452 Prof. McLennan and Mr. Young on the 



when suitable densities were used, absorption was always 

 obtained for the frequency v = (l'5, S) — (2,p 2 )? which in the 

 magnesium spectrum is the series frequency of the line 

 X = 4571 A.U. 



Some difficulty seems to have been experienced in observing 

 the spectrum of magnesium in the region lying between 

 7^ = 2600 A.U. and \ = 2000 A.U., for while Handke *, 

 Lyman f , and Q Saunders J record lines between A, = 2000 A.U. 

 and X = 1700 A.U., the only one who appears to have recorded 

 any line in the first-mentioned region is Saunders. 



Lorenser § in his Inaugural Dissertation gives the second 

 line of the series v=(l'5 S) — (m, P) as \ = 2026 A.U., and 

 ihis line Saunders || reports that he observed. 



From the illustrations given in fig. 1 it will be seen that 

 in the region between \=2500 i.U. and X = 2000 A.U. the 

 spark-lines are rather faint. In the spectrum of the arc in 

 air the line A. = 2026 A.U. is very strong, but the rest of 

 "the spectrum in this region is faint. In the spectrum of the 

 arc in vacuo the line \ = 2026 A.U. is clearly marked, as also 

 are some lines between A, = 2500 A.U. and \=2200 A.U. 

 The rest of the spectrum below X = 2600 A.U. is faint. 



In working out the spectrum of magnesium in the present 

 investigation a great many plates were taken with each 

 source of light, and different samples of magnesium were 

 used in order to eliminate any lines due to impurities which 

 might be present as traces in the metal. In determining 

 the wave-lengths four of the best plates obtained with each 

 source were selected, and only lines common to all four were 

 measured up. 



The wave-lengths of the lines were determined from a 

 calibration curve constructed for the spectrograph by using 

 the following prominent zinc spark-lines : — 

 Wave-lengths of Zinc lines ^[. 



A= 6588-65 A.U. 



6362-98 „ 



6103-58 „ 



5675-30 „ 



4925-37 „ 



3988-75 „ 



3282-49 „ 



3076-03 „ 



2801-15 „ 



X = 2658-27 A.TJ. 



2502-20 „ 



2418-95 „ 



2346-80 „ 



2265-08 „ 



2138-66 ., 



210006 „ 



2062-08 „ 



2025-56 „ 



* Handke, Inaug. Diss. Berlin, 1909, p. 18. 



t Lyman, i Spectroscopy of the Extreme Ultra- Violet ' (Longmans, 

 Green & Co.), p. 117. 



% Saunders, The Astrophys. Jl. vol. xliii. no. 3, p. 234 (1916). 

 § Lorenser, Inaug. Diss. Tubingen, 1913. 

 || Saunders, loc. cit. 

 51 Eder and Yalenta, Atlas Typischer Spectren, Wien. 



