Resonance Spectra of Sodium Vapour. 595 
The wave-lengths are recorded below. 
It is easy to pick out the three series of equidistant lines 
excited by the three magnesium lines, but it is impossible to 
tell anything about the extra lines or to which series any one 
of them belongs. Lines belonging to the same series are 
indicated by the same letter, while the exciting lines are 
underscored. The differences for the three series a, b, and c 
are given in a separate table. The series " a " is the best one 
obtained, so far as constancy of wave-length difference goes. 
The slight discrepancy at the middle of the table compensates 
itself, and is doubtless due to an error of half an Angstrom 
unit in the determination of one of the lines. 
5342 b 
5145*5 b 
Differences 
5327 a 
51427 
5305 
5140 
a. 
6. 
c. 
5302-5 b 
5136 a 
39 
39-5 
38-8 
5297-2 
5134-5 
38 
39 
38-7 
5296 
5133 
38 
39-5 
39 
5288 a 
5129 c 
38-5 
39-5 
38-5 
5286 
5124 
37-5 
39 
39-2 
5284 c 
5106-5 6 
38 
39 
40 
5263-5 b 
5101 
38 
38-5 
5258 
5098 a 
38 
38 
5250 a 
5095-2 
38 
39 
5245-2 c 
5089-8 c 
38 
5224 b 
5075 
5222 
5068 b 
5212 a 
5060 a 
5206-5 c 
5056 
5201-5 
5049-5 c 
5190 
5030 b 
5184-5 5 
5022 a 
5181 
5179 
5173-5 a 
4991 b 
4964 a 
4978-5 
4957 
5167-5 c 
4946 a 
Lead Excitation. 
The lead arc causes the sodium vapour to emit a well- 
marked series of lines, with a wave-length difference of 
39 Angstrom units. These lines are accompanied by com- 
panion lines sometimes on one side and sometimes on both 
sides, as will be seen from the chart. The Jaw governing 
the distribution of the companion lines and the possible cause 
of their appearance cannot be determined until a larger 
collection of photographs of the resonance spectra has been 
made and the results compared and tabulated. 
