﻿Spectra of Helium ami Hydrogen. 287 



As the factor outside the bracket differs from that in 

 formula (6) only by a small correction due to the difference 

 in the masses of the hydrogen and helium nuclei, it will be 

 seen that equation (7) approximately represents the series of 

 lines given by (2), (3), and (4). Bohr therefore suggested 

 that the lines in question were not due to hydrogen but to 

 helium. This conclusion was not disproved by previous 

 observations and experiments, for helium was always known 

 to be present when the lines appeared. Bohr also pointed 

 out that the reason why the lines considered are not observed 

 in ordinary helium tubes may be that in such tubes the 

 ionization of the gas is not as complete as in the star 

 £ Puppis or in Fowler' experiments with the condensed 

 discharge. According to the theory, the presence of helium 

 atoms which have lost both electrons is necessary for the 

 appearance of these lines in a spectrum-tube. 



Preliminary experiments by the author * gave strong- 

 support to Bohr's conclusions. A helium spectrum was 

 obtained showing the first line (X 4686) of the series 

 represented by equation (3) very brightly, but no trace of 

 the ordinary hydrogen lines of the Balmer series. Also 

 this line could not be observed in the spectrum obtained by 

 passing a strong discharge through mixtures of hydrogen 

 and neon, and of hydrogen and argon. Later Stark f 

 observed the 4686 line in a helium tube in which the 

 hydrogen lines did not appear. Further evidence pointing 

 to the same conclusion has been given by Rau i, who has 

 made some interesting experiments on the voltage necessary 

 for the production of spectrum lines. 



In a discussion in ' Nature/ Fowler § pointed out that the 

 lines observed by him were not accurately represented by 

 formulae (2), (3), and (4), but Bohr || subsequently showed 

 that the deviations from the values given by the formulae 

 could be accounted for within the limits of experimental 

 error by taking into account the correction due to the mass 

 of the central nucleus. This influence of the mass of the 

 nucleus was not considered in Bohr's original paper. 

 Recently Fowler^f has published a very important paper on 

 series lines in spark spectra. He concludes from analogy 

 with spark spectra of the enhanced type, that the lines 

 represented by equations (2), (3), and (4) are enhanced lines 



* Evans, ' Nature/ xcii. p. 5. 



t Stark, Verh. d. Deutsch. Phys. Ges. xvi. p. 468 (1914). 



+ Rau, Sitzungsb. d. Phys.-Med. Ges. Wurzburff, 1914. 



§ Fowler, ' Nature,' xcii. p. 95. || Bohr. ' Nature/ xcii. p. 231, 



51 Fowler, Phil. Trans. A. vol. ccxiv. p. 225. 



