CELESTIAL CHEMISTRY. 243 



nebula consists of one line only. A prismatic examination of the 

 light from different parts of this object showed that it is through- 

 out of a similar constitution. 



"The most widely known, perhaps, of all the nebulae is the 

 remarkable cloud-like object in the sword-handle of Orion. 



"This object is also gaseous. Its spectrum consists of three 

 bright lines. Lord Rosse informs me that the bluish-green matter 

 of tlie nebula has not been resolved by his telescope. In some 

 parts, however, he sees a large number of very minute red stars, 

 whicli, though apparently connected with the irresolvable matter 

 of the nebula, are yet doubtless distinct from it. These stars 

 would be too faint to furnish a visible spectrum. 



" I now pass to some examples of the other great gi'oup of 

 nebulas arid clusters. 



" All the true clustei'S, which are resolved by the telescope into 

 distinct bright points, give a spectrum, which does not consist of 

 separate bright lines, but is ajaparently continuous in its light. 

 There are many nebuljB which furnish a similar spectrum. 



"I take as an example of these nebulae, the great nebula in 

 Andromeda, which is visible to the naked eye, and is not seldom 

 mistaken for a comet. The spectrum of this nebula, though 

 apparently continuous, has some suggestive peculiarities. The 

 whole of the red and part of the orange are wanting. Besides 

 this character, the brighter parts of the spectrum have a very 

 unequal and mottled appearance. 



"It is remarkable that the easily-resolved cluster in Hercules 

 has a spectrum precisely similar. The prismatic connection of 

 this cluster witli the nebula in Andromeda is confirmed by tele- 

 scopic observation. Lord Rosse has discovered in this cluster 

 dark streaks or lines, similar to those which are seen in the nebula 

 in Andromeda. 



" In connection with these observations, it was of great intei*est 

 to ascertain whether the broad classification afforded by the prism 

 of the nebulas and clusters, would correspond with the indications 

 of resolvability furnished by the telescope. Would it be found 

 that all the unresolved nebuljB are gaseous, and that those which 

 give a continuous spectrum are clusters of stars ? 



"Half of the nebulie which give a continuous spectrum have 

 been resolved, and about one-third more are probably resolvable ; 

 while of the gaseous nebulse none have been certainly resolved, 

 according to Lord Rosse. 



" Comets. — There are objects in the heavens which occasionally, 

 and under some conditions, resemble closely some of the nebulte. 

 In some positions in their orbits some of the comets appear as 

 round vaporous masses, and, except by their motion, cannot be 

 distinguished from nebulas. Does this occasional general i-esem- 

 blance indicate a similarity of nature ? If such be the case, if the 

 material of the comets is similar to that of the nebulas, then the 

 study of the wonderful changes which comets undergo in the 

 neighborhood of the sun may fui'nish useful information for a 

 more correct interpretation of the structure and condition of the 

 nebulae. In 1864, Donati found that the spectrum of a comet, 

 visible in that year, consisted of bright lines. 



