1889.] Magnetic Rotation of the Plane of Polarisation. 65 



May 9, 1889. 



Professor G. G. STOKES, D.O.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The Right Hon. Baron Henry de Worms was admitted into the 

 Society. 



The following Papers were read : — 



I. " On the Magnetic Rotation of the Plane of Polarisation of 

 Light in donbly refracting Bodies." By A. W. Ward. 

 Communicated by Professor J. J. Thomson, F.R.S. Re- 

 ceived April 13, 1889. 



In repeating Villari's* experiment on the rotation of the plane of 

 polarisation of light in a spinning disk of heavy glass, placed with its 

 axis of rotation perpendicular to the lines of force in a magnetic field, 

 it was observed that the incident plane polarised light became ellip- 

 tically polarised. The elliptic polarisation was due to the centrifugal 

 force which had the effect of stretching the glass along the radii of the 

 disk and compressing it parallel to the axis of rotation. The strained 

 glass in the magnetic field has, therefore, the double property of 

 elliptically polarising plane polarised light, and at the same time 

 rotating the plane of polarisation. The strained glass therefore acted 

 like a crystal placed in a magnetic field, and so before Villari's experi- 

 ment could be properly interpreted, it was necessary to examine how 

 the elliptic polarisation and magnetic rotation affect each other. The 

 following investigation is an attempt to solve this question, and its 

 conclusions show that the apparent magnetic rotation in a doubly 

 refractive medium is a periodic function of the length of the path of 

 light in the medium. This result entirely accounts for the effects 

 observed by Villari, and those observed by Liidtge in a piece of com- 

 pressed glass. 



Let the axes of the doubly refracting diamagnetic medium be taken 

 as those of x and y, and let the axis of z be the direction in which the 

 light travels. 



Let a. be the inclination of the plane of vibration of the incident 

 light to that of xz. The equation of the incident light may be written 



# Yillari, ' Reiidiconti del Istituto Lornbardo/ 9 June, 1870. 

 VOL. XLTI. P 



