Radiation and the Structure of the Atom. 401 



connexion with ordinary mechanics in the region of slow 

 vibration, mentioned in the former section, is satisfied. 



Putting N = l in (3) we get the ordinary series spectrum 

 of hydrogen. Putting N = 2 we get a spectrum which, on 

 the theory, should be expected to be emitted by an electron 

 rotating round a helium nucleus. The formula is found very 

 closely to represent some series of lines observed by Fowler * 

 and Evans f. These series correspond to ?^ = 3 and 7^ = 4 J. 

 The theoretical value for the ratio between the second factor 

 in (3) for this spectrum and for the hydrogen spectrum is 

 1*000409 ; the value calculated from Fowler's measurements 

 is 1*000408 §. Some of the lines under consideration have 

 been observed earlier in star spectra, and have been ascribed 

 to hydrogen not only on account of the close numerical rela- 

 tion with the lines of the Balmer series, but also on account 

 of the fact that the lines observed, together with the lines 

 of the Balmer series, constitutes a spectrum which shows a 

 marked analogy with the spectra of the alkali metals. This 

 analogy, however, has been completely disturbed by Fowler's 

 and Evans' observations, that the two new series contain 

 twice ns many lines as is to be expected on this analogy. In 

 addition, Evans has succeeded in obtaining the lines in such 

 pure helium that no trace of the ordinary hydrogen lines 

 could be observed ||. The great difference between the con- 

 ditions for the production of the Balmer series and the series 

 under consideration is also brought out very strikingly by 

 some recent experiments of KaulTon the minimum voltage 

 necessary for the production of spectral lines. While about 

 13 volts was sufficient to excite the lines of the Balmer series, 

 about 80 volts was found necessary to excite the other series. 

 These values agree closely with the values calculated from 

 the assumption E for the energies necessary to remove the 

 electron from the hydrogen atom and to remove both electrons 

 from the helium atom, viz. 13*6 and 81'3 volts respectively. 

 It has recently been argued ** that the lines are not so sharp 

 as should be expected lrom the atomic weight of helium on 

 Lord Rayleigh's theory of the width of spectral lines. Tnis 

 might, however, be explained by the fact that the systems 



* Fowler, Month. Not. Koy. Astr. Soc. lxxiii. Dec. 1912. 



t Evans, Nature, xcii. p. 5 (1913) ; Phil. Mag. xxix. p. 284 (1915). 



X For v ] = 2 we get a series in the extreme ultraviolet of which some 

 lines have recently been observed by Lyman (Nature, xcv. p. 343, 1915). 



§ See Nature, xcii. p. 281 (1913). 



|| See also Stark, Verh. d. I). Phys. Ges. xvi. p. 468 (1914). 



% llau, Site. Ber. d. Phys, Med. Ges. Wurzbury (1914). 



Merlon, Nature, xcv. p. 65 (1916); Proc. Roy. Soc. A. xci. p. 389 

 (1915). 



Phil Mag. S. 0. Vol 30. No. 177. Sept. 1915. 2 D 



