258 
PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
where c is the velocity of light, h is Planck’s constant, e and m the charge and mass 
of the electron, and E and M the charge and mass of the nucleus of the hydrogen 
atom. In the case of proto-helium, the value of E is doubled and M is increased four 
times. The theoretical ratio between N' and N is therefore 
N„ He _ 16(M + m) 
N„ “ 4(M + m)' 
Introducing the recognised value M/m = 1835, the resulting ratio is 4‘001635, as 
compared with 4'001632 derived from the observations. 
A spectroscopic method of determining the mass of the electron, from the constants 
of the hydrogen and proto-helium series, is thus suggested. The value of N' obtained 
from X 4685‘98 + O'Ol (Rowland’s scale) is 438,879'1 + l'O, and combining this with 
hydrogen 109,675, the resulting value for the mass of the hydrogen atom in terms of 
that of the electron is 1836 + 12. A similar calculation from wave-lengths on the 
International scale, using Mr. Curtis’s new value of the hydrogen constant (109,679'3) 
gives the ratio M/m = 1855 + 12. This result, however, is only to be regarded as 
provisional ; more exact measures of the proto-helium lines may be possible, and the 
formula employed may require a small correction for the alteration in the mass of the 
electron due to its velocity. 
The assignment of the “ 4686 ” series to proto-helium may nevertheless be considered 
to be independent of Bohr’s theory. It really depends upon the recognition of the 
new class of series associated with enhanced lines, and the better accordance with 
experimental results. Apart from the theory it might still be considered possible that 
the hydrogen spectrum, under appropriate conditions, would include the Rydberg 
lines. The well-known line about X 4686 which occurs in the solar chromosphere and, 
in some of the nebulee, however, is undoubtedly the proto-helium line, and there are no 
indications in these spectra of another line near 4688 which would correspond with 
the hypothetical line of hydrogen. Until other evidence is forthcoming, it may 
therefore be considered that the line spectrum of hydrogen, as required by Bohr’s 
theory, consists only of the Balmer series and parallel series in the infra-red 
and Schumann regions. These are all included in the simple formula previously 
given. 
The proto-helium spectrum is apparently of the same simple character as that of 
hydrogen, but the series in the infra-red and Schumann regions have not yet been 
investigated. 
§ 13. Enhanced Lines in Relation to Bohr’s Theory. 
The apparently successful application of Bohr’s theory to the lines of proto-helium 
renders it desirable to consider briefly the implications of this theory as regards other 
systems of enhanced lines. 
