252 PROF. A. FOWLER ON SERIES LINES IN SPARK SPECTRA. 
Either of the groups might be approximately determined in this way from the 
other, but an accurate knowledge of all the data for one of them can only be obtained 
by utilizing observations of both. 
An interesting extension of the investigation of magnesium has been made possible 
by the production of a number of new lines of moderate intensity, which occur under 
the same experimental conditions as the “F.P.” group (Plate 3, figs. 1, 2, 3). For 
some time the appearance of these lines seemed rather capricious, and it was thought 
that they might possibly be due to impurities. It was eventually found, however, 
that they could always be obtained in a small region near the negative pole of the 
arc in vacuo, provided a sufficiently heavy current (7 or 8 amperes) were employed. 
The lines were shown clearly in many photographs, where the only lines 
traceable to impurities were faint ones of Na, Ca, and Ba. No further doubt can 
be felt as to the magnesium origin of the lines, in view of the manner in which 
their positions can be calculated from the constants for the doublets, taken from 
Table XI. 
One of the combination terms has already been utilised in the discussion of the 
p series. This is represented by the combination 2d — 3p 1} 2d — 3p 2 , giving the lines 
XX 3850'40, 3848'24, which form a well-marked doublet, shown very clearly in Plate 3, 
fig. 8. 
In addition, two well defined Combination series have been identified. A few of 
the lines involved appear in a list given by Lorenser, but their connections were not 
then recognised. One of these series is somewhat stronger than the other and 
may be distinguished as Series A, the weaker being designated Series B. In 
each case the intensities degrade in the usual way in passing to the violet 
(Plate 3, figs. 1, 2, 3). Series A results from the combination 3 d—mf, and B from 
3 f—mf. Details of the observations and comparisons with the calculations are given 
in Tables XII. and XIII. 
Table XII.—Magnesium Combination Series A (3 d—mf). 
Limit, 3 cl = 27,957-8. 
m. 
A (I. A.) 
(Fowler). 
n 
(observed). 
n 
(calculated). 
O-C, 
An. 
Remarks. 
5 
6346-67 
15,752-0 
15,751-2 
+ 0-8 
Lor. gives A 6347-27 (Row.). 
6 
5264-14 
18,991-3 
18,990-4 
+ 0-9 
„ „ A5264-2 
7 
4739-59 
21,093-1 
21,092-4 
+ 0-7 
8 
4436-48 
22,534-2 
22,533-8 
+ 0-4 
9 
4242-47 
23,564-7 
23,564-5 
+ 0-2 
„ „ A 4242-60 „ Mg? 
10 
4109-54 
24,326-9 
24,327-0 
-o-i 
11 
4013-80 
24,907-1 
24,907-0 
+ 0-1 
