256 
PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
which the ordinary lines of hydrogen could not be detected.* In accordance with 
the convenient nomenclature of Lockyer it is suggested that the lines in question 
should be designated “ proto-helium ” lines. 
Analogy with the magnesium “ 4481 ” series suggests that the “4686 ” series 
should be regarded as one of the F type, and the question arises as to how the 
associated P, S, and D series are represented in the proto-helium spectrum. A 
knowledge of the F series alone does not permit the complete calculation of any of the 
three associated series, but only indicates the “ variable part ” of the first line of the 
D series. In the case of proto-helium, the limit of the “ 4686 ” series, N'/3 2 , shows 
that the variable part of the D formula is identical with that of the F, but the limit 
of D remains undetermined. A consistent arrangement is possible, however, if the 
D be regarded as having the same limit as the F series, in which case the four series 
would coalesce in the formula 
n = N' 
where N' = 438,879'1. This only requires that the zero values of n given by m — 3 
be regarded as negative terms, so that if the formula be taken to represent the 
coalesced D and S series, the limit of P derived from S, and that of F derived from 
D would also be N'/3 2 . The lines of the four series would thus be coincident. The 
great strength of the “ 4481 ” series of magnesium as compared with that of the other 
series of the “ F.P.” group, however, suggests that the “ 4686 ” series is primarily of 
the F type. 
As pointed out by Bohr, the substitution of 4 for 3 in the above formula for the 
“ 4686 ” series yields a series of lines of which the Pickering (£ Puppis) lines, also 
formerly attributed to hydrogen, are alternate members, the remainder being nearly 
coincident with the Balmer lines of hydrogen. It is probable, therefore, that the 
Pickering lines are also due to proto-helium, as they have only been observed in 
association with 4686. The calculated wave-lengths of the earlier members of the 
extended series are compared in Table XV. with the Balmer series, and with the 
hypothetical “ sharp ” series of hydrogen calculated by Rydberg. 
Table XV.—The “ Pickering ” Series. 
Proto-helium, 
“ Pickering ” series, 
A (calculated). 
Hydrogen, 
Balmer series, 
A. 
“ Hydrogen,” 
Rydberg’s S series, 
A (calculated). 
6560-37 
6563-07 
5411-74 
5412-8 
4859-53 
4861-52 
4541-79 
4543-3 
4338-86 
4340-64 
4200-02 
4201-5 
* ‘Nature,’ 1913, September 4, p. 5. 
