Line in the Spectrum of the Nebula in Orion. 209 



(1.) That the principal line is not coincident with, but falls within, 

 the termination of the magnesium-flame band.* 



(2.) That in the nebula of Orion this line presents no appearance 

 of being a " fluting." 



It is scarcely needful to say that, in the face of the observations 

 recorded in this paper, we are not able to accept the conclusions 

 arrived at by Professor Lockyer in his recent communicationsf to the 

 Royal Society. From them it would appear that Professor Lockyer 

 confirms my statement made in 1874,J that the second line " is 

 sensibly coincident with an iron wave-length 4957 " (Thalcn, \ 4956'8 ; 

 Liveing and Dewar, X 4956 9) ; and also that Professor Lockyer's 

 photographs confirm my photographs of 1882, 1888, and 1889, in that 

 it is a single strong line, nnd not a triplet, which appears in the ultra- 

 violet region, and that this strong line is more refrangible than tho 

 first component of the magnesic oxide triplet. 



Addendum. Received June 6, 1890. 



1. Addendum on the Position of the Line. 



One of the planetary nebulae, in the spectra of which I found in 

 my earlier comparisons with lead|| in 1874, that the principal line had 

 sensibly the same position as the corresponding line in the nebula of 

 Orion was 2. 5 (G. C. 4234). We have now compared again the 

 principal line in this nebula with the lead line X 5004'5 with the same 

 spectroscope (spectroscope B, 3rd eye-piece) and an arrangement for 



* [Even if the nebular line appeared to be sensibly coincident under the amount 

 of dispersion which can be brought to bear upon the nebula?, for reasons stated in 

 our paper of last year (loc. cit., p. 55, foot-note), the evidence would be strongly in 

 favour of the view that the coincidence was apparent only, and against the assump- 

 tion that the nebular line was to be regarded as the " remnant" of the magnesium- 

 flame band. We did not, however, give suflictent prominence to the fact of the 

 great brilliancy of the line in many nebulae, without the faintest traces of the 

 second and third flutings. The relative intensities of the brightest ends of these 

 flutings are : 



1st fluting .. .. .. .. .. 8 



2nd 7 



3rd 5 



(Watts, ' Index of Spectra,' p. 175). However, the position of the nebular line at a 

 measurable distance from the terminal line of the magnesium-flame band towards 

 the blue makes such considerations superfluous, and disposes finally of the assertion 

 that the nebular line is the " remnant " of the magnesium fluting. July 4.J 



t ' Roy. Soc. Proc.,' vol. 47, p. 129 and p. 189, &c. 



J ' Roy. Soc. Proc.,' vol. 22, p. 252. 



' Koy. Soc. Proc.,' vol. 46, p. 64. 



|| ' Boy. Soc. Proc ,' vol. 22, 1874, p. 254. 



