AND MODEKX PHYSICS. 201 



some close connection between electric waves and 

 those of light Faraday's experiments on the rota- 

 tion of the plane of polarisation by magnetic force 

 shew one phenomenon in which the two are con- 

 nected, and Maxwell endeavoured to apply his theory 

 to explain this. Here, however, it became necessary 

 to introduce an additional hypothesis there must be 

 some connection between the motion of the ether 

 to which magnetic force is due and that which con- 

 stitutes light. It is impossible to give a mechanical 

 account of the rotation of the plane of polarisation 

 without some assumption as to the relation between 

 these two kinds of motion. Maxwell, therefore, 

 supposes the linear displacements of a point in the 

 ether to bo those which give rise to light, while the 

 components of the magnetic force are connected with 

 these in the same way as the components of a vortex 

 in a liquid in vortex motion are connected with the 

 displacements of the liquid. He further assumes the 

 existence of a term of special fonu in the expression 

 for the kinetic energy, and from these assumptions he 

 deduces the laws of the propagation of polarised 

 light in a magnetic field. These laws agree in the 

 main with the results of Verdet's experiments. 



