1876.] 



On the Plane of Polarization of Light. 



447 



December 21, 1876. 

 Dr. J. DALTON HOOKER, C.B., President, in tlie Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. *^0n the Potation of the Plane of Polarization of Light by 

 Peflection from the Pole of a Magnet.^^ By Oeorge Francis 

 Fitzgerald, M.A. Commnnicated by G. Johnstone Stoney, 

 F.P.S. Received November 14, 1876. 



At a meeting of the Dublin Scientific Club on Monday the 6th IN'o- 

 vember, Professor Barrett gave the Club an account of Mr. Kerr's 

 experiments on the rotation of the plane of polarization of a ray of Hght 

 when reflected from the surface of the end of a magnet, to which addi- 

 tional interest was attached by the reading of a letter from Mr. Kerr to 

 Professor Barrett giving an account of the mode of making and of the 

 last results of his experiments. At the time I proposed trying whether 

 any similar effects would be produced by reflection from the surface of a 

 crystal of quartz cut perpendicularly to the axis, as I was led to think 

 there might be, owing to the similarity of the rotatory polarization of 

 quartz .and of substances under magnetic action. Following out that 

 clue, I obtained the following explanation of Mr. Kerr's experiment, and 

 was enabled, through Professor Barrett's kindness in helping me to 

 verify my recollections of Mr. Kerr's letter, to make sure that my theory 

 explains the facts. 



Faraday has shown, in the nineteenth series of his experimental re- 

 searches, that a ray of plane-polarized light, when transmitted through 

 any solid (diamagnetic ?) transparent medium under the action of a 

 powerful magnet, has the plane of its polarization rotated in that direc- 

 tion in which a positive current must circulate round the ray in order to 

 produce a magnetic force in the same direction as that which actually 

 exists in the medium. Yerdet, however, discovered that in certain /^rro- 

 magnetic media (as, for instance, a strong solution of perchloride of iron 

 in wood-spirit or ether) the rotation is in the opposite direction to the 

 current which would produce the magnetic force. 



Now Fresnel's explanation of the rotatory power of quartz has been 

 applied by Professor Maxwell, in his * Electricity and Magnetism/ 

 vol. ii. p. 402, to explain the similar, though not identical, phenomenon 

 of magnetic rotation of light. He there, in § 812, gives this explanation 

 - in the following words : — "A plane-polarized ray falls on the medium. 



