TOO PROF. W. M. HICKS: A CRITICAL STUDY OF SPECTRAL SERIES. 



take wave-numbers as of equal value. It would, at any rate, have made the 

 calculation simpler. The results are as follows : 

 With AMES' HD(5) 



n = 27419-22- 1097087/(m + -000412) 2 , 

 without 



n = 27419-21 -109705-8/(TO+'000374) 2 . 



The limits are practically the same. They are 1097087/(2'000264) 2 . From the 

 latter formula the values of obs. calc. up to m = 10, and then at intervals of 5 to 30, 

 and the last 31 are as follows : 



m =3 4 



01 II --07R 

 25 30 



-03 -'20 



TO = 5 is the doubtful measurement. In spite of the good agreement, however, the 

 observations on which the determination of N is based are not at all as accurate as is 

 desirable. PICKERING'S measurements of HS in Puppis are far too inexact to give 

 any information as to the value of N. The most accurately measured spectra are 

 those of helium by RUNGE and PASCHEN, to which we next proceed. The HD and 

 HF series should be almost identical, so that the observed lines should be close 

 multiple lines, for which, as is well known, there is good evidence. 



The combination principle in H has recently been illustrated by the discovery of 

 two ultra-red lines by PASCHEN belonging to VP (3) VD (m), m = 4, 5. 



Helium. 



The observations for the spectrum of helium are due to RUNGE and PASCHEN* 

 for the visual and photographic region, PASCHENf for the ultra red, and measurements 

 of certain visual lines by interferential methods by Lord RAYLEIGH.J R. and P. 

 estimate their possible errors to amounts of the order of '005 A.U. There is no 

 doubt but that their measurements give by far the. most accurate values for any 

 complete spectrum yet obtained, but the possible errors are certainly several times 

 larger than their estimates. They estimated their possible errors for P' (l) and P" (l) 

 to be respectively 80 and 48. PASCHEN'S later and very accurate determinations 

 show that actually those errors were respectively 182 and 340. It is true that these 

 were bolometric measurements, and naturally cannot be compared with the others, still 



* ' Astro. Journ.,' iii., p. 4. 



t ' Ann. d. Phys.,' 27, p. 552 (1908). 



| ' Phil. Mag.,' xv. (6), p. 548 (1908). 



