10 Proceedings of the Royal Society of Victoria. 



lines, while A5007 and X4959 are the " chief " nebular lines) 

 together with the helium line A4713, were the strongest lines in 

 the spectrum. Omitting A-i959, these appear alone in the repro- 

 duction^ of one of Sidgreaves' plates, showing that they were 

 then brighter than the hydrogen lines lying near them. 



But a comparison of Sidgreaves' results with those of the Lick 

 observers shows that in some respects the spectrum differed 

 widely from that of a typical nebula. In the first place, the 

 relative intensity of the lines was always altering, and was 

 never that of a typical nebular spectrum. Throughout August 

 and September Sidgreaves- and Lockyer^ found that A.3869 

 was the brightest line in the spectrum, though A5007 is not 

 merely the brightest line in the spectrum of ordinary nebulae, 

 but absolutely dwarfs all others. However, by 6th October, 

 when Sidgi'eaves brought his series to a close, H/? and Hy 

 were the only surviving hydrogen lines, while A5007 and A4959 

 had asserted their superiority both over them and over A3869, 

 though not to anything like the normal extent. 



Another pronounced difference between the two spectra was 

 the great width and difl'useness of the lines, combined with their 

 complex structure. This showed that the hot gases, though 

 similar in chemical nature to those of nebulae, were in a 

 condition of far greater density. This is further borne out by 

 the fact that, of the helium lines present, every one without 

 exception^ belongs to Kayser and Runges' "high density" series. 



A further difference of a very important character consisted in 

 the continued existence of a continuous spectrum by no means 

 faint for a 6th magnitude star,^ crossed by the old series of 

 narrow dark lines, the displacements of these lines indicating the 

 same steady but low velocity as they had done early in the year. 



The discovery by Ritchey," on 20th September, of a fairly large 

 nebula surrounding the star, falls within the period covered by 

 the later spectrographic i-esearches. In itself a most interesting 



1 All the lines are stated by Sidgreaves to be visible on the negative, but obviously those 

 wiped out in the reproduction must be the weakest. 



2 Loc. cit. 



3 P.R.S., Ixix., p. 137. 



4 Shown by a table given by the Lick observers ; loc. cit., p. 290. 



5 Sidgreaves, loc. cit., p. 150. 



6 Ap. J., xiv., p. 167. 



