710 



Professor Andrew Gray 



[April 29, 



which we have hardly time to make, to exclude all extraneous light 

 from the screen. 



It would perhaps be incorrect to say that the plane of polarisation 

 has been rotated in this case, as it has been asserted by Eighi that the 

 light after reflection is no longer plane polarised, but that there are two 

 components of vibration at right angles to one another, so related that 

 the resultant vibration is not rectilinear but elliptical. There is there- 

 fore no position in which the analysing prism can be placed so as to 

 extinguish the reflected light. The transverse component necessary 

 to give the ellijDtic vibration is, however, in this case, if it exists, very 

 small, and very nearly complete extinction of the beam can be obtained 

 by turning the analysing prism round so as to stop the other com- 

 ponent vibration. The angle through which the prism must be turned 

 to effect this is the amount of theaj^parent rotation. The direction 

 of rotation is reversed by reversing the magnetism of the reflecting 

 pole. Dr. Kerr found that the direction is always that in which the 

 current flows in the coils producing the magnetisation of the pole. 



Fig. 7. 



Dr Kerr also made experiments with light obliquely incident on a 

 pole-face, with the arrangement of apparatus shown in this other dia- 

 gram (Fig. 7). He found that the previously plane polarised light was 

 by the reflection rendered slightly elliptically polarised. A slight 

 turning of the analysing Nicol was necessary to place it so as to stop 

 the vibration corresponding to the long axis of the ellipse and so 

 secure imperfect extinction. 



These effects are, like those of normal incidence, very small, and 

 they can hardly be shown to an audience. 



I must now endeavour to give some slight account of the theories 

 that have been put forward in explanation of magneto-optic rotation. 

 There is an essential distinction between it and what is sometimes 

 called the natural rotation, the plane of polarised light produced 

 by substances, such as solutions of sugar, tartaric acid, quartz, &c., 

 some of which rotate the plane to the right, some to the left. When 



