﻿Spectra of Helium and Hydrogen. 285 



obtained. The formula for this series was discovered by 

 Balmer, and the series is usually referred to as the Balmer 

 series. If ?z 1 = 3 a series of lines in the ultra-red is obtained, 

 and two members of this series have been observed by 

 Paschen. Also the series calculated by putting n 1 = l has 

 been recently observed by Lyman * in the extreme ultra- 

 violet. In addition to the lines represented by formula (1), 

 some other series of lines have generally been ascribed to 

 hydrogen. In 1896 Pickering f discovered in the spectrum 

 of the star f Puppis a series of lines, which are closely 

 represented by the formula 



"= K {i-^Ti)4 ( " 2) 



On account of the numerical relationships existing between 

 this series and the Balmer series, Pickering attributed the 

 lines to hydrogen, and later Rydberg {, from analogy with 

 the spectra of the alkali metals, considered the Balmer and 

 Pickering series to be the diffuse and sharp series of hydrogen- 

 He further concluded that the complete hydrogen spectrum 

 should contain another series of lines, corresponding to the 

 principal series in the spectra of the alkali metals, and given 

 by the formula 



" =K {W 2 -(^TI?} (3) 



Rydberg's conclusions were apparently strongly supported 

 by the observation of a strong line in the spectra of certain 

 stars and nebulae at the place calculated for the first line in 

 formula (3). 



A few years ago, Fowler § obtained the two series of lines 

 represented by equations (2) and (3) by passing a condensed 

 discharge from a 10-inch coil through mixtures of helium 

 and hydrogen contained in an ordinary Plucker tube. 

 Further, he observed in the spectrum of the mixed gases a 

 third series related to the series represented by equation (3) 

 in the same way as the Pickering series is related to the 

 Balmer series, and which was approximately represented by 

 the formula 



v=K {(i-(Ap} (4) 



* Lyman, 'Nature/ xciii. p. 5241. 



t Pickering, Astro-Phys. Journ. vol. iv. p. 369 (1896) : vol. v. p. 92 

 (1897). 



% Rydberg, Astro-Phys. Journ. vol. vii. p. 233 (1899V 

 § Fowler, Monthly Notices R. A. S. } Dec. 1912. 



