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LXXIV. The Spectrum of Helium in the Extreme Ultra- 

 violet. By Hugo JFkicke and Theodore Lyman *. 



THE study of the Spectrum of Helium in the Schumann 

 region, m which was published some five years ago, 

 yielded results which were difficult to interpret f ; and the 

 conclusions drawn from a more recent attack on the same 

 subject {, though interesting, were by no means final. Both 

 investigations suffered from the same defect, for to obtain 

 any lines at all when hdium was used in the vacuum tube it 

 was necessary to employ a strong disruptive discharge, an 

 electrical condition sure to introduce impurities by its 

 action on the walls of the tube and upon the electrodes. 



The upshot of the whole matter was that, though the 

 region betwen 1700 and 600 A.U. contained a considerable 

 number of lines, only those at 1640 and 1215 could be 

 attributed to helium §. It has been pointed out that these 



wave-lengths fit the relation y = 4N ( — 2 A and therefore 



probably form members of the enhanced spectrum series. 

 Even now, however, the origin of these lines cannot be 

 regarded as perfectly certain. 



It has become increasingly evident during the progress 

 of the present investigation that the arrangement proposed 

 by Hicks || by which a considerable number of lines, many 

 of them certainly due to impurities, were made to fit 

 the formula for the enhanced spectrum, is without justifi- 

 cation If. 



The immediate cause of the renewal of the attack on the 

 problem was the discovery by several investigators of a 

 resonance potential in helium corresponding to a wave- 

 length of about 600 A.U. ** 



The improvements introduced were two in number : first, 

 by the employment of a vacuum spectroscope with a grating 

 of but 20 cm. radius the gas absorption was greatly reduced ; 



* Communicated by the Authors. 



f T. Lyman, Astrophysical Journal, xliii. No. 2, p. 89 (1916). 



t T. Lyman, Science, xlv. p. 187, Feb. 1917. 



§ T. Lvman, Nature, civ. p. 314 (1919). 



I Hicks, Nature, civ. p. 393 (1919). 



H T. Lvman, Nature, civ. p. 565 (1920). 



** F. Horton & A. C. Davies, Proc. Eoy. Soc. xcv. p. 408 (1919). 

 F. Horton & D. Bailev, Phil. Mag. xl. p. 440 (1920). J. Franck & 

 P. Knipping, Phys. Zeitsch. xx. p. 481 (1919) ; Zeitsch. f. Physik, i. 

 p. 320 (1920"). K. T. Conipton, Phil. Mag. xl. p. 553 (1920). 



