PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
259 
Since enhanced lines have been found to occur in families of series similar to those 
previously recognised in arc spectra, the only new theoretical problem presented by 
them is to account for the appearance of the series constant with four times the value 
associated with arc lines. Otherwise, the extension of the simple formulae for proto- 
helium to enhanced lines in general is identical with the extension of Balmer’s 
formulae for hydrogen to the ordinary systems of series of arc lines. Hydrogen and 
proto-helium may thus be regarded as limiting cases of the two kinds of series. 
Independently of the series evidence there are numerous indications that enhanced 
lines originate in vibrating systems differing from those which give rise to arc lines. 
The mode of occurrence of the two sets of lines in the spectra of celestial bodies 
furnishes important evidence in favour of this view, and experimental evidence in the 
same direction is abundant. The extensive researches of Hemsalech on the phenomena 
of the spark discharge, for example, have revealed striking differences in the behaviour 
of the two classes of lines. # 
It has been shown that in a single discharge, the durations of the enhanced lines of 
Ca, Sr, and Mg, are much shorter with respect to their intensities than those of the 
arc lines of the same elements; and in another investigation it was shown that the 
velocities of the particles emitting the enhanced lines of Ca were nearly twice as great 
as those of the particles producing the arc lines.'!' 
These experiments suggested that centres of emission of different constitution were 
concerned in the production of the two classes of lines, but Hemsalech hesitated to 
adopt this view until other experiments had been made. 
The series investigation not only points to the existence ot different emitting 
systems, but, in the light of Bohr’s theory, may possibly give a clue to the nature of 
the difference. The appearance of 4N in the formulae for series of enhanced lines 
suggests that, as in the case of proto-helium, such series are produced during the 
binding of an electron by each of the atoms from which two electrons have been 
removed by the exciting source ; while arc series lines are emitted during the re-forma¬ 
tion of atoms from each of which only one electron has been removed. Although 
the neutral atoms of different elements contain different numbers of electrons, the 
approximate constancy of N for arc series, according to Bohr, is explained by the 
fact that the force on an electron entering a system consisting of a positive nucleus 
and a number of electrons one less than that required to render it neutral would not 
be very different from the force in the case of the binding of an electron by a hydrogen 
atom. A similar explanation would evidently account for the constancy of 4N in the 
formulae for series of enhanced lines. Variations from the simple series conditions 
presented by hydrogen and proto-helium would be expected in consequence of the 
presence of additional electrons, but these have not yet been worked out theoretically 
by Dr. Bohr. 
* ‘ Comptes Rendus,’ vol. 151, pp. 220 and 668 (1910). 
t ‘ Comptes Rendus,’ vol. 154, p. 873 (1912). 
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