368 The Followers of Maxwell. 



light, Fitz Gerald proceeded to extend it so as to take account 

 of a closely related phenomenon. In 1876 J. Kerr* had shown 

 experimentally that when plane-polarized light is regularly 

 reflected from either pole of an iron electromagnet, the reflected 

 ray has a component polarized in a plane at right angles to the 

 ordinary reflected ray. Shortly after this discovery had been 

 made known, Fitz Geraldf had proposed to explain it by means 

 of the same term in the equations which accounts for the mag- 

 netic rotation of light in transparent bodies. His argument was 

 that if the incident plane-polarized ray be resolved into two 

 rays circularly polarized in opposite senses, the refractive index 

 will have different values for these two rays, and hence the 

 intensities after reflexion will be different; so that on re- 

 compounding them, two plane-polarized rays will be obtained 

 one polarized in the plane of incidence, and the other polarized 

 at right angles to it. 



The analytical discussion of Kerr's phenomenon, which was 

 given by Fitz Gerald in his memoir of 1879, was based on these 

 ideas ; the most essential features of the phenomenon were 

 explained, but the investigation was in some respects imperfect.} 



Anew and fruitful conception was introduced in 1879-1880, 

 when H. A. Eowland suggested a connexion between the 

 magnetic rotation of light and the phenomenon which had been 

 discovered by his pupil Hall.|| Hall's effect may be regarded 



* Phil. Mag. (5) iii (1877), p. 321. 



t Proc. 11. S. xxv (1877), p. 447 ; Fitz Gerald's Sclent. Writings, p. 9. 



J Cf. Larmor's remarks in his Report on the Action oj Magnetism on Light, 

 Brit. Assoc. Kep., 1893 ; and his editorial comments in Fitz Gerald's Scientific 

 Writings. Larmor traced to its source an inconsistency in the equations hy which 

 Fitz Gerald had represented the boundary-conditions at an interface between the 

 media. Fitz Gerald had indeed made the mistake, similar to that which was so often 

 made hy the earlier writers on the elastic-solid theory of light, of forgetting that when 

 a medium is assumed to be incompressible, the condition of in compressibility must 

 be introduced into the variational equation of motion (as was done supra, p. 172). 

 Larmor showed that when this correction was made, new terms (resembling the 

 terms in p, supra, p. 172) made their appearance; and the inconsistency in the 

 equations was thus removed. 



Amer. Jour. Math, ii, p. 354, iii, p. 89; Phil. Mag. xi (1881), p. 254. 



|| Cf. p. 321. 



