Resonance Spectra of Sodium Vapour. 593 
In this table I have marked the lines which form the equi- 
distant set with a star *. The last three belong to a different 
series not continuous with the first. It will be seen that as 
we go up the spectrum other lines come in on the short wave- 
length side of the principal lines, as in the case of the series 
of doublets excited by the green cadmium line. 
A discussion of the spectrum excited by the blue- violet 
lithium line will be postponed for the present, as I prefer to 
wait until photographs have been obtained with the large 
Barium Excitation. 
The excitation by the strong barium line 1934: is of great 
interest as the same series is brought out as in the case of the 
lithium excitation. The point of excitation is at the first 
line below to the exciting line of lithium, and the resonance 
spectra are identical except that the companion lines which 
appear on the short wave-length side of the principal lines 
are not identical. The wave-lengths are given in the 
following table, the principal lines being marked with stars. 
Differences. 
4896-2* 
49340* %% 
4972-3* o8d 
5008-9 3S-7 
5011-0* 
5046-6 337 
5049-7 * oo.- 
5088-4* d5/ 
51230 39-2 
5127-6 * 
5130-8 
51609 39-5 
5163-8 
5167-1 * 
5192-0 
5199-3 395 
52023 
5206-6 * 
5238 
52400 
5299-5 
5327-7 
5336-2 
5340-0 
The three strong lines of a different series, which appeared 
with the lithium excitation (double starred in the table), do 
not appear with the barium excitation. 
