Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/233

 After working strenuously for the ten years which followed the discovery of induced currents, Faraday found in 1841 that his health was affected; and for four years he rested. A second period of brilliant discoveries began in 1845.

Many experiments had been made at different times by various investigators with the purpose of discovering a connexion between magnetism and light. These had generally taken the form of attempts to magnetize bodies by exposure in particular ways to particular kinds of radiation; and a successful issue had been more than once reported, only to be negatived on re-examination.

The true path was first indicated by Sir John Herschel After his discovery of the connexion between the outward form of quartz crystals and their property of rotating the plane of polarization of light, Herschel remarked that a rectilinear electric current, deflecting a needle to right and left all round it, possesses a helicoidal dissymmetry similar to that displayed by the crystals. "Therefore," he wrote, "induction led me to conclude that a similar connexion exists, and must turn up somehow or other, between the electric current and polarized light, and that the plane of polarization would be deflected by magneto-electricity."

The nature of this connexion was discovered by Faraday, who so far back as 1834 had transmitted polarized light through an electrolytic solution during the passage of the current, in the hope of observing a change of polarization, This early attempt failed; but in September, 1845, he varied the experiment by placing a piece of heavy glass between the poles of an excited electro-magnet; and found that the plane of polarization of a beam of light was rotated when the beam traveiled through the glass parallel to the lines of force of the magnetic field.