220 Scientific Proceedings, Royal Dublin Society. 
lengths are given in the following table; and in the subsequent 
tables these numbers are inserted in their proper place. Thus in 
the case of chromium, we see from the table that the first triplet 
of persistency ~ lies between 11 and 12, and we know where to 
look for it on the plate. 
NumsBerep Goup Lines. 
No. Wave-length. | No. | Wave-length. 
9 4792°8 18 3029°3 
10 4488-4 19 2913°6 | 
11 4315-4 20 2825-6 
12 4065-2 21 2748°3 
13 3898°0 22 267671 
14 3586°5 23 2641-6 
16 3380°0 24 2590-2 
16 3280-8 25 2544-3 | 
7 3122-9 
Tron. (Plate XII.) 
Spectra 1 and 2 show the lines of the metal, with gold lines 
first long and then short, and there are so many coincident lines, 
that in the second spectrum the gold lines have almost all 
disappeared. Spectrum 3 shows the lines of a strong solution 
long, and the metal short; and as no short lines stand out it is 
clear that all iron lines are developed, both by the metal and a 
strong solution. 
Spectrum 4 gives the lines of a strong solution short, with 
gold long, and is practically identical with spectrum 1. 
Spectra 5, 6, 7, 8 show the lines of dilute solutions, and 
their progressive disappearance is given in the following table ; 
but there are so many 9 lines that it was not thought necessary 
to record more than the strongest. Some of the lines that show 
well with a strong solution, but are not seen with dilute solutions, 
are marked o. 
