

236 Mr. S. IS. Richardson on Magnetic Rotary 



will not contain a , and therefore the electrons of extra-high 

 frequency have no influence on magnetic rotation. 



The effect of molecular distortion on the periods of the 

 vibrating particles may be represented by a small change in 

 the impressed field, as it must be proportional to the field in 

 order that the direct proportion between 8 and H observed 

 experimentally may be retained. Representing the effective 

 field by (l + f)H, we have 



We may note in passing that the effect denoted by £ is 

 similar to the effect of solvents or reagents in opening up 

 the electromagnetic force-fields of a molecule, which has been 

 shown by Prof. Baly to account for the development of new 

 bands in the absorption spectrum. 



Reference must here be made to Drude's earlier or mole- 

 cular-current theory of magnetic rotation *. The phenomenon 

 is there attributed to the displacement of the molecular 

 orbits for orbits of revolving electrons) about positions of 

 equilibrium by the passing of the light- wave, such displace- 

 ment being always in the wave-front. If, for simplicity, we 

 confine ourselves to the case where the magnetic field is 

 directed along Oz, and to plane waves propagated in this 

 direction, an examination of Drude's analysis shows that the 

 term responsible for the magnetic rotation arises from the 

 periodic variation of the mean molecular field through the 

 element of area dydz. This variation, as Drude states, must 

 be due to a rotation of the mean molecular field about the 

 axis Oy, and this rotation is then represented by 



d( rff\ 



dydz-^t, 



dt 



where 7! is the density of the molecular field and (f, tj, f) 

 the displacement of the orbit. Such a term, however, 

 represents merely a shearing of the molecular orbits parallel 

 to the plane asz, and would not produce a variation in the 

 flux through the element dydz unless accompanied by rota- 

 tion of the orbits themselves about axes parallel to Oy. Such 

 rotation is, however, not assumed by Drude, for as he points 

 out the magnetic field in the light- wave is too small in com- 

 parison with the field impressed in actual experimental 

 measurements. Hence we may conclude that no effect 

 arises in ordinary cases from orbital displacements. More- 

 over, as is well known, the theory requires opposite rotations 

 on the two sides of an absorption band, and the only known 

 * ' Theory of Optics'; or Optik, 2te Aufl. S. 417. 



