BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 543 



Herschel, W.: Synopsis of the Writings of— Continued. 



A. D. Vol. p. 



1789 79 214 Systems of stars— globular clusters and definition of a cluster. 



216 Admitting tliat a cluster is real, not apparent, the stars composing it 



are about of equal magnitude. 



217 At the same distance fi-om the centre an equal scattering takes place. 



217 These clusters are of a globular form. 



218 They are more condensed at the centre than at the surface. 



219 Form I of nebula? [P/(i/. Trans., 1785, p. 214], is thus shown to exist 



in the heavens. 



219 Such clusters are subject to central powers. 



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



tion of the sidereal heavens, was given in certain mathematical 

 papers delivered to the Philosophical Society of Bath [and is yet 

 entertained]. 



221 Not only were round nebula and clusters formed by central powers, 



but likewise every cluster of stars or nebula that shows a gradual 

 condensation, or increasing brightness towards a centre. 



222 This theory of central power is fully established on grounds [of 



observation] which cannot be overturned. 



223 Clusters can be found of 10' diameter W'itli a certain degree of com- 



pression 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 resolvable nobulaj by imperceptible stej>8, to the 

 smallest and faintest [and most distant] nebula). 



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



more compressed or are composed of larger stars. Spherical clus- 

 ters are probably not more dift'erent in size among themselves than 

 dilferent individuals of plants of the same sxiecies. As it has been 

 shown that the spherical figure of a cluster of stars is owing to 

 central powers, it follows that those clusters which, cmteris 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 sidereal system may thus bo judged from the dis- 



jjosition of the component parts. Planetaiy nebulaj may be looked 

 on as very aged, 



226 This method of viewing the heavens seems to throw them into a new 



kind of light. They are now seen to resemble a luxuriant garden 



which contains the greatest variety of productions in different 



flourishing beds; and one advantage we may at least reap from it 



is that we can, as it were, extend the range of our experience to au 



immense duration. For, is it not almost the same thing whether 



we live successively to witness the germination, blooming, foliage, 



fecundity, fading, withering, and corruption of a plant, or whether 



a vast number of specimens, selected from every stage through 



which the plant passes in the course of its existence, be brought at 



once to our view ? 



William Herschel. 



[Dated] Slough, near Windsor, May 1, 1789. 



226 Catalogue : 



226 Class I No. 94 to No. 215 



229 II No. 403 to No. 768 



238 III No. 377 to No. 747 



346 IV No. 30 to No. 58 



248 V No. 25 to No. 44 



250 VI No. 20 to No. 35 



