Part II. — 3Itiffnctic Perturhations of the Spectral Lines. 17 



case depicted in fig. 7, although, as I have said before, my observations do not 

 confirm the existence of this case. 



The supposition made above to account for tlie doubling of the middle line, 

 viz, that the amplitude of the z component vibration varies periodically, is one 

 which appears to be justified when we consider the nature of the moving system 

 and the forces which control it. For the revolving ion is part of some more or 

 less complex system which must set in some definite way under the action of the 

 magnetic field — say with its axis along the direction of the magnetic force — and, 

 in coming into their position, the inertia of the system will cause it to vibrate 

 with small oscillations about the position of equilibrium, and this vibration 

 superposed on the precessional motion of the ionic orbit gives the motion 

 postulated above to explain the quartet. 



This, indeed, comes to the same thing, as a suggestion made by Prof. G. F. 

 Fitz Gerald about a year ago — shortly after I discovered the existence of the 

 quartet form (October, 1897). In Prof. Fitz Gerald's view, the ion revolving 

 in its orbit is equivalent to an electric current round the orbit, and therefore the 

 revolving ion and tlie matter with which it is associated behaves as a little 

 magnet, having its axis perpendicular to the plane of the orbit. The action of 

 the magnetic field will be to set the axis of this magnet along the lines of force, 

 and, in taking up this set, the ionic orbit will vibrate about its position of 

 equilibrium, just as an ordinary magnet vibrates about its position of rest under 

 the Earth's magnetic force. 



In a similar way a periodic change in the elli2)ticity of the orbit produces a 

 doubling of the spectral lines previously existing, wliile a periodic oscillation 

 in the apsidal motion renders tlie lines nebulous or diffuse ; and by treating these 

 cases in the foregoing manner the corresponding forces may be discovered. 



It is clear, therefore, that perturbations of this kind are sufficient to account 

 for all the observed phenomena, and that the theor}^ is ready to meet the 

 demands of more complicated types than have yet been observed. It is 

 legitimate to expect that perturbations of this kind will occur in some, at 

 least, of the ionic motions, and, in fact, that they must occur, and that the 

 perfect freedom required for the production of the pure precessional trijolet 

 cannot exist in all cases. 



The existence of all these variations of the normal triplet is a matter of great 

 interest, not only as showing that the perfect uniformity required for the 

 production of the normal triplet is not maintained in all cases, but also as giving 

 us a further insight into the nature of the conditions under which the ionic 

 motions take place, as well as demonstrating that the causes supposed by 

 Dr. Stoney, in 1881, to be operative in producing doublets and satellites in the 

 natural spectra of gases may be really the true causes by which they are produced. 



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