170 Dr. T. Prestori on Radiation Phenomena 



the side lines are broad and weak, while the centre is much 

 denser. It is just possible, and indeed probable, that these 

 modifications may be really quartets, of which the side lines 

 B and C are broad and weak, and overlap at their inner 

 edges, giving the appearance of a bright central line winged 

 with two weaker bands. 



The distance between the side lines B and is about 

 the same as that between the components of A, and when the 

 double-image prism is not used, the lines in question photo- 

 graph as triplets, i. e. as bands having three ribs or denser 

 parts running along them lengthwise. With a much stronger 

 field it could be determined whether the part BC in these 

 lines is really a triplet or an overlapping doublet; but as they 

 are all weak lines requiring long exposure (four hours in my 

 case), it is not easy to arrange to have a very strong field for 

 such a long time. However, it is a matter of very little 

 importance at present, for if we can explain the quartet we 

 are on the highway to the explanation of all the various 

 modifications. 



For this purpose let us revert to Dr. Larmor's paper 

 already cited. In this investigation he considers merely the 

 simple case of a single ion describing an elliptic orbit under 

 a central force directly proportional to the distance. This 

 electric charge, when subject to the influence of the magnetic 

 field, is so acted on that its elliptic orbit is forced into 

 precession round the direction of the magnetic force, that is, 

 as a first approximation. For the equations of motion of the 

 ion moving round a centre of force in a magnetic field are, 

 as a first approximation, the same as those which obtain for 

 a particle describing an elliptic orbit under a central force 

 when the orbit precesses or revolves round a line through its 

 centre drawn in the direction of the lines of magnetic force. 

 If N be the natural frequency of revolution of the particle in 

 its orbit and n the frequency of revolution of the orbit in its 

 precessional movement, the combined movement is equivalent 

 to three coexisting motions of frequencies N + w, N, and 

 'N — n respectively. When n is small compared with N so 

 that its square may be neglected, the equations of motion of 

 the particle in the revolving orbit become identical with those 

 of the moving ion in the magnetic field. 



This simple theory therefore predicts that a single spectral 

 line should be converted into a triplet by the action of the 

 magnetic field, and that the constituents of this triplet should 

 be plane-polarized when viewed across the lines of force. It 

 teaches us that the cause of the tripling is the forced precession 

 of the ionic orbits round the lines of magnetic force, and it 



