the Spectrum of the Nebula in Orion. 201 



the compound carbon fluting. It seems, therefore, probable that all 

 the fainter lines are either due to carbon or to low-temperature 

 metallic lines. 



It is a very striking fact that some of the chief lines are ap- 

 parently coincident, although the statement is made with reserve, 

 with the chief bright lines in P Cygni, a magnificent photograph 

 of which I owe to the kindness of Professor Pickering ; it is one of 

 the Henry Draper Memorial photographs. 



The wave-length of the line about \373 may, perhaps, even yet 

 not be considered quite settled ; but this much may be said, that, in 

 those photographs in which the chief nebula line is sensibly coinci- 

 dent with the magnesium fluting, the ultra-violet line is very nearly, 

 if not quite, coincident with the least refrangible member of the mag- 

 nesium triplet (X3730). This, however, is somewhat uncertain, 

 because of the over-exposure of the magnesium spectrum. It is 

 certainly not coincident with either of the more refrangible lines of 

 the triplet, as the measured distance between the two lines of the 

 nebula is almost the same as that between the least refrangible line 

 and the fluting near X 500 of the magnesium spectrum. 



The most satisfactory determination of the position of the ultra- 

 violet line has been made by a comparison of the two photographs of 

 February 10 and 11. The magnesium spectrum in the latter photo- 

 graph is more clearly defined than in the former one, the slit being 

 narrower, and the other instrumental conditions remaining the 

 same. 



The distance between the fluting near 500 and the least refrangible 

 member of the triplet on the photograph of February 11 was found to 

 be very slightly less than that between the two nebula lines on the 

 photograph of February 10. According to these measures, the 

 nebula line falls between the two magnesium lines at 3730 and 3724, 

 about one-sixth of the distance between them from the former, giving 

 its wave-length as nearly 37^9. These measures, however, must only 

 be regarded as preliminary. 



A complete map is being prepared by Mr. .Fowler, but, as it 

 requires careful manipulation of the incident light for the detection of 

 the more delicate lines, it is not yet completed. I have asked Mr. 

 Fowler to take complete charge of this work, for the reason that the 

 sensitiveness of my own eyes is somewhat impaired. 



I have finally to express my great obligations to Mr. Fowler for the 

 zeal and patience which he has displayed in taking the photographs. 

 He is entirely responsible for those taken on February 2, 10, and 11, 

 when I was away from Westgate. 



YOL. XLVJII. 



