1016] on The Spectra of Hydrogen and Helium 049 



unique. The helium hands are not known to occur in any celestial 

 spectrum. 



The line spectrum of helium, however, is abundantly represented 

 in the heavens. All the principal lines occur as bright lines in 

 the chromosphere, though they do not ordinarily appear among the 

 Fraunhofer lines in the spectrum of sunlight. They are also the 

 characteristic feature of the " helium stars," many of which are found 

 in the constellation of Orion. 



A very important chapter in the story of hydrogen and helium 

 was opened in 1896 by Professor Pickering's discovery of a new 

 series of lines in the spectrum of the star ^ Puppis (Fig. 4). These 

 Hnes occupy positions intermediate between the lines of the Balmer 

 series, and W'ithin the limits of error of the measurements, the two 

 series appeared to converge to the same limit. Also, if in the Balmei 

 formula m be given successive values 3J, 4|, &c., the resulting 

 wave-lengths are in close agreement with those of the Pickering- 

 lines. Now, these relations are appropriate to two subordinate series 



fc^ PU PPl 5 



I 



Fig. 4.— Spectrum of C Puppis 



of the same element, and it was at once suggested by Pickering that 

 the new series might also be due to hydrogen. Rydberg took up the 

 question, and concluded that the Pickering lines formed the Sharp 

 and the Balmer lines the Diffuse series of hydrogen, and he 

 proceeded to calculate the Principal series from the Sharp in the way 

 I have already indicated. Rydberg's scheme for hydrogen, in 

 wave-numbers, thus became 



n = 109,675 |1 - -J- (Balmer series) 



n = 109,675 |^^ - ^^^^^^jt^,} (Pickering series) 



_ lAQCTP; (1 1 ) f Calculated \ 



n - 109,675 -^ ^^^ - -^^, - VPrincipal series/ 



giving P(l) = A 4688 ; P(2) = A 2734, &c. 



