

Faraday. 213 



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,f " 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 1834J 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 

 travelled through the glass parallel to the lines of force of the 

 magnetic field. 



*e.g. by Morichini, of Rome, in 1813, Quart. Journ. Sci. xix, p. 338; by 

 Samuel Hunter Christie, of Cambridge, in 1825, Phil. Trans., 1826, p. 219 ; and 

 by Mary Somerville in the same year, Phil. Trans., 1826, p. 132. 



t Sir. J. Herschel in Bence Jones's Life of Faraday, p. 205. 



lExp. Res., 951. \Ib., 2152. 



