Note 6 — § 1. The Emission of a Line Spectrum.
Reading edition
A series of lines, first observed by Pickering in stellar spectra and recently by Fowler in vacuum-tubes containing a mixture of hydrogen and helium, is generally also ascribed to hydrogen. These lines, however, can be accounted for on the present theory, if we ascribe them to helium. (Phil. Mag. loc. cit. p. 10: comp. also ‘Nature’ xcii. p. 231 (1913).)