264 Wisconsin Academy of Sciences, Arts^ and Letters, 



where g^ is the value of q when the magetic force is zero. The 

 angle, 6, through which the plane of polarization is turned in pass- 

 ing through a thickness, c, of the medium, is half the sum of the 

 positive and negative values of q c, the sign of the result being 

 changed, because the sign of q is negative in equations (14). We 

 thus obtain 



a do 4:71 C i^ { . , di ' 



dr vp ' ;.2 ( ' dX) l-27rO}'-L 



VpA 



which is the complete form of the equation for determining the- 

 angle, through which the plane of polarization has been turned 

 bj the magnetic force while passing through a thickness of the 

 medium equal to c, and is, in its modified form the one with which 

 Yerdet's results have been compared. From this comparison of 

 the consequences of assuming the motions of light to be capable 

 of composition with the motions caused by electric currents, with 

 what experiment shows to be true of bodies conveying circularly 

 polarized light when also placed under magnetic strain, we have 

 probably good evidence for the opinion that some phenomenon of 

 rotation is going on in the magnetic field, that this rotation is per- 

 formed by a great number of very small portions of matter each 

 rotating on its own axis, this axis being parallel to the direction 

 of the magnetic force, and that the rotations of these different 

 vortices are made to depend on one another by means of some 

 kind of mechanism connecting them. The problem of determining- 

 the rL^echanism required to establish a given species of connection 

 between the motions of the parts of a system always admits of 

 an infinite number of solutions. Of these some may be more 

 clumsy tlian others, but all must satisfy the conditions of mechan- 

 ism in general."— Maxwell— .E'^eci;ric% and Magnetism, Chap, 

 xxi. 



Note.— On page 246 the radical sign ( v) should be the Greek letter 

 gamma {j^^ in formula No. (2). 



