$ 3i?] Photometry : the Sidereal System 405 



317. The great problem to which Herschel gave so 

 much attention, that of the general arrangement of the 

 stars and the structure of the system, if any, formed 

 by them and the nebulae, has been affected in a variety 

 of ways by the additions which have been made to our 

 knowledge of the stars. But so far are we from any 

 satisfactory solution of the problem that no modern theory 

 cm fairly claim to represent the facts now known to us as well 

 as Herschel's earlier theory fitted the much scantier stock 

 which he had at his command. In this as in so many 

 cases an increase of knowledge has shewn the insufficiency 

 of a previously accepted theory, but has not provided a 

 successor. Detailed study of the form of the Milky Way 

 (cf fig. 104) and of its relation to the general body of stars 

 has shewn the inadequacy of any simple arrangement of 

 stars to represent its appearance ; William Herschel's cloven 

 grindstone, the ring which his son was inclined to substitute 

 tor it as the result of his Cape studies, and the more 

 complicated forms which later writers have suggested, alike 

 fail to account for its peculiarities. Again, such evidence 

 as we have of the distance of the stars, when compared 

 with their brightness, shews that there are large variations 

 in their actual sizes as well as in their apparent sizes, and 

 thus tells against the assumption of a certain uniformity 

 which underlay much of Herschel's work. The " island 

 universe " theory of nebulae, partially abandoned by 

 Herschel after 1791 (chapter xn., 260), but brought into 

 credit again by Lord Rosse's discoveries ( 310), scarcely 

 survived the spectroscopic proof of the gaseous character 

 of certain nebulae. Other evidence has pointed clearly to 

 intimate relations between nebulae and stars generally ; 

 Herschel's observation that nebulae are densest in regions 

 farthest from the Milky Way has been abundantly verified 

 as far as irresoluble nebulae are concerned while 

 obvious star clusters shew an equally clear preference for 

 the neighbourhood of the Milky Way. In many cases again 

 individual stars or groups seen on the sky in or near a 

 nebula have been clearly shewn, ekher by their arrangement 

 or in some cases by peculiarities of their spectra, to be really 

 connected with the nebula, and not merely to be accident- 

 ally in the same direction. Stars which have bright lines 



