Part I1.—Magnetic Perturbations of the Spectral Lines. 19 
subsidiary series of Kayser and Runge), and when the action of the magnetic 
field is recorded, it is found that the magnetic effect increases with the 
refrangibility for the members of a triplet of the second series (fig. 8), whereas 
the reverse is the case for a triplet of the first series (fig. 9). Examples of the 
former class occur in the natural triplets of cadmium, zinc, and magnesium, 
already mentioned ; and further examples of these and other peculiarities I hope 
to give in the near future, as soon as I have fully examined and verified them. 
GENERAL Law. 
The first general survey of the magnetic effect on the spectral lines of any 
given substance did not appear to favour the view that the phenomena are 
subject to any simple law. According to the electro-magnetic theory the 
separation 6A, of the side lines of a magnetic triplet, should, under the same 
conditions, vary directly as )’, as we pass from line to line of the same spectrum. 
The possibility of such a law as this seemed to be refuted by the fact that some 
lines are largely affected in the magnetic field, while others, of nearly the same 
wave-length in the same spectrum, are not appreciably affected under the same 
circumstances. In this connexion, however, I pointed out * that ‘it is possible 
that the lines of any one substance may be thrown into groups for each of 
which 6\ varies as )’, and each of these groups might be produced by the 
motion of a single ion. The number of such groups in a given spectrum would 
then determine the number of different kinds of ions in the atom or molecule. 
‘‘ Homologous relations may also exist between the groups of different spectra, 
but all this remains for complete investigation.” 
Although the investigation referred to in the foregoing is still far from 
complete, yet the measurements so far made uniformly tend to confirm the above 
speculation. For the corresponding lines of the natural groups into which a 
given spectrum resolves itself possess the same value of e/m or d5\/’ ; and further, 
this value is the same for corresponding lines in homologous spectra of different 
substances. 
To illustrate the meaning of this, take the case of magnesium, cadmium, and 
zinc, which are substances possessing homologous spectra, and belonging to the 
same chemical group (Mendelejefi’s second group). The spectra of these metals 
consist of a series of natural triplets. The first triplet of the series in magnesium 
is the green 6 group, consisting of the wave-lengths 5183°8, 5172°8, 5167°5, 
while the first cadmium triplet consist of the lines 5086, 4800, 4678, and the first 
zinc triplet consists of the lines 4810°7, 4722, 4680. ach of these triplets 
* Phil. Mag., April, 1898, p. 337. 
