SPECTRUM OF THE GREAT NEBULA IN ORION. 79 
This was confirmed by Messrs. FowLer and Baxanpatt at Kensington, with the 
10-inch equatorial, on October 31 and November 1, and again by Mr. Fow er with 
the 30-inch on November 2. 

Fig. 2. Difference in the appearance of the lines at 4862 (F) and 5006'5. 
It is also recorded in the Observatory note-book that at times these lines appeared 
of unequal length, the spectroscope employed in the observations having a long slit, 
and that sometimes 500 and 495 were seen without H,. In the photographic 
investigation of variations in the spectrum, the question is complicated by differences 
in sensitive films, and in the case of a silver-on-glass reflector by the conditions of 
the mirrors. My own experience has shown that when mirrors are tarnished, the © 
ultra-violet portion of the spectrum is weakened in greater proportion than the violet 
and blue. 
One of the most striking variations previously recorded is that of the strong line 
in the ultra-violet near \ 373. This was the strongest line in the photograph taken 
by Dr. Hucerns in March, 1882,* but it was not shown in Dr. Draper's photograph 
taken in the same year.t 
Dr. DRAPER says: “I have not found the line at 3730, of which he (Dr. Hucers) 
speaks, though I have other lines which he does not appear to have photographed. 
This may be due to the fact that he had placed his slit on a different region of the 
nebula, or to his employment of a reflector and Iceland spar prism, or to the use of a 
different sensitive preparation. Nevertheless, my reference spectrum extends beyond 
the region in question.” 
A later photograph (1889), taken by Dr. Huaains,t did not show the line in 
question, the slit being placed on a different part of the nebula. As already stated, 
the line is one of the strongest in my photographs, though it is not quite as strong as 
Hy. The spectrum photographed by Dr. Huccins, in 1889, differed entirely from 
those photographed by him in 1882 and 1888, the slit being again placed on a 
different region of the nebula. 
My own photographs are specially interesting, as they indicate differences even in 
the small area of the nebula which is covered by the slit during a single exposure. 
* ‘Roy. Soc. Proc.,’ vol. 33, p. 427. 
+ ‘Amer. Journ. of Science,’ vol. 23, p. 339. 
ft ‘Roy. Soc. Proe.,’ vol. 46, p. 41. 
