M 3IO 312] Double Stars and Nebulae 401 



and had accordingly put into " the shining fluid " class 

 (chapter XIL, 260). This last discovery, being exactly 

 analogous to Herschel's experience when he first began to 

 examine nebulae hitherto only observed with inferior tele- 

 scopes, naturally led to a revival of the view that nebulae 

 are indistinguishable from clusters of stars, though many 

 of the arguments from probability urged by Herschel and 

 others were in reality unaffected by the new discoveries. 



311. The question of the status of nebulae in its simplest 

 form may be said to have been settled by the first 

 application of spectrum analysis. Fraunhofer ( 299) had 

 seen as early as 1823 that stars had spectra characterised 

 like that of the sun by dark lines, and more complete 

 investigations made soon after KirchhofFs discoveries by 

 several astronomers, in particular by Sir William Huggins 

 and by the eminent Jesuit astronomer Angela Secchi 

 (1818-1878), confirmed this result as regards nearly all 

 stars observed. 



The first spectrum of a nebula was obtained by Sir 

 William Huggins in 1864, and was seen to consist of three 

 bright lines; by 1868 he had examined 70, and found in 

 about one-third of the cases, including that of the Orion 

 nebula, a similar spectrum of bright lines. In these cases 

 therefore the luminous part of the nebula is gaseous, and 

 Herschel's suggestion of a " shining fluid " was confirmed 

 in the most satisfactory way. In nearly all cases three 

 bright lines are seen, one of which is a hydrogen line, while 

 the other two have not been identified, and in the case of 

 a few of the brighter nebulae some other lines have also 

 been seen. On the other hand, a considerable number of 

 nebulae, including many of those which appear capable of 

 telescopic resolution into star clusters, give a continuous 

 spectrum, so that there is no clear spectroscopic evidence 

 to distinguish them from clusters of stars, since the dark 

 lines seen usually in the spectra of the latter could hardly 

 be expected to be visible in the case of such faint objects 

 as nebulae. 



312. Stars have been classified, first by Secchi (1863), 

 afterwards in slightly different ways by others, according to 

 the general arrangement of the dark lines in their spectra ; 

 and some attempts have been made to base on these 



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