Faraday. 215 



plane of polarization rotates in unit length of path is a numerical 

 multiple of 



where T denotes the period of the light. Now it was shown by 

 Verdet* that the magnetic rotation is approximately proportional 

 to the inverse square of the wave-length ; and hence we must 



have 



r + s= 3; 



so that the only equations capable of correctly representing 

 Faraday's effect are either 



w ^dx-dt 



or 



dt* W * dt 



d'Z _ 2 d'Z 

 W '" l ~W~^'W 



The former pair arise, as will appear later, in Maxwell's 

 theory of rotatory polarization : the latter pair, which were 

 suggested in 1868 by Boussinesq,f follow from that physical 

 theory of the phenomenon which is generally accepted at the 

 present time.* 



Airy's work on the magnetic rotation of light was limited 

 in the same way as MacCullagh's work on the rotatory power 

 of quartz ; it furnished only an analytical representation of the 

 effect, without attempting to justify the equations. The earliest 

 endeavour to provide a physical theory seems to have been 

 made in 1858, in the inaugural dissertation of Carl Neumann, 



* Comptes Rendus, Ivi (1863), p. 630. 



t Journal de Math., xiii (1868), p. 430. 



J Fand Z being interpreted as components of electric force. 



