264 
PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
of proto-helium is primarily of the Fundamental type; the three associated series 
may be regarded as coincident with it. 
The “ Pickering ” lines associated with 
to proto-helium, in which case the series would include lines nearly coincident with 
the Balmer series. The observational evidence on this point is incomplete, but the 
assignment of these lines to proto-helium is supported by the fact that one of the new 
combination series of magnesium is related to the “ 4481 ” series exactly as the 
extended Pickering series would be related to the “ 4686 ” series. 
(10) The slight differences between the observed positions of alternate lines of the 
“ 4686 ” series and those calculated by Rydberg for the “ Principal ” series of hydrogen 
are in very close agreement with Bohr’s theoretical formulae for hydrogen and proto- 
helium. Adopting these formulae, the spectroscopic data give a provisional value for 
the mass of the hydrogen atom, in terms of that of the electron, as 1836 + 12; or 
1855 ±12 when the data are corrected to the International scale of wave-lengths. 
(11) The appearance of the Ptydberg constant in enhanced lines series with four 
times its usual value may be explained on Bohr’s theory by supposing that series of 
the arc type are produced when only one electron is removed from each of the atoms 
involved, while in the case of enhanced lines two electrons are removed. 
(12) A preliminary examination of terrestrial and celestial spectra has given no 
indications of series requiring still greater multiples of the Rydberg constant in the 
formulae representing them. 
The author has pleasure in acknowledging his obligations to Mr. W. Jevons, 
A.R.C.S., D.I.C., B.Sc., who has taken the new photographs of the magnesium 
spectrum, and has given much valuable assistance in other ways. 
APPENDIX. 
The wave-lengths (on the International scale) of the enhanced lines of magnesium, 
as observed in the arc in vacuo in the course of the foregoing investigation, are 
brought together in Table XYI. The letters indicating the series to which the lines 
belong have the significance given in the description of Plate 3. For complete¬ 
ness additional lines observed by Lorenser and by Lyman are also included. 
the “ 4686 ” series are also probably due 
