18 Preston—fadiating Phenomena in a Strong Magnetic Field. 
Nevertheless, Dr. Stoney’s explanation of the natural doublets is opposed by 
a serious difficulty in the fact that the two lines of a given doublet, say the two D 
lines of sodium, behave in different ways, as if they arose from different sources 
rather than from the perturbation of the same source. For, in addition to the 
differences previously known to exist, there is the difference of behaviour in 
the magnetic field. Thus, D, is a wide-middled quartet, in which the distance 
between the central lines A (fig. 5) is nearly as great as the distance between 
the side lines B and C, while D, shows as a sextet of uniformally-spaced lines. 
In a similar manner individual members of the natural triplets which occur in 
the natural spectra of the zine, cadmium, magnesium, &c., groups, behave 
differently. Thus, if we denote the numbers of one of the natural triplets by 
the symbols 7, 7), 7’; in ascending order of refrangibility (for example, the 
triplet 5086, 4800, 4678 of cadmium, or the triplet 4811, 4722, 4680 of zine, or 
the green 0} triplet of magnesium), we find that 7 in all cases, in the magnetic 
field, shows as a pure triplet, or suffers, according to the foregoing, merely 
precessional perturbation. On the other hand, 7, shows in each case as a quartet, 
while 7 is a more or less diffuse triplet, in which each of the members may 
prove to be complex on further resolution.* This would seem to point to an 
essential difference in the characters of the lines 7, 7, 73, as if they sprang from 
different origins rather than immediately from the same origin. 
DIFFERENT CLAsses oF NATURAL GROUPS. 
It is also of great interest to note that, so far as my observations yet show, 
the natural groups into which the spectral lines arrange themselves show, as 
te 
a 
Red. Violet. 
Fic. 8.—TRieLet or SECOND SERIES. Fic. 9.—Trietet or First Series. 
[The ordinates represent the separations 5A of the side lines by the same magnetic field. ] 
groups, a characteristic difference in their behaviour in the magnetic field. 
Thus, if we take the case of the natural triplets in the spectrum of zinc, we 
find that these triplets arrange themselves into two series (the first and second 
* See Addendum, p. 21. 
