Magnetic Field on Radiation Frequency. 515 



alter the free periods of the spectral lines of the substance. Bat the 

 actual phenomena do not seem to be thus reciprocal. On the 

 electric theory of light it is only the dispersion in material media that 

 arises from direct influence of the free molecular periods, the main 

 refraction arises from the static dielectric coefficient of fche material. 

 which is not connected with the periods of molecules.* From the 

 phenomena of magneto-optic reflexion it may be shown that, on the 

 hypothesis that the Faraday effect is due to regular accumulated 

 influences of the individual molecules, it must be involved in the 

 relation between the electric force (PQK) and the electric polarisa- 

 tion of the material (/V V), of type 



.^, 



4?r dt dt 



where (ciC 3 c 3 ) is proportional to the impressed magnetic field. This 

 relation, interpreted in the view that the electric character of a 

 molecule is determined by the orbits of its electrons, simply means 

 that the capacity of electric polarisation of the molecule depends on 

 its orientation with regard to the imposed magnetic field, that, in fact, 

 the static value of K, depending on the molecular configurations jnst 

 as much as do the free periods, is altered by the magnetic field. This 

 relation agrees with the main feature of rotatory dispersion, namely, 

 that it roughly follows the law of the inverse square of the wave- 

 length. The specific influence of the molecular free periods, that is, 

 of the ordinary dispersion of the material, on the Faraday effect, is 

 presumably a secondary one ; though it, too, follows the same law 

 for different wave-lengths, in the case of substances for which Cauchy's 

 dispersion formula holds good. It is this latter part of the Faraday 

 effect that is reciprocal to Dr. Zeeman's phenomenon. 



The question is fundamental how far we can proceed in physical 

 theory on the basis that the material molecule is made up of revolv- 

 ing electrons and of nothing else. Certain negative optical experi- 

 ments of Michelson almost require this view ; at any rate, they have 

 not been otherwise explained. It may be shown after the manner of 

 1 Phil. Trans.,' 1894, A, p. 813 (and Dr. Zeeman's calculation, in fact, 

 forms a sufficient indication of the order of magnitude of the result), 

 that in an ideal simple molecule consisting of one positive and one 

 negative electron revolving round each other, the inertia of the 

 molecule would have to be considerably less than the chemical 

 masses of ordinary molecules, in order to lead to an influence on the 

 period, of the order observed by Dr. Zeeman. But then a line in the 

 spectrum may be expected to arise rather from one of the numerous 

 epicycles superposed on the main orbits of the various electrons ia 

 the molecule than from a main orbit itself. 



* Loo. cit., ' Phil. Trans.,' 1894, A, p. 820 ; and 1895, A, p. 713. 



