Closing Years of the Nineteenth Century. 453 



value as found from the first term, namely, eE/(ic z - run*). The 

 equation thus becomes 



r = K 2 - mn* + (i'-wm 1 ) 1 ^'^' 

 If P denote* the electric moment" per unit volume, we have 



P = ei x the number of such systems in unit volume of the 

 medium ; 



so P must be of the form 



where e evidently represents the dielectric constant of the 

 medium, and o- is the coefficient which measures the magnetic 

 rotatory power. In the magneto-optic term we may replace 

 H by K, the external magnetic force, since this is large com- 

 pared with the magnetic force of the luminous vibrations. 

 Thus if D denote the electric induction, we have 



D = fE/47rc 2 + <r [E . K]. 



'Combining this with the usual electromagnetic equations, 



curl H = 47ri>, 



curl E = - H, 

 we have 



- curl curl E = *E/c 2 + 4:r<r [E . K]. 



When a plane wave of light is propagated through the 

 medium in the direction of the lines of magnetic force, and 

 the axis of x is taken parallel to this direction, the equation 

 gives 



(VEy 



.and these equations, as we have seen,f are competent to explain 

 the rotation of the plane of polarization. 



*Cf. p. 428. t Cf. p 215. 



