Closing Years of the Nineteenth Century. 449' 



certain definite wave-length in the medium, the period of the 

 ray absorbed from a beam of circularly polarized white light 

 will not be the same when the polarization is right-handed 

 as when it is left-handed. "Thus," wrote Tait, "what was 

 originally a single dark absorption-line might become a double 

 line." 



The effect anticipated under different forms by Faraday and 

 Tait was discovered, towards the end of 1896, by P. Zeeman.* 

 Eepeating Faraday's procedure, he placed a sodium flame 

 between the poles of an electromagnet, and observed a widen- 

 ing of the D -lines in the spectrum when the magnetizing 

 current was applied. 



A theoretical explanation of the phenomenon was imme- 

 diately furnished to Zeeman by Lorentz.f The radiation i& 

 supposed to be emitted by electrons which describe orbits 

 within the sodium atoms. If e denote the charge of an electron 

 of mass ra, the ponderomotive force which acts on it by virtue 

 of the external magnetic field is e [r . K], where K denotes the 

 magnetic force and r denotes the displacement of the electron 

 from its position of equilibrium; and therefore, if the force 

 which restrains the electron in its orbit be &, the equation of 

 motion of the electron is 



mi? -t- K 2 r = e [f . K]. 



The motion of the electron may (as is shown in treatises 

 on dynamics) be represented by the superposition of certain 

 particular solutions called principal oscillations, whose distin- 

 guishing property is that they are periodic in the time. In order 

 to determine the principal oscillations, we write T^e nt ^- * for r, 

 where r denotes a vector which is independent of the time, and 

 n denotes the frequency of the principal oscillation : substitut- 

 ing in the equation, we have 



( K - - mn*) r c = en^/^~l [r, E]. 



* Zittingsverslagen der Akad. v. "Wet. te Amsterdam v (1896), pp. 181, 242 ; 

 vi (1897), pp. 13, 99 ; Phil. Mag. (5) xliii (1897), p. 226. 

 t Phil. Mag. xliii (1897), p. 232. 



2 G 



