EFFECT OF PRESSURE UPON ARC SPECTRA. 
141 
shifts suggests that this group may be capable of resolution into simpler groups. 
The measurements of these lines were carefully repeated, but their diffuseness under 
pressure prevented great accuracy in the true setting of the lines, and all that could 
be definitely ascertained is that in both sets of photographs a few lines, hi, cl, d 1, 
are less shifted than some others, for example the lines c4, g2, h 3, l\A,jl,j2. See 
Diagrams VII. and VIIL, pp. 150 and 151, in which are plotted curves for the lines 
which reverse for a portion of the range of jDressure. 
Diagram II. 
The employment of poles not consisting entirely of iron, but of an alloy containing 
iron, or of an iron wire in a cored carbon, might facilitate the subdivision of this 
group, since it would decrease the widths of the lines and occasion greater accuracy 
in the measurements ; still it is not yet clear that the presence of another element is 
without effect upon the displacement of the lines. This is at present under investi¬ 
gation ; in the meantime it was considered advisable to avoid the use of an alloy. 
The reversed lines of Group I. are plotted separately in Diagrams III. and IV. 
Group II. contains fewer members and is more compact, all the lines showing 
