Part IT.—Magnetic Perturbations of the Spectral Lines. 13 
equivalent to three coexisting motions of frequencies V + n, NV, and NV — n, 
respectively, and consequently a spectral line of frequency WV becomes resolved 
into a triplet of frequencies V +n, NV, 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 be 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 precessional 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 produced by these 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 precessional 
motion, the orbit be forced into an apsidal motion, or a motion of revolution in 
its own plane, then each member of the precessional 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 precessional motion round another axis, then each member of 
the precessional 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. Stoney, Trans. Roy. Dub. Soc., vol. iv., p. 563, 1891—‘‘On the Cause of Double Lines 
and Equidistant Satellites in the Spectra of Gases.” 
