716 Professor Andrew Gray [April 29, 



We pass now to the other case, that of magneto-optic rotation. 

 Let us suppose, to fix the ideas, that the right-banded circular ray- 

 travels faster than the other, and that whether direct or reversed. 

 Here, as in the other case, the elastic reaction of the medium on the 

 displaced particles depends only on the distortion, and if there be no 

 structural peculiarity in the medium there must be the same reaction 

 in the particles in both the circular waves which combine to make 

 up the plane polarised one. 



Thus the actions on the particles being the same for both waves, 

 and the velocities of propagation being different, the motions con- 

 cerned in the light propagation cannot be the same. There must in 

 fact be a motion already existing in the medium which, compounded 

 with the motions concerned in light propagation, give two motions 

 which give equal reactions in the medium against the equal elastic 

 forces, applied to the particles in the case of equal helical displace- 

 ments. 



Thus Lord Kelvin supposes that in the medium in the magnetic 

 field there exists a motion capable of being compounded with the 

 luminiferous motion of either circularly polarised beam. The latter 

 is thus only a component of the whole motion. 



In the very important paper in which he has set forth his theory 

 Lord Kelvin expresses his strong conviction that his dynamical 

 explanation is the only possible one. If this view be correct, 

 Faraday's magneto-optic discovery affords a demonstration of the 

 reality of Ampere's theory of the ultimate nature of magnetism. For 

 we have only to consider the particles of a magnetised body as 

 electrons or groups of charges of electricity, ultimate as to smallness, 

 rotating about axes on the whole in alignment along the direction of 

 the magnetic force, and with a preponderance of one of the two 

 directions of rotation over the other. Each rotating molecule is an 

 infinitesimal electro-magnet, of which the current distribution is 

 furnished by the system of convection currents constituted by the 

 moving charges. 



The subject of magneto-optic rotation has also been considered by 

 Larmor, and two types of theory of these effects have been indicated 

 by him in his report on the ' Action of Magnetism on Light.' One 

 is represented by Lord Kelvin's theory, which is illustrated by 

 Maxwell's chapter on molecular vortices in his ' Electricity and 

 Magnetism.' FitzGerald's paper " On the Electromagnetic Theory 

 of the Reflection and Refraction of Light," in the ' Philosophical 

 Transactions' for 1880, is related to Maxwell's theory, and ex- 

 plains the rotation produced by reflection from the pole of a magnet 

 by means of the addition of a term to the energy of the system. 

 The other theory is also a purely electromagnetic one, and supposes 

 that the effects are due to a kind of seolotropy of the medium set up by 

 the magnetisation, and so attributes them to a change of structure 

 which introduces rotational terms into the equations connecting 

 electric displacements and electric forces. This latter theory therefore 



