that electrical and magnetic actions take place through a universal medium. In order to determine whether this medium may not be identical with the luminiferous ether, he investigated its properties when a
