and Magnetic Rotation of Polarized Light. 259 



W = 1_ 



i dX 



and the angle of rotation 6 will become 



which is of the same form as Maxwell's expression. Now 

 Maxwell's equation is obtained from considerations entirely 

 different from any which I have used in this paper. In ob- 

 taining them, Maxwell made no assumption as to the kind of 

 motion which constitutes light, but merely assumed that the 

 magnetic lines of force were vortices, and that the motion of 

 the vortices caused a rotation of the motion constituting light. 

 In my theory I have used no hypothesis as to the nature of 

 magnetic force; but have simply calculated, from the known 

 laws of magnetism and electricity, the action in this case 

 according to Maxwell's theory of light. And the conclusion 

 which we draw is that the effect discovered by Mr. Hall is the 

 same, or due to the same cause, as the rotation of the plane of 

 •polarization of light. 



It is interesting to repeat here the comparison made by 

 Verdet between the various formulas and observation. 



The formulas of Maxwell and Rowland, of Airy, and of 

 Neumann are 



'- M 50- x S^ (L) 



^ M iO-4lK en-) 



e=M(i-\ c ^)i)c 1 (in.) 



The comparison of these formulae with the experiments of 

 Verdet* are as follows: — 



Bisulphide of Carbon. 



0. 



D. 



E. 



F. 



G. 



Observed rotation . 0*592 



0*768 



1*000 



1*234 



1*704 



Calculated, formula I. 0*589 



0*760 



1*000 



1*234 



1*713 



„ „ II. 0*606 



0*772 



1*000 



1*216 



1*640 



„ III. 0*943 



0*967 



1-000 



1*034 



1*091 



Verdet, (Euvres, vol. i. p. 262, or Maxwell's ' Electricity/ art. 830. 



