452 The Theory of Aether and Electrons in the 



inward to the axis of z (so as to shorten the period), the charge 

 on the electron must be negative. 



The value of e/m for this negative electron may be determined 

 by measurement of the separation between the components of 

 the triplet in a magnetic field of known strength ; for, as we 

 have seen, the difference of the frequencies of the outer com- 

 ponents is eKjm. The values of e/m thus determined agree well 

 with the estimations* of e/m for the corpuscles of cathode rays. 



The phenomenon discovered by Zeeman is closely related to 

 the magnetic rotation of the plane of polarization of light. f 

 Both effects may be explained by supposing that the molecules 

 of material bodies contain electric systems which possess 

 natural periods of vibration, the simplest example of such a 

 system being an electron which is attracted to a fixed centre 

 with a force proportional to the distance. Zeeman's effect 

 represents the influence of an external magnetic field on the 

 free oscillations of these electric systems, while Faraday's effect 

 represents the influence of the external magnetic field on the 

 forced oscillations which the systems perform under the stimulus 

 of incident light. The latter phenomenon may be analysed 

 without difficulty on these principles, the equation of motion of 

 one of the electrons being taken in the form 



mr + K 2 r = eE + e[r.H], 



where m denotes the mass and e the charge of the electron,, 

 r its distance from the centre of force, K 2 r the restitutive force, 

 E and H the electric and magnetic forces. When the electron 

 performs forced oscillations under the influence of light of 

 frequency n, this equation becomes 



( K 2 -m?i 2 )r = eE + e[r.H]. 



The influence of the magnetic force on the motion of the 

 electron is small compared with the influence of the electric 

 force, i.e. the second term on the right is small compared with 

 the first term ; so in the second term we may replace r by its 



* Cf. p. 405. t Cf. pp. 213-216, 307-309, 367-370. 



