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PROF. A. FOWLER.ON SERIES LINES IN SPARK SPECTRA. 
§ 1. Introductory. 
The classical work of Rydberg, and that of Kayser and Runge, dealt mainly with 
series lines in spectra which are developed in the flame or electric arc, or in vacuum 
tubes with discharges of moderate intensity. The lines to be discussed in the present 
communication are some of those which belong to the class of enhanced lines as defined 
by Lockyer ; that is, they are relatively intensified in passing from arc to spark 
conditions. 
In considering these lines it is necessary to take account of the fact that their 
behaviour in the arc is different for different elements. At least three classes may be 
recognised :—(I.) Enhanced lines like the H and K lines of calcium, which are well 
developed in the ordinary arc ; (II.) Lines which only appear with small intensities in 
the arc, such as the enhanced lines of iron ; (III.) Lines which do not appear in the 
ordinary arc (except very locally near the poles), but are strongly developed under 
spark conditions, as in the case of the well-known magnesium line at X 4481. 
Until very recently the only evidence that enhanced lines may belong to series was 
that afforded by Runge and Paschen’s observations of the spectra of magnesium, 
calcium, strontium, barium, and radium, under the influence of a magnetic field. In 
the case of each of these elements, three pairs of lines of Class I. were observed, and 
though no series formulae could be calculated for them, the magnetic resolutions 
proved that one pair belonged to the Principal series, another to the Sharp, and the 
third to the Diffuse series. Further discussion of these series has become possible 
through the valuable work of Lyman in the Schumann region, published in 1912. 
The present investigation was undertaken in connection with the new series of lines 
which were produced in 1912 by passing strong condensed discharges through helium 
tubes containing hydrogen as an impurity. # These lines, of which the strongest is at 
X 4686, are of considerable importance in celestial spectroscopy because of their 
occurrence in the spectra of some of the nebulse, and in stars which are generally 
regarded as representing the earliest stages of stellar condensation. One of the series, 
only feebly visible, was in apparent agreement with a series of lines first observed in 
i Puppis by Pickering, and attributed to hydrogen because of their simple relation to 
the Balmer hydrogen series. The other was a strong series, which included the lines 
assigned by Rydberg to the Principal series of hydrogen from analogy with the spectra 
of the alkali metals. In addition to the Rydberg lines, however, the “ 4686 ” series 
included intermediate lines, which the then recognised formuke suggested was a second 
Principal series related in a simple manner to the first. As the new lines could not be 
obtained from hydrogen alone, and in consideration of the occurrence of lines not 
anticipated by Rydberg, it was soon felt that further inquiry should be made as to 
the value of the numerical evidence on which their assignment to hydrogen was chiefly 
* ‘Monthly Notices, R.A.S.,’ vol, 73, p. 62 (December, 1912). 
