ROTATION OF THE YLANE OF POLARIZATION. 627 



and the rotation w, for unit difference of potential, becomes 



If the refractive index were independent of the dispersion of 

 the medium, the rotation of the plane of polarization will be 

 inversely as the square of the wave length, which is approximately 

 the law of magnetic rotation. 



We have seen (597) that, to allow for the dispersion of the 



medium, this result must be multiplied by the factor (i ) ; 



\ n A/ 



we get then 



irn 



This, apart from the factor /*, is the formula at which Maxwell 

 had arrived by the theory of molecular vortices, and that which 

 best accords with experiment. We see that, other things being 

 equal, the rotatory power is inversely as the coefficient of per- 

 meability /A. 



s s 2 



