18 POPULAR SCIENCE MONTHLY. 



four others, shone through a nebula, so that the space around them 

 seemed far brighter than the rest of the heavens, which was entirely 

 clear, and appeared quite black, the effect being that of an opening in 

 the sky, through which a brighter region was visible." 



For a century after Huyghens made this observation it does not ap- 

 pear that these objects received special attention from astronomers. 

 The first to observe them systematically on a large scale was Sir Wm. 

 Herschel, whose vast researches naturally embraced them in their scope. 

 His telescopes, large though they were, were not of good defining 

 power and, in consequence, Herschel found it impossible to draw a cer- 

 tain line in all cases between nebula? and clusters. At his time it was 

 indeed a question whether all these bodies might not be clusters. This 



Fig. 8. The Great Nebula of Orion, as Photographed by A. A. Common with 



a Four-foot Reflector. 



question Herschel, with his usual sagacity, correctly answered in the 

 negative. Up to the time of the spectroscope, all that astronomers 

 could do with nebula? was to discover, catalogue and describe them. 



Several catalogues of these objects have been published. The one 

 long established as a standard is the General Catalogue of Nebula? and 

 Clusters, by Sir John Herschel. With each object Herschel gave a 

 i ondensed description. Recently Herschel's catalogue has been super- 

 seded by the general catalogue of Dreyer, based upon it. 



Some of the more conspicuous of these objects are worthy of being 

 individually mentioned. At the head of all must be placed the great 

 nebula of Orion. This is plainly visible to the naked eye and can be 



