1889.] On the Spectrum of the Great Nebula in Orion. 41 



nebula was obtained with a narrow slit ; the same apparatus, so far 

 as the essential parts, which were described in my paper on the 

 "Photographic Spectra of the Stars,"* being employed. 



In this photograph, in addition to the strong line about X 3730, a 

 pair of less conspicuous lines is seen on the less refrangible side of 

 the strong line. 



The continuous spectra due to the two of the four bright stars of 

 the Trapezium which fell upon the slit are present. 



Across these continuous spectra at least four groups of bright 

 lines can be seen, of which the greater number can be traced into the 

 nebula for some little distance from the stellar spectra. 



It is scarcely necessary to state the importance of this observation 

 as showing that these stars of the Trapezium are not merely optically 

 connected with the nebula, but are physically bound up with it, and 

 are very probably condensed out of the gaseous matter of the nebula. 

 This observation would seem also to show that the nebula, as a whole, 

 may not be at a distance from us greater than that which we should 

 attribute to such stars, if they occurred alone in the heavens. 



The first group, of six lines, occurs between X 4116 and 4167. 

 The lines of this group do not extend far from the continuous star 

 spectra, with the exception of two lines. These can be seen 

 faintly in another photograph taken in 1889. Beyond there is a fainter 

 group, probably of four lines a little beyond h. I am pretty sure 

 that these lines extend into the nebula. The third group from 

 X 3896 to 3825, of which I have endeavoured to measure ten lines, is 

 faint, but here there is no doubt that the same lines are present in 

 the adjoining nebular matter. There are two lines a little more 

 refrangible than the strong line seen in 1882, at about X 3709 and 

 X 3699. I have a suspicion of a faint group about this place, and also 

 of another group on the less refrangible side of G. 



I shall discuss further on the probable chemical significance of 

 these lines. The lines of this photograph are shown in Spectrum 

 ETo. 1 of the folding Plate. 



During the time that Orion was favourably situated for observation 

 in the season of 1888 and in that of the present year, the unusual con- 

 tinuance of bad weather has made it impossible for me to give so 

 complete an account of the spectrum of the nebula in the photo- 

 graphic region as a few really fine nights would have enabled me to 

 do. However, on February 28th of the present year I obtained 

 another photograph, the slit being very narrow, which gives some 

 more new information of the nature of its spectrum. I was astonished 

 on looking at the photograph not to see the strong line about X 3730, 

 which was by far the most conspicuous feature of the photograph 

 taken in 1888. The pair of lines near it on the less refrangible side, 

 * ' Phil. Trans.,' 1880, p. 672. 



