5 26i] Nebulae and Star Clusters 339 



explanations Herschel chose the latter, considering the 

 nebulosity to be " a shining fluid, of a nature totally un- 

 known to us." One exception to his earlier views being 

 thus admitted, others naturally followed by analogy, and 

 henceforward he recognised nebulae of the "shining fluid" 

 class as essentially different from star clusters, though it 

 might be impossible in many cases to say to which class 

 a particular body belonged. 



The evidence accumulated by Herschel as to the distri- 

 bution of nebulae also shewed that, whatever their nature, 

 they could not be independent of the general sidereal 

 system, as on the " island universe " theory. In the first 

 place observation soon shewed him that an individual nebula 

 or cluster was usually surrounded by a region of the sky 

 comparatively free from stars ; this was so commonly the 

 case that it became his habit while sweeping for nebulae, 

 after such a bare region had passed through the field of 

 his telescope, to warn his sister to be ready to take down 

 observations of nebulae. Moreover, as the position of a 

 large number of nebulae came to be known and charted, 

 it was seen that, whereas clusters were common near the 

 Milky Way, nebulae which appeared ii, capable of resolution 

 into clusters were scarce there, and shewed on the contrary 

 a decided tendency to be crowded together in the regions 

 of the sky most remote from the Milky Way that is, round 

 the poles of the galactic circle ( 258). If nebulae were 

 external systems, there would of course be no reason why 

 their distribution on the sky should shew any connection 

 either with the scarcity of stars generally or with the position 

 of the Milky Way. 



It is, however, rather remarkable that Herschel did not 

 in this respect fully appreciate the consequences of his 

 own observations, and up to the end of his life seems 

 to have considered that some nebulae and clusters were 

 external " universes," though many were part of our own 

 system. 



261. As early as 1789 Herschel had thrown out the 

 idea that the different kinds of nebulae and clusters were 

 objects of the same kind at different stages of develop- 

 ment, some " clustering power " being at work converting 

 a diffused nebula into a brighter and more condensed 



