OPTICAL PHENOMENA. 



601. OBSERVATIONS OF SIR W, THOMSON. According to Sir 

 W. Thomson, the phenomena of magnetic rotatory polarization 

 would appear to confirm Ampere's ideas on the ultimate nature of 

 magnetism. 



"The magnetic influence on light, discovered by Faraday, de- 

 pends on the direction of motion of moving particles. For instance, 

 in a medium possessing it, particles in a straight line perpendicular 

 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 current in the magnetising coil), or in the 

 contrary direction. But the elastic reaction of the medium must 

 be the same for the same displacements, whatever be the velocities 

 and directions of the particles; that is to say, the forces which 

 are balanced by the 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 component in one direction, 

 compounded with a motion existing in the medium when trans- 

 mitting 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 polarized 

 light transmitted through magnetised glass parallel to the lines of 

 magnetising 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 demon- 

 strated 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 magnetisation in the dynamical 

 theory of heat. 



"The introduction of the principle of moments of momenta 

 ('the conservation of areas') into the mechanical treatment of 

 Mr. Rankine's hypothesis of ' molecular vortices,' appears to in- 

 dicate a line perpendicular to the plane of resultant rotatory 

 momentum (' the invariable plane ') of the thermal motions, as the 



