456 Prof. McLennan and Mr. Young on the 



one most easily stimulated. It is also the only line which 

 comes out in the spectrum of the light from a gently burning 

 bunsen flame * fed with magnesium vapour. All these 

 considerations point to the series i> = (l*5, S) — (m, P) as 

 given above by Lorenser as being correct. The evidence 

 goes to show further that this series represents frequencies 

 of fundamental importance in the spectrum of magnesium 

 just as corresponding series have been shown to do for the 

 spectra of mercury, zinc, and cadmium. 



Assuming the wave-lengths given above for the series 

 v = (l , 5, S) — (m, P) as being correct, one may calculate the 

 series given by v = (l*5, S) — (nt,p 2 ). 



This has been done by Lorenser, and the wave-lengths of 

 the different members are as follows : — 



m— 2 3 4 



X=4571'27 . 2090-08 1843*08 1621-00 



Although series given by v = (l*5, S) — (m,p 2 ^ have been 

 identified in the spectra of mercury, zinc, and cadmium, no 

 such series, Q with the exception of the first member of 

 X = 45 7 1*2 7 A.U., has as yet been observed with magnesium. 

 The real existence of the series in the spectrum of magnesium, 

 moreover, has been questioned. It will be noted, however, 

 that both in the spectrum of the arc in air, and in that of 

 the spark in air, a faint line was observed at \ = 2091 A.U. 

 As the calculated value of the second member of the series 

 X = 2090*08 A.U. is within the possible error of measure- 

 ment of this line, it would appear therefore that it represents 

 the second member. The series would then seem to have a 

 real existence. 



Summary. 



1. The spectra of magnesium for (a) the spark in air, 

 (6) the arc in air, and (c) the arc in vacuo, have been 

 investigated in the region between X= 2852*22 A.U. and 

 A = 2000 A.U., and in all some fifty-eight new lines have been 

 measured. 



2. The existence of the line \ = 2026 A.U., first measured 

 by Saunders, has been confirmed and considerations have 

 been brought forward supporting the view that the series 

 i/ = (l*5, S) — (m, p 2 ) has a real existence. 



* McLennan and Thomson, Proc. Roy, Soc. A. vol. xcii. p. 584. 



