Spectrum of Hie, Great Nebula in Orion. 215 



exactly on account of the email scale on which, from the faintuess 

 of the light of the nebula, it is desirable with the telescope at our 

 disposal to take the photographs, and also because in the nebular 

 spectrum itself we had no fiducial line nearer than H>/. In the photo- 

 graphs taken this year we have the advantage of the known position 

 of the hydrogen line at H, and with the help of this line our recent 

 measures show that the " about " must be interpreted as slightly less 

 refrangible than X 3724. Without attempting to fix its position abso- 

 lutely, we believe that the line will be found to fall between X 3725 

 and X 3726. It is not needful to point out that measures of these 

 little photographs cannot compare in accuracy with direct com- 

 parisons with considerable dispersion, as in the case of our observations 

 of the chief line of the nebula by eye. It is, however, now certain 

 that the line does not coincide with any one of the three components 

 of the magnesic oxide triplet, but is less refrangible than the middle 

 line at \ 3724, and falls between this line and the first line of the 

 triplet at X 3730. 



In these photographs there is a strong line, besides many faint lines, 

 on the less refrangible side of G. 



The background of the spectrum is seen to contain numerous faint 

 lines, which, as far as we have been able to identify them, are the 

 same as those seen in our earlier photographs, of some of which ap- 

 proximate measures were given in our paper, but they are, possibly on 

 account of a slightly wider slit, not so easily measured as they were 

 in the former photographs, in which no traces of the hydrogen lines at 

 h and at H could be detected. 



A marked feature of the lines consists of their abruptly different 

 intensities at different parts of their length, giving the blotchy appear- 

 ance which is characteristic of the lines in the visible spectrum, and 

 which we have described in our recent paper " On a Bedetermination of 

 the Position and Character of the Principal Line in the Spectrum of 

 the Nebula in Orion." The length of the slit takes in a large angular 

 extent of the nebula, and, therefore, usually includes within it one or 

 more of the brighter " mottlings " which are so well shown in photo- 

 graphs of the nebula. It is to be remarked that these brighter 

 blotches are sharply bounded, showing that the different parts of the 

 nebula are to some extent distinct and often become suddenly brighter 

 than the neighbouring parts. 



The lines of the new photographs contain two very strong and 

 abruptly-bounded blotches, and a third one less marked. 



These brighter blotches, corresponding to different conditions of 

 closely-adjacent nebular matter, give an explanation of an appearance 

 which we recorded last year in speaking of the strong line " about 

 X 3724." " On one side of the star-spectra this line is a little broader 

 than on the other side ; but, as a similar appearance is presented by 



