PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
227 
founded. A search for other series of similar character was therefore undertaken, in 
the hope that some generalisation with regard to such series might be arrived at. 
The mode of production of the “4686 ” lines suggested spark spectra as the most 
promising source of such series, and, shortly afterwards, experiments on magnesium 
yielded some new lines of that element which were obviously associated with the spark 
line 4481 in series of the kind looked for. No satisfactory evidence of relation to other 
series of magnesium lines, however, was then obtained, and it seemed possible that 
both the “4686 ’’ and the “ 4481 ” series were of a new type, having no necessarily 
simple relation to other known series in the respective spectra A 
The lines of the “ 4686 ” and the associated Pickering series have since become of 
increased importance, in connection with theories of the constitution of the atom, 
through the theoretical investigations of Dr. Bohr, f Beginning with the “ Rutherford ” 
model of the atom, and introducing Planck’s quantum, Dr. Bohr has derived a formula 
for the hydrogen spectrum which excludes these lines, while agreeing closely with the 
hydrogen series about which there can he no doubt. The “ 4686 ” and Pickering 
series, however, are included in another formula, derived for the emission of helium 
atoms during the first stage of their reformation when both electrons are supposed to 
have been removed by the strong discharges employed. This formula is identical, in 
a first approximation, with that for hydrogen except that the Rydberg constant 
“ N ” (= 109,675 for Rowland’s scale) has four times its ordinary value. The two 
“ Principal ” series previously assigned to hydrogen were thus united in a single 
formula and attributed to helium, while the Pickering series was made to include 
intermediate lines coincident with the Balmer series of hydrogen. As the lines in 
question, whatever their origin, must be regarded as enhanced lines, it was evidently 
desirable to continue the general investigation of lines of this class. 
Further investigation of magnesium, in particular, was also suggested by the 
discovery made by Paschen,J and by King,§ that the line 4481 is a close doublet 
o 
(dA = about 0‘2 A.U.), as it appeared that a valuable indication of the type of series 
to which it belongs might be obtained if other members could be resolved. Photo¬ 
graphs with high dispersion have accordingly been taken for the resolution of the lines 
and to provide more accurate data for testing the formula for enhanced lines which 
was suggested by the work of Bohr. Other photographs, taken with smaller 
dispersion, have resulted in the detection of additional lines which clearly belong to 
series related to that beginning with 4481. The discussion of the new data for mag¬ 
nesium, and of the data for calcium and strontium given by other observers, has led 
to some conclusions which are probably of general application to enhanced line series. 
The chief results of the investigation are summarised in § 14. 
* ‘Roy. Soc. Proc.,’ A, 1913, vol. 89, p. 133. 
f ‘Phil. Mag.,’ vol. 26, p. 1 and p. 476 (1913). 
X Communicated privately (1913, August). 
§ ‘Astrophys. Jour.,’ vol. 38, p. 327 (1913). 
2 G 2 
