CELESTIAL CHEMISTRY. 229 



gas. The examination of easily resolved clusters by spectrum 

 analysis was a sure means of doing this. 



2 and 15 M, and 4678 and 4670 in Sir John Herschel's cata- 

 logue, both bright clusters, were chosen. Both these clusters 

 gave a continuous spectrum. 



The Great Nebula in the Sword-handle of Orion was next ex- 

 amined. The telescopic observations of this nebula seem to 

 show that it is suitable for observation, as a crucial test of the 

 correctness of the usually received opinion that the resolution of 

 a nebula into bright points is a certain and trustworthy indication 

 that the nebula consists of stars. Would the brighter portions of 

 the nebula adjacent to the trapezium which have been resolved, 

 according to Sir John Herschcl and others, pi'esent the same 

 spectrum as the fainter and outlying portions ? In the brighter 

 parts, would the existence of closely-aggregated "stars" be re- 

 vealed to us by a continuous sijectrum, in addition to that of the 

 true gaseous matter? These are suggestive questions, which it 

 was desirable to answer. 



The light from the brightest parts of the nebula near the trape- 

 zium was resolved into the three bright lines, to which we have 

 before drawn attention. These three lines, indicative of a gaseous 

 constitution, appeared. when the slit of the ajjparatus was made 

 narrow, veiy sharply defined, and free from nebulosity ; the in- 

 tervals between the lines were quite dark. 



When either of the four bright stai's of the trapezium Avas 

 brought ujDon the slit, a continuous spectrum of considerable 

 brightness, and nearly linear (the cylindrical lens of the appara- 

 tus having been removed), was seen, together with the bi-ight 

 lines of the nebula, which were of considerable length, corre- 

 sponding to the length of the opening slit. A fifth star y' and a 

 sixth a' are seen in the telescope, but the spectra of these are too 

 faint for observation. 



The positions in the spectra of a, j9, y, d trapezii, which corre- 

 spond to the positions in the spectrum of the thi*ee bright lines of 

 the nebula, were carefully examined, but in no one of them were 

 dark lines of absorption detected. 



The part of the continuous spectra of the stars a, ^, y, near the 

 position in the spectrum of the brightest of the bright lines of the 

 nebula, appeared, on a simultaneous comparison, to be more bril- 

 liant than the line of the nebula, but in the case of ^ the diflerence 

 in brightness was not great. The corresponding part of d was 

 perhaps fainter. In consequence of this small difference of bril- 

 liancy, the bright lines of the adjacent nebula appeared to cross 

 the continuous spectra of y and d trapezii. 



Other portions of the nebula were then brought successively 

 iqjon the slit ; but, throughout the whole of those portions of the 

 nel)ula which are sufficiently bright for this method of observa- 

 tion, the spectrum remained unchanged, and consisted of the three 

 bright lines only. The whole of this great nebula, as far as it lies 

 within the power of an eight-inch achromatic, emits light identi- 

 cal in character. The light from one part differs from the light of 

 another in intensity alone. 

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