120 MAGNETISATION OF LIGHT [CH. XL 



At first sight one might be inclined to suppose 

 that the orbits would all face round and set 

 themselves normal to the lines of force, like so 

 many circular currents ; but that is to forget the 

 inerbia of the travelling electron. It is manifest that 

 since a revolving electron constitutes a circular 

 current, its tendency will be to set itself with its 

 plane normal to the lines of force ; but since by 

 hypothesis the revolving electron has inertia, the 

 current will not so set itself, but will yield to 

 the deflecting force in an indirect manner, as a top 

 does ; or as the oblate spinning earth does as 

 explained by Newton in the Principia, the axis of 

 rotation having a conical motion round the lines 

 of force: a motion which is called "the precession 

 of the equinoxes" in the case of the earth, and 

 " the Zeeman effect " in the case of a radiating atom. 



This is an account of the chief part of the Zeeman 

 effect, and may be regarded as the most fundamental 

 kind of disturbance caused by a magnetic field on a 

 source of radiation. But there may be other minor 

 disturbances, just as in the case of the earth, whose 

 axis is not only subject to precession, but also to 

 nutation a nodding movement superposed upon the 

 main motion. It is also quite possible for the middle 

 line, or for the two outer lines, or indeed for all three 

 lines, to be doubled ; thus giving rise not to the 

 standard triplet, but to a quartet or a quintet or 

 even a nonet, appearances seen and photographed 

 by Zeeman, Preston and others. The remarkable 

 * echelon ' spectroscope of Michelson has been in- 

 vented just in time for application to phenomena 

 of this kind its special function being the close 

 examination in detail of a minute portion of a 

 spectrum otherwise produced. 



