TRANSACTIONS OF SECTION A. 443 



tinuous spectrum on one side. The head of the comet was in sharp focus upon the 

 slit, and the continuous spectrum with defined edges corresponds to the nucleus, 

 which in this comet was very distinct. Tlie side of tlie spectrum on wliich these 

 suspected groups project corresponds to the liglit of the corona on the side of the 

 nucleus next the sun. We learn, therefore, that the light of this part of tlie coma 

 consisted chiefly, in this region of the spectrum, of these groups ; as, on the plate, 

 only a very faint continuous spectrum can be seen between these projected parts 

 of the bright spaces. 



It is not possible to measure with accuracy the beginnings and endings of 

 these groups. Measures as accurate as the circumstances would permit have been 

 taken, therefore, of the brightest parts of the groups. The wave-lengths of these 

 brightest parts are : — 



X4253 



„ 4412 



„ 4507 



„ 4634 



„ 4769 

 As the lines of sodium were strong in the visible part, it may be that some of 

 the light producing some of these groups may be due to that substance. 



Mr. Hind has kindly furnished me with the distance of this comet from tlie sun 

 at the time the photograph was taken. The comet was then 42,380,000 miles 

 from the sun, while the comet of last year was distant 69,420,000 when I obtained 

 the photograph of its spectrum. Considering the fact that the presence of sodium 

 and the absence of the hydrocarbon groups were observed some time previously to 

 the taking of the photograph, when the comet was much farther from the sun, we 

 cannot attribute this difl'erence of spectrum directly to greater heat through a 

 nearer approach to the sun, but must regard it as arising from a diflerent chemical 

 constitution of the cometary stuff. Professor A. Ilerschel and Dr. von Konkoly 

 showed long ago that the spectra of the periodic meteors are different for 

 different swarms, and it is not surprising that the nuclei of comets should differ 

 chemically. 



If the light of comets is due to electric discharges, then, although these dis- 

 charges woidd owe their existence to solar heat, their heating effect upon the 

 cometary matter might not be strictly proportional to the nearness of the comet 

 to the sun. In this case too the absence of the liydrocarbon lines might not neces- 

 sarily indicate the freedom of the cometary matter from these substances. It has 

 long been known how preferential is an electric discharge when several substances 

 are present at the same time, and Dr. Hasselberg has recently shown by some 

 laboratory experiments that in the presence of sodium vapour the hydrocarbon 

 spectrum fades out. Notwithstanding these considerations, it is probable that 

 the comet of this year owed the exceptional character of its spectrmn to some 

 unusual chemical condition of the meteoric matter which forms its nucleus. 



[P.S.] — November 14. — The spectrum of the great comet which was discovered 

 near the sun in September was similar in the visible region to that of comet 

 Wells. Now that this comet has receded from the sun the lines of sodium are 

 fainter, and the hydrocarbon groups seen in former comets have made their appear- 

 ance. These observations support the view taken in this paper, but suggest that 

 the chemical state of the matter of these two comets is not essentially different from 

 that of former comets, but is probably a modification of the same fundamental 

 constitution. — W. H.] 



9. Oil the Photographic Spectfum of the Great Nebula in Orion. 



By William HuggixXs, D'.O.L., LL.V., F.B..S. 



[Plate X.] 



In September 1864 ^ I had the honour to announce to the Royal Society that 



I had discovered that certain of the Nebuloe (up to that time, eight in number), 



give a spectrum of bright lines. In one nebula four bright lines were observed. 



> PJdl. Trans. 1864, p. 437. 



