250 Prof. J. J. Thomson, On the rotation of the plane [April 20, 



These formulae may however be derived at once from the series 

 for G by simple differentiation, since 



or, even more simply, by multiplying the series for — by 2 sin 20 

 and using the formula 



2 sin 20 sin (4m, + 1)0 = cos (4m -1)0- cos (4n + 3) 0. 



April 20, 1885. 

 Prof. Foster, President, in the Chair. 



The following communications were made to the Society : — 



(1) Note on the rotation of the plane of polarization of light 

 by a moving medium. By Prof. J. J. Thomson, M.A. 



In a paper in the Philosophical Magazine for April 1880, I 

 considered, assuming the Electromagnetic Theory of Light, some 

 of the effects produced by the motion of the medium which is the 

 seat of the electrostatic action. The motion was then supposed 

 to be translational. In this note I shall consider the case when 

 the motion of the medium is of the most general character which 

 a rigid body can possess. 



The notation is as follows : 



f g, h are the components of the electric displacements parallel 

 to the axes of x, y, z respectively. 



a, b, c the components of magnetic induction. 



F, G, H the components of the vector potential. 



P, Q, R the components of the electromotive force. 



p, q, r the components of the velocity of the medium; if as we 

 shall suppose the medium moves like a rigid body, we may look 

 on the velocity as made up of a motion of translation whose 

 components are u, v, w and a rotatory motion the components of 

 whose angular velocity are co 1} co 2 , co 3 . 



H is the magnetic permeability and K the specific inductive 

 capacity of the medium. 



