1898.] on Magneto-Optic Rotation. 707 



If the room is sufficiently darkened, I think all will see that when the 

 magnet is excited there is a very perceptible brightening of the dim 

 patch of light on the screen, and that this brightening disappears 

 when the current is removed from the magnet. This is Faraday's 

 discovery. 



How are we to describe this result ? What effect has been pro- 

 duced by the magnetic field? It is clear that the direction of 

 vibration of the light emerging from the specimen of heavy glass has 

 been changed relatively to the prism so that the light now readily 

 passes. It is found, moreover, that the amount of turning of the direc- 

 tion of vibration round the ray is proportional to the length of the 

 specimen, so that the directions of vibration at different points along 

 the wave within the specimen lie on a helically twisted surface, and 

 may be regarded as represented by the straight rods in the model 

 before you on the table (Fig. 5), 



Fig. 5. 



It is also found that the amount of the turning depends on the 

 intensity of the magnetic field — is, in fact, simply proportional to that 

 intensity. Hence the turning is proportional to the mean intensity of 

 the field, and to the length of the path in the medium, that is, to the 

 product of these two quantities. It also depends on the nature of the 

 medium. The angle of turning produced by a field of known intensity 

 when the ray passes through bisulphide of carbon has been very 

 carefully measured by Lord Kayleigh, whose results are of great value 

 for other magnetic work. 



The law of proportionality of the amount of turning of the plane 

 of polarisation to the intensity of the magnetic field in the space 

 in which the substance is placed, is not, however, to be regarded 

 as established for strongly magnetic substances, such as iron, nickel 

 or cobalt. The matter has not yet been completely worked out, but 

 the turning in such cases seems to be more nearly proportional to 

 the intensity of magnetisation, a different quantity from the intensity 

 of the magnetic field producing the magnetisation. If this law be 



