Resonance Spectra of Sodium Vapour. 587 
quite possible that the apparent position of an emission-line 
of finite breadth may be to a slight extent modified by 
absorption. If, for example, there is a strong absorption- 
line on the violet side of and close to the emission-line, the 
centre of the emission-line will appear a little on the red 
side of its true position. Very slight departures from the 
law of equal spacing need not be regarded as fatal. A portion 
of one of the spectrograms is reproduced on PI. XIX. 
fig. 2. Exciting line 5086 shows and 8 or 10 of the resonance 
lines — Comparison spectrum of iron. 
The wave-lengths of the lines in the series excited by 
stimulation at wave-length 4800 are given in the following 
table, together with the wave-length differences. 
X. X differences. 
4765-5 34.5 
4800 .; 5 . 8 
4835-8 
74-2 
^=36-2 
49082 q fi .q 
4945-1 Ill 
4982-2 o 7 .* 
5019-7 375 
^=37-3 
5094-27 37.23 
5;3i-5 I;;::::;::;;::;:::::::;.;;; 3 s-oo 
5169-5 Qn.aa 
5207-46 ' o 7 . 7 . 
5245-2 :;:;;;;;::;;;;;;;;;;;;;;:;; 37-53 
5282-76 qs-Aq 
5321-39 H%1 
535942 3803 
Two only of the lines in the series are missing. In the 
previous investigation there were three that escaped detection. 
With longer exposure I feel confident that all would appear. 
It is clear that the lines are closer together in the region of 
shorter wave-lengths. The slight variations in the differences 
above the line 5019*7 are, in my opinion, due to the cause 
mentioned above. If we take the differences between five 
lines in the series, instead of between two, we find the spacing 
averages remarkably constant. Starting at the top (longest 
line) of the series, the differences between each line and the 
fifth below it run as follows:— 151-96, 151*89, 151*26, 150'93, 
151-46, 149*8, 149-3, and 149'17. 
The spectra are now being photographed with the 12-foot 
grating, and it is quite possible that still greater regularity 
will result. There seems to be no escape from the fact that 
the spacing widens slightly as we ascend the spectrum. 
We will now consider the resonance spectrum excited by 
