Prof. Pi. W. AVood on the 
power of the spectroscope employed. The silver line excites 
a series, the individual lines of which are about 1*5 Angstrom 
units longer than the corresponding lines* excited bv the 
cadmium line. The silver arc employed in exciting the 
:• train was formed between carbons, the lower 
one of which was furnished with a core of metallic silver. 
Xo prismatic separation was required, as the line 5209 was 
found to be the only one capable of exciting the vapour. 
The wave-lengths of the lines in the resonance spectrum are 
given in the following table. The first six lines in the table 
were verv faint, and their wave-lengths are not so accurate 
as is the case with the remainder. 
I';:- 
4985-£ "_~ 
5 --3 . 
5059-5 •:: 
5097-0 
5122-0 a og 
5i >0-0a :- I 
".' - - 
5198-0 a 38-2 
5209-0* 
5236-0 a 38 
52470* 
5285<T* ••- * 
53-45^ **° 
The strong lines which have been measured with consider- 
able accuracy, and which form a series, I h ve indicated with 
stars. The first five lines in the table also belong to the 
same series I think, though there seems to be a slight 
discrepancy in the table of differences. Four of the strong, 
equidistant Hues are accompanied by faint lines on the short 
wave-length side. These four companion lines are also equi- 
distant. They are marke I "in the table, and their 
wave-length differences are exactly 3<S A.E. 
. 
The resonance spectra excited by the light of the arc 
between electrodes of metallic copper are reproduced on 
PI. XVIII. fig. 3. The three green lines are alone operative, 
and though the li:_ - appears on the plate I am of the 
opinion that no fluorescence is excited by it. Spectrograms 
have been obtained of the spectra excited by the three lines 
separatelv. but thev are n satisfactory, as the slits of 
the monochromatic illuminator had to be made very narrow 
and there was but little light available. To sej irate close 
