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.

Faraday did not seem to thoroughly understand this phenomenon.  He spoke as if he thought the lines of magnetic force had been rendered luminous by the light rays; for, he announced his discovery in a paper entitled, “Magnetization of Light and the Illumination of the Lines of Magnetic Force.”  Indeed, this discovery was so far ahead of the times that it was not until a later date that the results were more fully developed, first by Kelvin, and subsequently by Clerk Maxwell.  In 1865, two years before Faraday’s death, Maxwell proposed the electro-magnetic theory of light, showing that light is an electro-magnetic disturbance.  He pointed out that optical as well as electro-magnetic phenomena required a medium for their propagation, and that the properties of this medium appeared to be the same for both.  Moreover, the rate at which light travels is known by actual measurement; the rate at which electro-magnetic waves are propagated can be calculated from electrical measurements, and these two velocities exactly agree.  Faraday’s original experiment as to the relation between light and magnetism is thus again experimentally demonstrated; and, Maxwell’s electro-magnetic theory of light now resting on experimental fact, optics becomes a branch of electricity.  A curious consequence was pointed out by Maxwell as a result of his theory; namely, that a necessary relation exists between opacity and conductivity, since, as he showed, electro-magnetic