22 INDEX CATALOGUE. 



Herschel, W. : Abstracts of Memoirs — Continued. 



A.D. Vol. P. 



1789 79 219 Form I of nebulas, [P. T., 1785, p. 214,] is thus shown to 

 exist in the heavens. 

 Such clusters are subject to central powers. 



220 The idea of other central forces [than that of gravity] in 



the construction of the siderial heavens, was given in 

 certain mathematical papers delivered to the Phil. Soc. 

 of Bath, [and is yet entertained.] 



221 Not only were round nebulae and clusters formed by cen- 



tral powers, but likewise every cluster of stars or nebula 

 that shows a gradual condensation, or increasing bright- 

 ness towards a centre. 



222 This theory of central power is fully established on grounds 



[of observation] which cannot be overturned. 



223 Clusters can be f >und of 10' diameter with a certain de- 



gree of compression and stars of a certain magnitude, 

 and smaller clusters of 4' 3' 2' in diameter, with smaller 

 stars and greater compression, and so on through resolv- 

 able nebulae by imperceptible steps, to the smallest and 

 faintest [and most distant] nebulae. 



224 Other clusters there are, which lead to the belief that either 



they are more compressed or are composed of larger stars. 

 Spherical clusters are probably not more different in 

 size among themselves than ditferent individuals of 

 plants of the same species. As it has been shown that the 

 spherical figure of a cluster of stars is owing to centra, 

 powers, it follows that those clusters which, caeteris 

 ■paribus, are the most complete in this figure, must have 

 been the longest exposed to the action of these causes. 



225 The maturity of a siderial system may thus be judged from 



the disposition of the component parts 

 Planetary nebulae may be looked on as very aged. 



226 Though we cannot see any individual nebula pass through 



all its stages of life, we can select particular ones in each 

 peculiar stage. 

 226 



255 



