18 Preston — Radiatmj Flienomcna in a Strong 3Iagnetic 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, Dx is a wide-middled quartet, in which the distance 

 between the central lines A (fig. 6) is nearly as great as the distance between 

 the side lines B and C, while Z>2 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 zinc, cadmium, magnesium, &c., groups, behave 

 differently. Thus, if we denote the numbers of one of the natural triplets by 

 the symbols T^,, Tj, T^ in ascending order of refrangibility (for example, the 

 triplet 5086, 4800, 4678 of cadmium, or the triplet 4811, 4722, 4680 of zinc, or 

 the green h triplet of magnesium), we find that T^ 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, T:,_ shows in each case as a quartet, 

 while Ti is a more or less diffuse trijjlet, 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 tlie lines Ti, T-,^ jTj, as if they spi'ang from 

 different origins rather than immediately from the same origin. 



Different Classics 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 



Red. Violet. 



Fig. 8. — Tkiplet of Second Series. Fig. 9. — Triplet of First Series. 



[The ordinates represent the separations 5\ 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. 



