434 TWO CLASSES OF NEBULAE. [SECT, xxxvir. 



whose places were computed from his observations, reduced 

 to a common epoch, and arranged into a catalogue, in order 

 of right ascension, by his sister, Miss Caroline Herschel, 

 who added lustre to the name she bore by her eminence in 

 astronomical knowledge and discovery. Sir John Herschel 

 revised his father's observations, and added 800 nebulse to 

 the catalogue before he went to the Cape of Good Hope, in 

 order to complete the survey of the heavens. On his return 

 he published a catalogue of 2049 nebulae of the southern 

 hemisphere, of which 500 were previously unknown, with 

 their position in the heavens. In a work unparalleled for 

 elegance of style, depth of knowledge, and originality of 

 views, he has given engravings from his drawings of the 

 most remarkable objects, so that whatever changes may 

 take place in their form, place, or condensation, will be 

 known by astronomers of future ages. 



Though infinite in variety, the nebulae are of two distinct 

 classes ; one consists of patches of great dimensions, capri- 

 ciously irregular, assuming all the fantastic forms of clouds, 

 now bright, now obscure ; sometimes like vapour flying before 

 the wind; sometimes stretching long arms into space. 

 Many present an ill-defined surface, in which it is difficult 

 to say where the centre of the greatest brightness is. Large 

 portions are resolvable into stars ; many have a granulated 

 appearance, as if they were resolvable ; and others probably 

 are not so merely from the smallness and closeness of the 

 stars, and possibly from their remoteness, indicating the 

 complex and irregular form the Milky Way would present if 

 seen from a distance. A wonderful nebula of this kind is 

 visible to the naked eye in the constellation of Orion ; it is 

 of vast extent, sending branches even into the southern 

 hemisphere ; and, although Lord Ross's telescope has resolved 

 much that had hitherto resisted others, there are parts that 

 still maintain their nebulous appearance from extreme re- 

 moteness, presenting a kind of mottled aspect, like flocks or 

 wisps of wool, or mackerel sky. There can be no doubt of 



