Part II. — Magnetic Perturhations of the S'pectral Lines. 13 



equivalent to three coexisting motions of frequencies N + n, iV, and iV — n, 

 respectively, and consequently a spectral line of frequency N becomes i-esolved 

 into a triplet of frequencies N + n, N, and N — n. This simple theory, therefore, 

 predicts that a single spectral line should become a triplet in the magnetic field, 

 and (since the magnetic force is fixed in direction) that the constituents of the 

 triplet should bo plane polarised 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 assigns a dynamical cause for 

 this precession in the action of the magnetic field on the ionic charge moving 

 through it. 



But this simple processional perturbation of the orbit is obtained on the 

 supposition that the ion is otherwise free from all constraint, or that its freedom 

 is the same in all directions, in the magnetic field. If other constraints come 

 into play on some of the ions, that is, if equal freedom in all directions does not 

 exist, then other perturbations of the ionic orbits must exist, and the spectral 

 lines which are jH'oduced by tlieso will show deviations from the normal triplet 

 type when subject to the magnetic field. And, as it is hardly to be expected that 

 perfect freedom from other perturbations should exist, we ought not to be 

 surprised that modifications, other than the normal triplets, are presented when 

 the subject is examined experimentally. Thus, if, besides having the processional 

 motion, the orbit be forced into an apsidal motion, or a motion of revolution in 

 its own plane, then each member of the processional triplet will become a doublet, 

 and the normal triplet will become a sextet ; such, for example, as that presented 

 by the line D^ of sodium. Again, if the inclination of the plane of the orbit to 

 the line round which precession takes place be subject to periodic variations, or 

 if it have another processional motion round another axis, then each member of 

 the processional triplet will itself be a triplet, and so on for other types of 

 perturbations. 



It is, however, unnecessary to enter into detail here concerning these and 

 other similar matters, as they have already been treated very fully by 

 Dr. Stoney* in the Transactions of this Society, and, indeed, published several 

 years before the facts here requiring explanation were discovered. Dr. Stoney's 

 object was to explain the existence of doublets and equidistant satellites in the 

 spectra of gases, that is in the natural spectra unaffected by the magnetic field — 

 for at that time the influence of the magnetic field was not known to exist. 

 Thus the character of certain spectra indicated that the lines resolved themselves 

 naturally into certain groups, or series of groups. For example, in the case of 

 the monad elements Na, K, &c., the spectrum resolves itself into three series of 



* Dr. G. J. Stouey, Trans. Eoy. Dub. Soc, vol. iv., p. 563, 1891— "On the Cause of Double Lines 

 and Ec^uidistant Satellites in the Spectra of Gases." 



