Dispersion in Relation to the Electron Theory. 237 



instances of this are exhibited by certain absorption bands 

 of one or two rare earths examined by G. J. Elias *, and in 

 those of crystals (tysonite, xenotime) at the temperature of 

 liquid air brought to light by the remarkable experiments of 

 J. Becquerel |. These isolated instances (the substances are 

 all paramagnetic) doubtless arise from some other cause. 

 Becquerel, in fact, explains them as due to positively charged 

 vibrators combined with differential absorption. 



In the case of very strongly magnetic substances cohesion 

 between the components of a molecular chain might induce 

 an appreciable variation of flux through orbital displacements, 

 and it is possible that the rotation observed by Kundt in 

 transparent films of iron and other magnetic metals may be 

 due to this cause. In view of the fact that the wave-length 

 of light is some 10 3 molecular diameters, no effect is to be 

 expected from such cohesion, except in the case of ferro- 

 magnetic metals- In general, therefore, the orbital dis- 

 placement will be without effect. 



A modification of equation (4) has been introduced by 

 Ulfilas Meyer J, namely, 



£_ en' dn' 



2n^A? n dX' 



In this expression n' is such that n" 2 — 1 is the value of the 

 right-hand member of (1) when the terms corresponding to 

 the infra-red periods are omitted. It is deduced from the 

 Hall-effect formula of Drude by omitting the infra-red and 

 high-frequency terms and assuming that e/m has the same 

 value for all the terms retained. Meyer obtains a good 

 agreement between theory and experiment in the case of 

 sylvin, rock-salt, and fluor-spar. 



In what follows we shall retain the summational formula. 

 Putting 



*-( l+ 6& «-( 1+ *)£ 



we have 



S= 55S? ( 6 ^fc=v) 2 + **(jfS?) + • • • } ' (7) 



or setting <£ = ?26\ 2 and &i= qTss, &c., 



* Ann. d. Phys. (35) p. 299 (1911). 

 t Phil. Mag. xvi. p. 153 (1908). 

 + Ann. d, Phys. (30), p. 607 (1909). 



