264 Scientific Proceedings, Royal Dublin Society. 
ments to wave-lengths by use of a curve. In the present set of 
measurements the micrometer was set at 280°0; and the gold-line, 
No. 9, brought into exact coincidence with the cross-wires, this 
being the reading corresponding to the wave-length of the line 
d 4792°8 on the curve. The micrometer was then screwed back 
to the edge of the plate, and the measurements proceeded with. 
It will be noted that only in one case did the micrometer return 
exactly to 280-0 for line No. 9; but the instrument was not 
perfect. With a quartz spectrograph all measurements on the 
less refrangible side of gold-line No. 9 are of little value, as 
there is not sufficient dispersion in this region of the spectrum. 
CrriuM. 
Plates XXII. and XXIII, spectra 3 and 4, show the spectra of 
solutions of cerium chloride with 10 per cent. and 1 per cent. of 
cerium respectively. There are a great number of lines in the 
spectrum of cerium ; but they are not well marked, and vanish very 
rapidly on dilution. Those lines marked y, corresponding to 
“1 per cent. of Ce, are scarcely visible on the negative, and could 
not possibly be seen in a reproduction; but they are the easiest 
lines by which to identify the presence of cerium, and they stand 
out fairly strongly with a 1 per cent. solution. 
QUANTITATIVE SpEcrRuM oF CERIUM CHLORIDE. 
me | aes | Teemgty | tees | aes | Ta 
ig inch. Persistency. ae Fart 2 Persistency 
249-6 5511°2 8x 289-9 4606°5 2 
26171 5472°8 6 290°7 4594-1 40 
255°3 5353°3 10 @ 291-9 4572-5 10¢ 
258°4 5274°3 10 » 292°6 4560°5 4 
261°9 5187°8 6 o 293°9 4539°9 4 
266°5 5079°5 6x 294°5 4527-5 10” x 
279-9 4792°8 Au No. 9 297°0 4488-4 Au No. 10 
284°0 4714-1 8 @ 298°0 4471-4 6 
288°7 4628°3 5 298°6 4460°4 8 
