CH. XL] MAGNETISATION OF LIGHT 117 



originate a doublet in the spectrum ; for of the two com- 

 ponent circular vibrations into which the motion can be 

 analysed, one has been made more rapid and therefore 

 its light raised in refrangibility, the other has been 

 made slower and therefore lowered in refrangibility. 



Another closely allied kind of perturbation, 

 analogous to precession of the equinoxes in the case 

 of the earth, would result in a triplet in the spectrum. 

 This precessional motion occurs in an orbit subject to 

 any oblique pull or deflecting force. Instead of 

 yielding directly to that pull, its effect is to make the 

 axis describe a kind of cone, the kind of motion 

 that one sees in an inclined spinning-top : the pull of 

 gravity on a spinning- top does not make it topple 

 over, but makes it precess. So also with a hoop or 

 bicycle when not vertical : instead of tumbling, it 

 turns round and round in a circuit, as long as its 

 motion continues ; only falling when the motion 

 ceases, or falls below a certain critical value. Hence 

 if the orbit of an electron were subjected to an 

 oblique or deflecting force, the effect would be, not 

 to place it directly in the desired position per- 

 pendicular to a line of force, but to cause it to 

 precess. And this motion might be analysed into 

 three components, the accelerated and retarded 

 circular orbits above-mentioned, which would result 

 in a doubling of the line, and a third component, 

 viz. the one parallel to the axis, which would be 

 unchanged and would therefore represent a spectral 

 line in its old position, the centre of a group of 

 three. All this was clearly perceived in connection 

 with Dr. Zeeman's discovery, with the assistance of 

 his great compatriot the eminent physicist, H. A. 

 Lorentz ; whose theory was in several respects 

 anticipatory of the experimental results. 



