Prof. Thomson on Polarized Light. 199 



predominate, cause a finite rotation of the plane of polarization of all 

 waves of which lengths are not infinitely great multiples of the steps 

 of the structural spirals. Thus a liquid filled homogeneously with 

 spiral fibres, or a solid with spiral passages through it of steps not 

 less than the forty-millionth of an inch, or a crystal with a right- 

 handed or a left-handed geometrical arrangement of parts of some such 

 lineal dimensions as the forty- millionth of an inch, might be certainly 

 expected to cause either a right-handed or a left-handed rotation of 

 ordinary light (the wave length being jj^th of an inch for homoge- 

 neous yellow). 



But the magnetic influence on light discovered by Faraday depends 

 on the direction of motion of moving particles. For instance, in a 

 medium possessing it, particles in a straight line parallel to the lines 

 of magnetic force, displaced to a helix round this line as axis, and 

 then projected tangentially with such velocities as to describe circles, 

 will have different velocities according as their motions are round in 

 one direction (the same as the nominal direction of the galvanic cur- 

 rent in the magnetizing coil), or in the contrary direction. But the 

 elastic reaction of the medium must be the same for the same dis- 

 placements, whatever 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, while the luminiferous motions are 

 unequal. The absolute circular motions being therefore either 

 equal or such as to transmit equal centrifugal forces to the particles 

 initially considered, it follows that the luminiferous motions are only 

 ^components of the whole motion ; and that a less luminiferous com- 

 ponent in one direction, compounded with a motion existing in the 

 medium when transmitting no light, gives an equal resultant to that 

 I greater luminiferous motion in the contrary direction com- 

 pounded vrith 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 polarized light transmitted through mag- 

 netized glass parallel to the lines of magnetizing force, with the same 

 quality, right-handed always, or left-handed always, is propagated at 

 different rates according as its course is in the direction or is contrary 

 to the direction in which a north magnetic pole is drawn ; but I 

 believe it can be demonstrated that no other explanation of that fact 

 is possible. Hence it appears that Faraday's optical discovery 

 affords a demonstration of the reality of Ampere's explanation of the 

 ultimate nature of magnetism ; and gives a definition of magnetiza- 

 tion in the dynamical theory of heat. The introduction of the prin- 

 ciple of moments of momenta ("the conservation of areas") into 

 the mechanical treatment of Mr. Kankine's hypothesis of " molecular 

 vortices," appears to indicate a line perj)endicular to the plane of re- 

 sultant rotatory momentum ("the invariable ])lane") of the thermal 

 motions as the magnetic axis of a magnetized body, and suggests the 

 resultant moment of momenta of these motions as the definite mea- 

 sure of the " magnetic moment." The explanation of all phenomena 

 of electro-magnetic attraction or repulsion, and of electro-magnetic 

 induction, ia to be looked for bimply in the inertia and pressure of 



