Closing Years of the Nineteenth Century. 45 1 



performed in a plane at right angles to the magnetic field K. 

 In order to determine the nature of these two principal oscilla- 

 tions, we observe that it is possible for the electron to describe 

 a circular orbit in 'this plane, if the radius of the orbit be 

 suitably chosen ; for in a circular motion the forces * 2 r and 

 .-e[r . K] would be directed towards the centre of the circle ; and 

 it would therefore be necessary only to adjust the radius so that 

 these furnish the exact amount of centripetal force required. 

 Such a motion, being periodic, would be a principal oscillation. 

 Moreover, since the force e [r . K] changes sign when the 

 sense of the movement in the circle is reversed, it is evident 

 that there are two such [circular orbits, corresponding to the 

 two senses in which the electron may circulate; these must, 

 therefore, be no other than the two principal oscillations of 

 frequencies K.m~^ el/2m. When the light received in the 

 spectroscope is that which has been emitted in a direction at 

 right angles to the external magnetic field, the circles, are seen 

 edgewise, and the light appears polarized in a plane parallel to 

 the field ; but when the light examined is that which has been 

 emitted in a direction parallel to the external magnetic force, 

 ,the radiations of frequencies Km~i eK/2m are seen to be 

 circularly polarized in opposite senses. All these theoretical 

 conclusions have been verified by observation. 



It was found by Cornu* and by C. G-. W. Konigf that the 

 more refrangible component (i.e., the one whose period is shorter 

 than that of the original radiation) has its circular vibration 

 in the same sense as the current in the electromagnet. From 

 this it may be inferred that the vibration must be due to a 

 resinously charged electron; for let the magnetizing current 

 and the electron be supposed to circulate round the axis of z in 

 the direction in which a right-handed screw must turn in order 

 to progress along the positive direction of the axis of z ; then 

 the magnetic force is directed positively along the axis of z, 

 jand, in order that the force on the electron may be directed 



* Comptes Rendus, cxxv (1897), p. 555. 



* Ann. d. Phys. Ixii (1897), p. 240. 



2G2 



