August 2 2, 1889] 



NATURE 



405 



SOCIETIES AND ACADEMIES. 

 London'. 



Royal Society, May 2. — '*On the Spectrum, Visible and 

 Photographic, of the Great Nebula in Orion." By William 

 Huggins, D.C.L., LL.D., F.R.S., and Mrs. Huggins. 



1 have added the name of Mrs. Huggins to the title of the 

 paper, because she has not only assisted generally in the 

 work, but has repeated independently the delicate observations 

 made by eye. 



In the year 1882 I had the honour to lay before the Royal 

 Society a note on the photographic spectrum of this nebula, in 

 which I described a new bright line in the ultra-violet, to which 

 I gave a wave-length of about 3730. In addition to this new 

 line, the lines of hydrogen, H;8 and H7, which I had discovered 

 by eye in my early observations on the visible spectrum, were to 

 be seen upon the plate. 



On account of the faintness of the object the slit had been 

 placed rather wide, and for this reason the character of the line 

 and its position, as I stated in the paper, could not be ascer- 

 tained with the accuracy which I desired. 



On February 5, 1888, a photograph of the spectrum of this 

 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" (Phil. Trans., 

 1880, p. 672) 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 ssen, 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 observa- 

 tion 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 \ 41 16 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 K 3896 to 3825, of which I hs-ve 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 A. 3709 and \ 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. 



During the time that Orion was favourably situated for ob- 

 servation in the season of 1888 and in that of the present year, 

 the unusual continuance of bad weather made it impossible for 

 me to give so complete an account of the spectrum of the 

 nebula in the photographic region as a few really fine nights 

 would have enabled me to do. However, on February 28 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 A. 3730, which was 

 by far the most conspicuous feature of the photograph taken in 

 1888. The pair of lines near it on the Itss refrangible side, 

 which I found for the first time in 1888, are present ; and on a 

 further scrutiny of the plate I discovered two other pairs of lines, 

 most probably rhythmically connected with them, in the still 

 more refrangible region, the last pair, accompanied by a third 

 line, being near the ultra-violet limit of ex-terrestrial light. 



I was also able to see faintly two of the bright lines which I 

 have described as present across the continuous spectra of the 

 brighter stars of the Trapezium in my photograph of 1888. It 

 is not quite certain whether these very faint and short lines are 

 really due to the matter of the nebula proper, or have come upon 

 the plate in consequence of the stars of the Trapezium having 



fallen accidentally upon the slit for a time loo short to impress 

 the continuous part of their spectra. No trace of a continuous 

 spectrum can be seen upon the plate, but these lines in the plate 

 of 1888 do extend beyond the condnuous spectra of the stars of 

 the Trapezium. 



I regret extremely that bad weather has made it impossible for 

 me to work out the circumstances on which depended the dis- 

 appearance of the strong line about A 3730. Both the photo- 

 graphs which show this line include two stars of the Trapezium, 

 and it may possibly be that this strong line is associated with the 

 groups near it in the spectra of the stars, and may therefore come 

 out in those parts of the nebula only which are more condensed. 

 A few photographs with the slit differently placed upon the 

 nebula would doubtless have thrown light upon this point. The 

 suggestion presents itself strongly that the mottled and broken- 

 up character of the nebular matter, shown in Lord Rosse's 

 drawing from eye observations, and much more strikingly 

 brought out in the recent photographs of Mr. Common and 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, ex- 

 cept, it may be, sime small differences of relative brilliancy of 

 the Hues. 



Until next winter we cannot go beyond the new information 

 which these photographs give to us. On the plate of the photo- 

 graph of 1889 two pairs of spectra for comparison were taken : 

 two spectra, one above and one below the nebular spectrum, of 

 burning magnesium ; and two spectra, similarly placed, of the 

 light of the sky. 



From the photographs of 1888 taken with a narrow slit, the 

 position which I gave in 1882 to this line is shown to be, as I 

 expected from the wide slit then used, approximate only. I find 

 from the later photograph that the wide slit had caused the 

 strong line to unite with a line near it, and that in 1882 I 

 measured the middle of the broad band produced by the union of 

 the wide images of two lines. Its position is about six tenth- 

 metres more refrangible. It does not therefore agree, as I then 

 suggested, with the hydrogen line C in my spectra of white stars. 

 A statement of the position of this line relatively to the mag- 

 nesium-flame triplet will be given further on, when I come to 

 discuss the comparison of this spectrum with that of the nebula. 



The position of the pair of lines a little less refrangible than 

 this strong line, seen with it in the photograph of 1888, and pre- 

 sent without the strong line in the photograph of 1889 ; and the 

 positions of the two other more refrangible pairs, presumably 

 connected with the first pair, are given in the following table : — 



. ^ • I, ^ r ^ 3752 o 



1st pair about -j ^^^j-q 



2nd pair about \ \ Jl.^ 



Line at about 3o6o'0 



3rd pair about -! -5 53 



In both photographs I suspect the indications of other lines, 

 which are too faint to permit any certain conclusion to be formed 

 about them, whether they are true lines, or imperfections only of 

 the film. 



[The continuous spectra of the stars of the Trapezium can be 

 seen on the plate from about F to about A 3570, but they are 

 very faint beyond A 3660. — May 18.] 



The Visible Speciruvi. 



a. Brightest line. — In 1872 (" On the Spectrum of the Great 

 Nebula in Orion, &c.," Roy. Soc. Proc, vol. xx. (1872) p. 383), 

 I stated, as the result of numerous direct comparisons of this line 

 with the brightest line in the spectrum of nitrogen, that the 

 nebular line was "sensibly coincident with the middle of the 

 less refrangible line of the double line of nitrogen." To avoid 

 repetition I will call this line Nj. Except where it is otherwise 

 stated, I use this line of nitrogen simply as a fiducial point in. 

 the spectrum, without any reference to its chemical significance. 



In a still more critical examination of the position of the 

 nebular line for the purpose of determining whether there 

 was any indication of relative motions of the gaseous nebulae in 

 the line of sight, I found some experimental difficulty from the 

 circumstance that the nebular line is narrow and defined, while Nj 

 is nebulous. I was fortunate to find a more suitable fiducial line 

 of comparison in a narrow line of lead, which falls almost upon 



