216 Faraday. 



of Halle.* Neumann assumed that every element of an electric 

 current exerts force on the particles of the aether ; and in parti- 

 cular that this is true of the molecular currents which constitute 

 magnetization, although in this case the force vanishes except 

 when the aethereal particle is already in motion. If e denote the 

 displacement of the aethereal particle ra, the force in question 

 may be represented by the term 



km [ e. K] 



where K denotes the imposed magnetic field, and k denotes a 

 magneto-optic constant characteristic of the body. When this 

 term is introduced into the equations of motion of the aether, 

 they take the form which had been suggested by Airy ; whence 

 Neumann's hypothesis is seen to lead to the incorrect conclusion 

 that the rotation is independent of the wave-length. 



The rotation of plane-polarized light depends, as Fresnel had 

 shown,f on a difference between the velocities of propagation of 

 the right-handed and left-handed circularly polarized waves into 

 which plane-polarized light may be resolved. In the case of 

 magnetic rotation, this difference was shown by Verdet to be 

 proportional to the component of the magnetic force in the 

 direction of propagation of the light; and CornuJ showed further 

 that the mean of the velocities of the right-handed and left- 

 handed waves is equal to the velocity of light in the medium 

 when there is no magnetic field. From these data, by Fresnel's 

 geometrical method, the wave- surf ace in the medium may be 

 obtained; it is found to consist of two spheres (one relating 

 to the right-handed and one to the left-handed light), each 

 identical with the spherical wave-surface of the unmagnetized 

 medium, displaced from each other along the lines of magnetic 

 force. 



The discovery of the connexion between magnetism and 



* Explicare tentatur, quomodojiat, ut lucis planum polarisationis per vires el. vel 

 mag. declinetur. Halis Saxonum, 1858. The results were republished in a tract 

 Die magnetische Drehung der Polarisationsebene des Lichtes. Halle, 1863. 



t Cf. p. 174. J Comptes Rendus, xcii (1881), p. 1368. 



Cornu, Comptes Rendus, xcix (1884), p. 1045. 



