$ 259, 26 ] Nebulae and Star Clusters 337 



new nebulae and clusters, three years later a second cata- 

 logue of the same extent, and in 1802 a third comprising 

 500. Each nebula was carefully observed, its general 

 appearance as well as its position being noted and described, 

 and to obtain a general idea of the distribution of nebulae 

 on the sky the positions were marked on a star map. 

 The differences in brightness and in apparent structure led 

 to a division into eight classes ; and at quite an early stage 

 of his work (1786) he gave a graphic account of the extra- 

 ordinary varieties in form which he had noted : 



" I have seen double and treble nebulae, variously arranged ; 

 large ones with small, seeming attendants ; narrow but much 

 extended, lucid nebulae or bright dashes ; some of the shape 

 of a fan, resembling an electric brush, issuing from a lucid 

 point; others of the cometic shape, with a seeming nucleus 

 in the center ; or like cloudy stars, surrounded with a nebulous 

 atmosphere ; a different sort again contain a nebulosity of the 

 milky kind, like that wonderful inexplicable phenomenon about 

 6 Orionis ; while others shine with a fainter mottled kind 

 of light, which denotes their being resolvable into stars." 



260. But much the most interesting problem in classifica- 

 tion was that of the relation between nebulae and star clusters. 

 The Pleiades, for example, appear to ordinary eyes as a 

 group of six stars close together, but many short-sighted 

 people only see there a portion of the sky which is a little 

 brighter than the adjacent region ; again, the nebulous 

 patch of light, as it appears to the ordinary eye, known as 

 Praesepe (in the Crab), is resolved by the smallest telescope 

 into a cluster of faint stars. In the same way there are 

 other objects which in a small telescope appear cloudy or 

 nebulous, but viewed in an instrument of greater power are 

 seen to be star clusters. In particular Herschel found that 

 i.iany objects which to Messier were purely nebulous 

 appeared in his own great telescopes to be undoubted 

 clusters, though others still remained nebulous. Thus in 

 'lis own words : 



"Nebulae can be selected so that an insensible gradation 

 shall take place from a coarse cluster like the Pleiades down 

 to a milky nebulosity like that in Orion, every intermediate step 

 being represented," 



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