NEB 



formly united, and of a milky appearance, it may be concluded to be 

 purely nebulous ; but when by using a superior instrument, its appear- 

 ance is a mixture of nebulosity and extremely fine points, so that we can 

 almost see stars, the nebula is said to be easily resolvable, and may be 

 concluded to be a cluster of stars. 



Conjecture on the nature of nebula, not resolvable. 



In the Philosophical Transactions for 1811, Herschel has started a new 

 conjecture respecting the nature of nebulae. He no longer considers 

 them as clusters of stars, which assume a nebulous appearance by reason 

 of their immense distance, but that they consist of a luminous and ex- 

 tremely rare substance. That this substance, at its first formation, is 

 pretty equally diffused through the nebula; but that in the course of 

 ages, this matter, by the preponderance of some part of it, formstme or 

 more centres, to which all the other matter gravitates ; that in conse- 

 quence of this, the nebula gradually decreases in size, and increases in 

 density, till at last a nucleus is formed ; and the nebula becomes plane- 

 tary surrounded by nebulous matter ; which last again is finally absorb- 

 ed by the central body ; and the whole then is, or has all the appearance 

 of, a fixed star. This connexion between nebulous matter and a fixed 

 star, and the conversion of the one into the other, he endeavours to 

 establish, by arranging the nebulae into classes, according to their sup- 

 posed age and degree of condensation, beginning with extensive and uni- 

 formly diffused nebulosity, and establishing the connexion between this 

 and a fixed star by such nearly allied intermediate steps, as makes it not 

 improbable that every succeeding state of the nebulous matter is the re- 

 sult of the action of gravitation upon it while in aforegoing one, and by 

 such steps the successive condensation of it has been brought up to the 

 planetary condition. From this the transit to the stellar form requires 

 but a very small additional compression of the nebulous matter; and in 

 Horschel's observations of many of these it became doubtful whether 

 they were not stars already. 



The steps by which he arrives at this conclusion are nearly as fol- 

 lows : 



1. Extensively diffused nebulosity. 



2. Nebulosities joined to nebulae. 



3. Nebulae of various shapes, but nearly uniform brightness, 



4. Nebulae that are gradually a little brighter in the middle. 



5. Nebulae which are gradually brighter in the middle. 



G. Nebulae which are gradually much brighter in the middle. 

 193 L3 



