Photographic Spectrunt of the Great Nebula in Orion. 213 



in the spectrum of 2. 5 with the magnesium fluting of nearly the 

 same wave-length, and I am glad to say that my observations were 

 in accordance with your own." . " On comparing the 



brightest line with the magnesium fluting, both directly and by aid 

 of the micrometer wire, the line was seen to be well within the 

 limits of the fluting, and separated by a small but unmistakable 

 interval from its bright lower edge. The appearance was the same 

 on both sides of the grating, and in the 3rd and 4th spectra. The 

 comparison apparatus was carefully adjusted, and no shifting of the 

 line was caused by changing the position of the spark. The edge 

 of the fluting could not be brought into coincidence with the nebular 

 line. No measurement of the difference of wave-length was made, 

 as my attention was directed to the main fact of the non-coincidence 

 of the line in all positions of the instrument. I will make such 

 measures as soon as possible." 



" Note on the Photographic Spectrum of the Great Nebula in 

 Orion." By WILLIAM HOGGINS, D.C.L., LL.D., F.R.S., and 

 Mrs. HUGGINS. Received April 1(5, Read June 12, 1890. 



From an examination of the photographs of the spectrum of the 

 nebula in Orion taken by us in 1882, 1888, and 1889, we suggested 

 in our paper " On the Spectrum, Visible and Photographic, of the 

 Great Nebula in Orion,"* "that the mottled and broken-up character 

 of the nebular matter shown in Lord Rosse's drawings from eye- 

 observations, and much more strikingly brought out in the recent 

 photographs of Mr. Common and of Mr. Roberts, may be connected 

 with differences of spectrum in the photographic region, though in 

 the visible region there is no known alteration of the spectrum of the 

 four bright lines, except it may be some Sinn 11 differences of relative 

 brilliancy of the lines. Until next winter we cannot go beyond the 

 new information which these photographs give to us." 



Unfortunately, the necessity thrown upon us of making a laborious 

 redetermination of the position and character of the principal line in 

 the visible spectrum,f which has confirmed in every point the results 

 contained in our paper of last year (loc. cit.), has deprived us of the 

 more favourable opportunities during the past season of carrying out 

 our intention of photographing the spectra of different parts of the 

 nebula. 



We have obtained two photographs only, one taken on March 14th 

 and 15th, and the other on March 17th ; but these suggest how much 



* ' Roy. Soc. Proc.,' vol. 46, p. 42. 



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



