We will first examine Faraday's discovery of the relations existing between light and magnetism. Though the discovery has not as yet borne fruit in any direct practical application, yet it has proved of immense value from a theoretical standpoint. In this investigation Faraday proved that light-vibrations are rotated by the action of a magnetic field. He employed the light of an ordinary Argand lamp, and polarized it by reflection from a glass surface. He caused this polarized light to pass through a plate of heavy glass made from a boro-silicate of lead. Under ordinary circumstances this substance exerted no unusual action on light, but when it was placed between the poles of a powerful electro-magnet, and the light was passed through it in the same direction as the magnetic flux, the plane of polarization of the light was rotated in a certain direction. Faraday discovered that other solid substances besides glass exert a similar action on a beam of polarized light. Even opaque solids like iron possess this property. Kerr has proved that a beam of light passed through an extremely thin plate of highly magnetic iron has its plane of polarization slightly rotated. Faraday showed that the power of rotating a beam of polarized light is also possessed by some liquids. But what is most interesting, in both solids and liquids, is that the direction of the rotation of the light depends on the direction in which the magnetism is passing, and can, therefore, be changed by changing the polarity of the electro-magnet.