On Rotation of the Plane of Polarization of Light. 529 



December 21. — Dr. J. Dalton Hooker, C.B., President, in the 



Chair. 



The following papers were read : — 



" On the Rotation of the Plane of Polarization of Light by 

 Reflection from tho Pole of a Magnet." By George Francis Pitz- 

 gerald, M.A. 



At a meeting of the Dublin Scientific Club on Monday the 6th 

 November, Professor Barrett gave the Club an account of Mr. 

 Kerr's experiments on the rotation of the plane of polarization of 

 a ray of light when reflected from the surface of the end of a 

 magnet, to which additional 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 experi- 

 ment, 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. 



Paraday has shown, in the nineteenth series of his experimental 

 researches, 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 direction 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. Verdet, 

 however, discovered that in certain ferro-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 Presnel's explanation of the rotatory power of quartz has 

 been applied by Professor Maxwell, in his ' Electricity and Mag- 

 netism,' 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. This is equivalent to two circularly 

 polarized rays, one right- and the other left-handed (as regards the 

 observer). After passing through the medium the ray is still plane- 

 polarized, but the plane of its polarization is turned, say, to the 

 right (as regards the observer). Hence of the two circularly pola- 

 rized rays, that which is right-handed must have had its phase 

 accelerated with respect to the other during its passage through 

 the medium. In other words, the right-handed ray has performed 

 a greater number of vibrations, and therefore has a smaller wave- 

 length within the medium than the left-handed ray which has the 

 same periodic time." This is the same as saying that the velocity 

 Phil Mag. S. 5. No. 21. Suppl. Vol. 3. 2 M 



