EFFECT OF PRESSURE UPON ARC SPECTRA. 
159 
Table IX. shows the characteristics of the different groups, and indicates that they 
may be the Principal, First and Second Subordinate Series into which the spectra ot 
other elements are generally divisible. 
Humphreys found that the ratios of the shifts for these series are for other elements 
1:2:4, and that in the iron spectrum two groups exist with shifts in the ratio 
of 1 : 3. 
As already suggested, the groups seem capable of further subdivision; for instance, 
cl and d 1 present differences in shift and in reversal from the majority of lines 
in Group I., Diagram VIII., p. 151. There are also very obvious triplets in the 
spectrum, as well as what are at present thought to he doublets. A detailed account 
of these is reserved for future publication, when a larger range of the spectrum has 
been investigated. # 
The photograph, Plate 5, fig. 2, shows the remarkable “ gregarious ” tendency of the 
lines of Groups II. and III. The members of the former all occur at the less 
refrangible end of the region investigated, and those of the latter within a small range 
of wave-lengths near the centre. 
Humphreys states that the lines belonging to any group behave similarly when 
the source is placed in a magnetic field, but the Zeeman effect upon the lines 
investigated in the present research is not to hand. 
The two groups into which Humphreys originally divided the lines of the iron 
spectrum are shown by the broken lines XX in Diagram YI. As already stated, 
Humphreys! has recently obtained a photograph at 37 atmospheres pressure; 
though the displacements given by it are of the same order as those obtained in the 
present research, the agreement between the readings is not as good as one would 
wish to find. 
The evidence for the existence of a Group IY. has already been given (see Table II., 
p. 138, lines g6 and g7, also p. 157). The displacements are roughly eight times 
those of lines belonging to Group I. 
In the present research the two sets of photographs agrfee in assigning the same 
lines to the same groups. A list of the lines belonging to each group is given in 
Table X., p. 1G0. 
* Attention may be drawn to the three lines dO, d’2, el (A. for d2 = 4308), which appear to form a 
triplet in which the frequency relation between its members is approximately the same as that for the 
triplets b3, bi, cO at 4404, and for j3, ji, IT at 4063, though the frequency differences are not the same. 
[t Note added December, 1907.—Subsequent to the presentation of this paper, a paper by Humphreys 
has appeared in the ‘ Astrophysical Journal,’ XXVI., 18, 1907, describing experiments in which photo¬ 
graphs of arc spectra were obtained under pressures of 42, 69, and 101 atmospheres.] 
