1889.] On the Spectrum of the Great Nebula in Orion. 53 



lines " (with two-prism train) " are very sharp. X 5005 showed a 

 faint fringe mainly on the side nearer the blue." 



Mr. Maunder has recently sent a note to the Royal Astronomical 

 Society, in which he explains that the observation was made with a 

 second half-prism added to the half-prism spectroscope. He says : 

 u The three principal lines of the nebular spectrum were seen as very 

 narrow bright lines, but none of them were perfectly sharp, each 

 showed a slight raggedness at both edges ; but in the case of the line 

 near X 5005 it was clear that this fringe, or raggedness, was more 

 developed towards the blue than towards the red. In the case of the 

 other two lines, they were not bright enough for it to be possible to 

 ascertain whether the fringes were symmetrical or not. But X 5005 

 was clearly a single line. There was no trace of any bright line, or 

 series of bright lines, close to it on either side ; no trace of a fluting, 

 properly so called. The entire line, fringes and all, was only a 

 fraction of a tenth metre in total breadth."* 



[It should be noticed that the instrumental conditions under which 

 Mr. Maunder observed showed the second and third line " not per- 

 fectly sharp, but with a slight raggedness at both edges." — May 13.] 



My own observations of this line, since my discovery of it in 1864, 

 with different spectroscopes up to a dispersion equal to eight prisms 

 of 60°, show the line to become narrow as the slit is made narrow, 

 and to be sharply and perfectly defined at both edges. 



As some importance attaches to the precise character of this 

 line, I wrote to Professor H. C. Yogel for permission to quote the 

 result of his experience, which has been nearly as long as my own, of 

 the character of this line. He says in his reply, dated 20th March, 

 1889 : " Beeile ich mich Ihnen mitzutheilen, dass meine langjahrigen 

 Beobachtungen iiber die Spectra der Gas-Nebel vollkommen mit den 

 Ihrigen darin iibereinstimmen, dass die Nebellinie X 5004 schmal, 

 scharf und nicht verwaschen ist. Auch D 'Arrest hat in seiner Unter- 

 suchung iiber die Nebel-Spectra (Kopenhagen, 1872) nicht erwahnt 

 dass die hellste Nebellinie unscharf sei." 



Dr. Copeland permits me to quote the following sentences of a 

 letter dated March 19, 1889 : — " Respecting the appearance of the 

 line X 5004 in the spectrum of the Orion nebula, I may say that I 

 have always drawn and seen it quite sharp and well defined on both 

 edges. About nine years ago I made a special effort to divide it, if 

 possible, with a large spectroscope in which the viewing telescope 

 was 3 inches in aperture. The lines were then seen as sketched." 

 (The diagram shows the nebular lines with sharply ruled lines for 

 edges.) " They were drawn by holding the note-book 10 inches from 



* < Monthly Notices R.A.S.,' vol. 49, 1889, p. 308. 



