PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
263 
doubtful, however, whether any change beyond that represented by enhanced lines 
should be expected. It is now easy to produce a spectum in which the proto-helium 
line 4686 exceeds any of the ordinary helium lines in brightness, and from the 
persistence of the ordinary helium lines in many of the Wolf-Rayet stars it seems 
reasonable to infer that even in these stars the conditions are not indefinitely in advance 
of those obtainable in laboratory experiments. 
14. 
Summary. 
(1) The enhanced (spark) lines of magnesium, calcium, strontium, and barium form 
series of doublets which occur in groups similar to those previously recognised in arc 
spectra. The formulae representing these series, however, differ from those employed 
for arc lines in that Rydberc4’s constant “N ” (= 109675 for Rowland’s scale) has 
a value equal to 4N. 
(2) The Fundamental series, in the Doublet systems of calcium, strontium, and 
barium, as shown by Saunders and Lorenser, derives its limit and the separation of 
its components from an observed negative term of the Diffuse series. In the case of 
magnesium, the corresponding negative term of the Diffuse series, and the Funda¬ 
mental series which might have been expected, do not exist. 
(3) A new group of series of narrower doublets has been found in magnesium, in 
which the separation of the Sharp and Diffuse pairs is identical with the (calculated) 
separation of the second pair of the Principal series of wider doublets. The new group 
is designated the “ F.P. system ; in addition to the ordinary series it includes two 
well-marked combination series, each consisting of seven observed lines. 
(4) The “ 4481 ” series of magnesium consists of very close doublets with constant 
separation, and forms the Fundamental series of the “ F.P. ’ system. Its limits are 
derived from the first negative term of the Diffuse series. 
(5) Corresponding series of the “F.P.’’ and Wide doublet systems run parallel to 
each other, but notwithstanding this numerical relation, the two systems do not 
necessarily co-exist, the wider doublets occurring in the ordinary arc as well as in the 
spark, while the “ F.P.” group occurs only under spark conditions or their equivalent 
in the region of the negative pole of an arc in vacuo. 
(6) No numerical relations have been traced between any of the enhanced line 
series and the series of arc lines of the same element. 
(7) The “ 4686 ” series produced in helium tubes is of the enhanced line (4N) type, 
and can no longer be considered to belong to the same group as the Balmer series of 
hydrogen, which is of the arc (N) type. It is concluded that these lines are due to 
helium, as indicated by Dr. Bohr from theoretical considerations. It is suggested 
that they should be designated “ proto-helium ” lines. 
(8) Analogy with the “ 4481 ” series of magnesium indicates that the “ 4686 ” series 
