254 



Wisconsin Academy of Sciences, Arts, and Letters. 



hj magnetism, "not as giving a mechanical explanition of the phe- 

 nomena, but as showing that the phenomena may be explained 

 by equations, which equations appear to be such as might possi- 

 bly be deduced from some plausible mechanical assumption, 

 although no such assumption has been made." 



This explanation of what rotatory polarization is, as it occurs in 

 bodies which of themselves rotate the plane of polarization, may 

 Fig. 16. help to an understanding of Fig- i^- 



the manner in which an 

 electric current, circulating / 

 around a medium throus-h 

 which circularly polarized 

 light is passing, may possi- 

 bly affect the velocity of 

 either circular component of the polarized light, and thus, accord' 

 ing as the direction of the current is with a circular componentr 

 as in Fig. 16, or against it, as in Fig. 17, produce a right-handed 

 or a left-handed rotation, according to the direction in which the 

 current circulates around the medium. 



Of this latter, Sir Wm. Thompson, in 1856, made the important 

 observation,^ which Prof. Clerk Maxwell has elaborated into the 



^"The magnetic influence on light, discovered by Faraday, depends on the 

 direction of motion of moving particles. For instance, in a medium possess- 

 ing it, particles in a straight line parallel to the lines of magnetic force, dis- 

 placed to a helix round this line as axis, and then projected tangentially with 

 such, velocities as to describe circles, will have diiferent velocities, according 

 as their motions are round in one direction (the same as the nominal direc- 

 tion of the galvanic current in the magnetizing coil) or in the contrary direc- 

 tion. But the elastic reaction of tlie medium must be the same for the same 

 displacements, vfhatever be the velocities and directions of the particles; that 

 is to say, the forces which are balanced by centrifugal force of the circular 

 motions are equal, w^hile the luminiferous motions are unequal. The absolute 

 circular motions being therefore either equal or such as to transmit equal cen, 

 trifugal forces to the particles initially considered, it follows that the lumi- 

 niferous motions are only components of the whole motion; and that a less 

 luminiferous component in one direction, compounded with a motion ex- 

 isting in the medium when transmitting no light, gives an equal resultant to 

 that of a greater luminiferous motion in the contrary direction compounded 

 with the same non-luminous motion. I think it is not only impossible to 

 conceive any other than this dynamical explanation of the fact that circularly- 



