216 Scientific Proceedings, Royal Dublin Society. 



distance in the ultra-violet to be observed, if the line have suflScient 

 intrinsic brightness. Professor Hartley has succeeded in photo- 

 graphing as far as A = 1800. 



6. That in Series D there are two new terms corresponding to 

 negative values of n, for one of which n is approximately - 860*6, 

 and for the other - 31-17. These positions are the same as 

 A = 1162, and A = 32082. The first of these is probably too far 

 in the ultra-violet and the other much too far in the ultra-red, to 

 be observed. 



7. Similarly, in the hydrogen spectrum there seems to be a 

 new line in the ultra-violet, viz. the line obtained by putting 

 m = 1 into Balmer's formula. This makes n = - 822'789 which 

 is the same asA = 1215,a position, however, which is probably too 

 far removed in the ultra-violet for observation. 



8. Lines corresponding to negative values of n do not appear 

 to have been observed in any of the monad elements except sodium, 

 but examples of them are met with in some of the triple-line series 

 of the dyads. Kayser and Runge record what is presumably one 

 triple group of this kind in the spectra of zinc, cadmium, and 

 mercury, and what is perhaps a second group in the spectra of 

 zinc and cadmium (see the photographs they give of a part of each 

 of these spectra, and the observations they make about them on 

 p. 71 of their fourth Paper, " tJber die Spectren der Elemente," in 

 the Transactions of the Berlin Academy for 1891). 



It is not yet known what kind of perturbation within the 

 molecules would be competent to affect the partials of the undis- 

 turbed motion of an electron so as to resolve the resulting lines into 

 triple lines. But it is, nevertheless, suggestive to find that in the 

 spectra of Zn, Cd, and Hg, the constituents of the triple line 

 corresponding to a negative value of n, are not reversed, but in the 

 same relative positions to one another as are those furnished by 

 positive values of n. If this non-reversal of position prevails among 

 those double lines of sodium which are due to negative values 

 of n, it will probably be indicated by the less refrangible constituent 

 of the double line. No. 1 of Series S being the brighter, as is said 

 to be the case with the double lines of the same series, which are 

 due to positive values of n. This would imply a physical fact of 

 importance, viz. that a change in the sign of n induces a change 

 in the direction of the apsidal perturbation as well as in the 



