151 



properties if the stress at any point of it was influenced only by 

 parts of the body touching it ; but if the stress at one point is 

 directly influenced by the strain in parts at distances from it finite 

 in comparison with the wave length, the he^oidal property might 

 exist, and the rotation of the plane of polarization, such as is observed 

 in many liquids and in quartz crystals, could be explained as a direct 

 dynamical consequence of the statical elastic reaction called into 

 play by such a strain as exists in a wave of polarized light. It may, 

 however, be considered more probable that the matter of transparent 

 bodies is really heterogeneous from one part to another of lineal di- 

 mensions not infinitely small in comparison with a wave length, than 

 that it is infinitely homogeneous and has the property of exerting 

 finite direct "molecular" force at distances comparable with the 

 wave length : and it is certain that any spiral heterogeneousness of a 

 vibrating medium must, if either right-handed or left-handed spirals 

 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 hi 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 



