88 Prof. Maxwell on the Theory of Molecular Vortices 



the vortices accounts for induced currents, is supported by the 

 opinion of Professor W. Thomson*. In fact the whole theory of 

 molecular vortices developed in this paper has been suggested to 

 me by observing the direction in which those investigators who 

 study the action of media are looking for the explanation of elec- 

 tro-magnetic phenomena. 



Professor Thomson has pointed out that the cause of the mag- 

 netic action on light must be a real rotation going on in the 

 magnetic field. A right-handed circularly polarized ray of light 

 is found to travel with a different velocity according as it passes 

 from north to south, or from south to north, along a line of mag- 

 netic force. Now, whatever theory we adopt about the direction 

 of vibrations in plane-polarized light, the geometrical arrange- 

 ment of the parts of the medium during the passage of a right- 

 handed circularly polarized ray is exactly the same whether the 

 ray is moving north or south. The only difference is, that the 

 particles describe their circles in opposite directions. Since, 

 therefore, the configuration is the same in the two cases, the forces 

 acting between particles must be the same in both, and the mo- 

 tions due to these forces must be equal in velocity if the medium 

 was originally at rest ; but if the medium be in a state of rota- 

 tion, either as a whole or in molecular vortices, the circular vibra- 

 tions of light may differ in velocity according as their direction 

 is similar or contrary to that of the vortices. 



We have now to investigate whether the hypothesis developed 

 in this paper — that magnetic force is due to the centrifugal force 

 of small vortices, and that these vortices consist of the same 

 matter the vibrations of which constitute light — leads to any 

 conclusions as to the effect of magnetism on polarized light. 

 We suppose transverse vibrations to be transmitted through a 

 magnetized medium. How will the propagation of these vibra- 

 tions be affected by the circumstance that portions of that me- 

 dium are in a state of rotation ? 



In the following investigation, I have found that the only effect 

 which the rotation of the vortices will have on the light will be 

 to make the plane of polarization rotate in the same direction as 

 the vortices, through an angle proportional — 



(A) to the thickness of the substance, 



(B) to the resolved part of the magnetic force parallel to theray, 



(C) to the index of refraction of the ray, 



(D) inversely to the square of the wave-length in air, 



(E) to the mean radius of the vortices, 



(F) to the capacity for magnetic induction. 



* Sec Nichol's Cyclopedia, art. " Magnetism, Dynamical Relations of," 

 edition I860; Proceedings of Roval Society, June 1856 and June 1861; 

 and Phil. Mag. 1857. 



