1872.] Spectrum of the Great Nebula in Orion, 



confirm. I was not then able to use a slit suffi- 

 ciently narrow to show that the nebular line is 

 single and not coincident with the middle of the 

 double line of nitrogen. 



I am still pursuing the investigation of the 

 parts of this inquiry which remain unsettled. 



Second line. — This line was found by my 

 former comparisons to be a little less refrangible 

 than a strong line in the spectrum of barium. 

 Three sets of measures give for this line a 

 wave-length of 4957 on Angstrom's scale ; this 

 would show that the line agrees nearly in posi- 

 tion with a strong line of iron. At present I 

 am not able to suggest to what substance this 

 line belongs. 



This line is also narrow and defined. I sus- 

 pect that the brightness of this line relatively to 

 the first line varies in different nebulae. 



Third and fourth line— My former obser- 

 vations show that these lines agree in position 

 with two lines of the spectrum of hydrogen, 

 that at F and the line near G. 



These lines are very narrow and are defined ; 

 the hydrogen therefore must be at a low tension. 



The brightness of these lines relatively to the 

 first and second lines varies considerably in dif- 

 ferent nebulae; and I suspect they may also vary 

 in the same nebulae at different times, and even 

 in different parts of same nebula ; but at present 

 I have not sufficient evidence on these points*. 

 I regret that, in consequence of a continuance 

 of bad weather, I have not yet been able to 

 obtain decisive observations as to the possible 



* Since writing this sentence, I have seen a note by 

 Prof. D' Arrest in the 4 Astronomische Nachrichten,' 

 No. 1885. Speaking of the nebula H. IV. 37, he says : 

 — " Sein Spectrum ist ausser von Huggins bisher nur 

 noch von Dr. H. Vogel untersucht worden. In No. 1864, 

 Ast. Nachr. theilt Letzterer mit, dass erim Jahre 1871, 

 im Widerspruch mit Huggins' Angabe, die Linie Neb. 

 (3) = (2), bisweilen sogar (2)<(3) gefunden habe. 

 Nach Huggins war dagegen im Jahre 1864 positiv 

 (2)>(3). Ist Vogel's Beobachtung, wio ich nicht be- 

 zweifle, zuverlassig, so wird seine Vermuthung einer 

 Veranderung hier in der That begrundet sein, denn 

 diesen Winter, namentlich im Februar und Marz 



H 



