428 SCIENCE PROGRESS. 



Zeeman effect in the substances already studied and from 

 the observed dispersion in air, we find that it is of about 

 the amount that has been observed for the Faraday effect 

 in air. So far, then, it appears probable that the whole 

 cause of the Faraday effect is to be looked for in each sub- 

 stance in a Zeeman change of the free vibrations of the 

 molecules of the substance by the action of magnetic force, 

 this change in the free periods reacting through their 

 resonances on the rate of propagation of the circularly 

 polarised components of light passing through the mag- 

 netised medium. 



A theory on similar lines may be worked out to explain 

 the Kerr effect and the Hall effect : this latter being an 

 action on electrons moving continuously in one direction, 

 not on simply vibrating ones. This has been gone into by 

 Mr. Larmor in his valuable papers on Electromagnetic 

 Theory in recent volumes of the Phil. Trans. 



A very interesting and fundamental question is raised 

 by all this theory as to the cause of the Faraday effect 

 when we compare it with former theories as to its cause. 

 Former theories all attributed it to a gyrostatic action 

 accompanying propagation of waves along lines of magnetic 

 force. The illustrations of magnetic rotation of plane of 

 polarisation have all been derived from the propagation of 

 waves through media in which rotating gyroscopes played 

 the part of the magnisation of the matter. It is, in fact, 

 impossible not to fall back directly or indirectly upon a 

 rotation when we look for an explanation of the Faraday 

 effect upon dynamical principles. Yet, where, in the theory 

 here profounded, is any rotation postulated or gyrostatic 

 action involved ? The action of magnetic force on a mov- 

 ing electron, which is postulated to be the same as on 

 an electric current, does not appear at first sight to involve 

 gyrostatic actions. Yet the whole theory given depends 

 on this action and on dynamical effects which do not in any 

 way involve gyrostatic effects. Mr. Larmor, for example, 

 supposes magnetic force to be due to an irrotational flow 

 in the ether and an electron to be a singular point sur- 

 rounded by a peculiar condition of twist. Where in this is 



