December 1, 1896.] 



KNOWLEDGE. 



285 



the principal series 1696019, almost exactly the difference 

 of the two. If we turn now to the cleveite gases we find 

 the limits of the two principal series to be 3202986 and 

 3845532, and the limits of the two pairs of subordinate 

 series 2717188 and 2921116. If we take the difference 

 of the first and third of these numbers, and also of the 

 second and fourth, we have 485798 and 92441G approsi- 

 matLag closely to the wave numbers of the first lines in 

 the two principal series. But to take together the first 

 and fourth, and the second and third, would give us wave 

 numbers not represented in the cleveite spectrum at all. 



These two leader lines were not discovered until after 

 the six series had been identified. Photography had shown 

 the ordered clusters of lines far in the unseen regions of 

 the ultra-violet, and the sequences so detected evidently had 

 their initial members deep m the equally invisible regions of 

 the infra-red. As the diagrams show, and as the formulae 

 require, the lines in the \'iolet are crowded together ; pro- 

 ceeding towards the red the intervals become very wide. 

 Hence there is no danger in the region of long wave 

 lengths of confusing a Une of one series for one of another. 

 The lines here are few, widely separated, and very intense. 



elements, had been arrived at considerably earlier by 

 several astronomers from quite different grounds. Prof. 

 Norman Lockyer had been supplied with a heUum tube 

 by Prof. Ramsay immediately the latter had made his 

 discovery, and Mr. Lockyer soon provided himself with 

 other tubes fiUed with gas from broggerite and urantnite. 

 Then, as he set to work on the examination of the spectrum 

 of the new gas, he found Une after Une to correspond with 

 lines of unknown origin in the spectrum of the solar 

 chromosphere, and of certain stars and nebulae, tiU he 

 concluded that " this gas is really the origin of most, 

 but certainly not of all, of the unknown iines which 

 have been teasing astronomical workers for the last 

 quarter of a century."* 



But the chromospheric Unes thus identified fell into two 

 very different categories. Of the six strongest lines of the 

 cli'veite gas spectrum, three, viz., A 7066, 5876 (D,), and 

 4472, are given the frequency number 100 in Young's 

 table of the chromospheric lines ; whilst those at X 6678, 

 5016, and 4922 have the numbers respectively 25, 80, 

 and 80. Prof. Lockyer, therefore, from the first ex- 

 pressed his opinion that the cleveite gas was not a 



6 7 S f \ / 2 3 At <i 7 a 9 I 2J1-I 6 7 8 9 13.31^6189 



1,10' 



ZxlO^ 



mil 



Jx/o« 



P AR HELIUM^ 



Su Ire t'ti' nair 



HELIUM 



Sane* 



1 M" 



tOGOO 8<>C3 



J^o 



I 



llv/Ta-Jl«a 



^J 



J^.d O'-anffc ye»o«'<»'.<«Si<" VIol.f VUra-VoJ.t 



So when Profs. Runge and Paschen, following the formulse 

 which the ultra-violet lines had given, found that ii = 2 for 

 the principal series of "helium'' and of " parhelium " 

 would indicate a line in each case far below the limits of 

 visibility in the red, and on searching with the bolometer 

 found each line close to its predicted place, bright, strong, 

 and intense, there was no ambiguity about the result. 

 The actual spectra corresponded to the theoretical, and 

 were complete from their rise far in the obscure regions of 

 the infra-red, tiU they died away in the darkness which 

 lies on the other side of the visible spectrum. One point 

 which the diagrams fail to indicate is of great significance, 

 viz., the way in which the brightness of the lines in any 

 particular series diminishes as the violet end of the 

 spectrum is approached. The smaller the value of n the 

 brighter the line. Then, too, the princii)al series is Une 

 for line brighter than either of the secondary series ; whilst 

 in the particular case of the cK-veite gas the various lines 

 of the "helium" spectra are brighter than the corre- 

 sponding lines of the " parhelium ' spectra. 



The belief which the two great German physicists had 

 thus reached, that the cleveite gas was a mixture of two 



simple one but a mixture,! and M. Deslandres was 

 scarcely behind him in arriving quite independently at 

 the same conclusion. 



It will be observed that the division which solar 

 physicists were led to make was precisely the same as 

 that which Profs. Runge and Paschen had reached by 

 so different a method. Those Unes of cleveite gas which 

 Young found "always visible " in the chromosphere are 

 all helium lines, the Unes " sometimes visible " are aU 

 parhelium lines. 



The evidence of stars and nebuhi^, so far as it goes, 

 tends to support the same division. Perhaps the strongest 

 instance is afl'orded us by the early spectrum of Nova 

 AurigiB. Here, lines \ 5016 and 4922 (both parhelium 

 Unes) shone out with great distinctness, whilst the helium 



• LockTcr, "The Story of Helium." 

 p .•?45. 



ynture, Vol. LIII , No. 1372, 



t Loi-kvcr, ' On tlio Now Gas obtaiiu'il from I'raiiinito ; " Socond 

 Xoto P'roc. Tfo.v"' Siicirlv. Vol. I. VIII., No. 34!», p. 113 



X Deslar.dros, " Coniparaison eutrc lea Spectres dii Oai de la 

 C'ltSveite et dc rAtnio.ipliiTe Solairc." Comples SfHjns, Tome CXX., 

 p. 1112. 



