Spectrum of the Great Nebula in Orion. 



These lines are very narrow and are defined ; 

 the hydrogen therefore mnst be at a low ten- 

 sion. 



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 W ] 

 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 

 motion of the nebula in the line of sight. With 

 spectroscope B and eyepiece 2, the lines appear to 

 be coincident with those of hydrogen. In con- 

 sequence of the uncertainty of the character of 

 the first line, which is single, while that of ni- 

 trogen is double, this determination can now 

 only be made by means of the comparison of 

 the third line with that of hydrogen. This 

 third line becomes very faint from the great 

 loss of light unavoidable in a spectroscope that 

 gives a sufficient dispersive power, and the com- 

 parison can only be attempted when the sky is 

 very clear and the nebula near the meridian. 



139 



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

 Prof. D'Arrest in the ' Astrononrische Nachrichten,' 

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

 — " Sein Spectrum ist ausser von Huggins bisher nur 

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

 Ast. Nachr. theilt letzterer init, dass er im 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). IstYogel's Beobachtung, wie ich nicht be- 

 zweifie, zuverlassig, so wird seine Vermuthung einer 

 Veranderung hier in der That begriindet sein, denn 

 diesen Winter, namentlich im Februar und Marz 

 1872, fand ich wiederum, so wie es Huggins friiher ge- 

 sehen hat, unzweifelhaft (2)>(3). Die relative Inten- ^ 

 sitat der drei Lichtarten habe ich mehrfaeh in Zahlen^ 

 geschatzt und erhielt z. B. in den letzten Nachten : 



Marz 6. Miirz 13. 



(1) 100 100 



(2) 58 63 



(3) 49 52" 



