1898.] 



on Magneto-Optic Rotation. 



in 



regards the magneto-optic rotation as only a secondary effect of the 

 magnetisation, which is not supposed to exert any direct dynamical 

 influence on the transmission of the light- waves. 



It is not possible here to enter into the subject of these theories, 

 but I should like to direct attention to a paper by Mr. J. G. 

 Leathem, just published in the ' Philosophical Transactions,' in 

 which the type of theory just referred to has been worked out and 

 compared in its results with the experiments of Sissingh and Zeeman 

 in reflection. These investigators made measurements of the phase 

 and amplitude of the magneto-optic component of the reflected light 

 for various angles of incidence. For both these quantities the 

 theoretical results of Leathem agree very well with the observed 

 values. 



Eeturning now to the gyrostatic medium, between which and the 

 electro-magnetic theory, it is to be remembered, there is a corre- 

 spondence, we may inquire in what 

 way the gyrostats, when moved by 

 the vibrations of the medium, react 

 upon it, and so affect the velocity 

 of propagation. The motion of a 

 gyrostat is often regarded as mys- 

 terious, and it can hardly be fully 

 explained except by mathematical 

 investigation. But the general na- 

 ture of its action may be made out 

 without much difficulty. First of 

 all, a gyrostat consists of a massive 

 fly-wheel running on bearings at- 

 tached to a case which more or 

 less completely encloses the wheel. 

 The mass of the wheel consists in 

 the main of a massive rim, which 

 renders as great as possible what 

 is called the moment of momentum 

 of the wheel when rotating about its axis. The diagram (Fig. 13) 

 represents a partial section of the case and fly-vfheel of a gyrostat, 

 showing the arrangement of fly-wheel and bearings. 



Now let the fly-wheel of such a gyrostat be rapidly rotated, and the 

 gyrostat be hung up, as shown in this other diagram (Fig. 14), with the 

 plane of the fly-wheel vertical, and a weight attached to one extremity 

 of the axis. The gyrostat is not tilted over, but begins to turn round 

 the cord by which it is suspended with a slow angular motion which 

 is in the direction of the horizontal arrow if the direction of rotation 

 is that of the circular arrow shown in the case. The same thing is 

 shown by the experiment I now make. I spin this gyrostat, and hang 

 it with the axis of rotation horizontal by passing a loop of cord round 

 one end of the axis so that the weight of the gyrostat itself forms the 

 weight tending to tilt it over about the point of suspension. The 



Fig. 13. 



