NEBULJE. 309 



layers of whose nuclei even a faint star continues visible, and 

 finally to that of discrete particles, so deficient in density 

 that their solidity, either within large or small dimensions, 

 can scarcely be characterized, except by the limits which 

 bound them. It was by such considerations as to the constitu- 

 tion of the apparently vaporous zodiacal light that Cassini, 

 long before the discovery of the so-called smaller planets be- 

 tween Mars and Jupiter, and prior to all conjectures regard- 

 ing meteor- asteroids, was led to the idea that there exist 

 cosmical bodies of all dimensions and all degrees of density. 

 We here almost involuntarily touch upon the old metaphy- 

 sical controversy regarding matter of primitive fluidity and 

 that composed of discrete molecular particles, and therefore 

 more amenable to mathematical treatment. From hence we 

 turn the more readily to our former consideration of the 

 purely objective part of the phenomenon. 



In the 3,926 (2,451+1,475) positions which belong 

 a. to the portion of the firmament visible at Slough, and 

 which we shall here for the sake of brevity term the northern 

 heavens, according to the three catalogues of Sir William 

 Herschel from 1786 to 1802, and the above-named great 

 exploration of the heavens published by his son in the Philos. 

 Transact, of 1833 ; and b. to the portion of the southern 

 heavens visible at the Cape of Good Hope, according to Sir 

 John Herschel' s African Catalogues, nebulae and clusters of 

 stars are set down indiscriminately together. I have, how- 

 ever, deemed it best, notwithstanding the natural affinity of 

 these objects, to enumerate them separately, in order to indi- 

 cate a definite epoch in the history of their discovery. I find 

 that the Northern Catalogue 35 contains 2,299 nebulae and 



35 The data on which these numbers are based require some 

 explanation. The three catalogues of the elder Herschel 



c 2 



