STETJCTUEE OF SPIRAL NEBULiE. 255 



medium to a common centre of gravity ; but they are such 

 as would be described by masses having their movements 

 modified by the rotation of the medium. 



In the centre of a spiral nebula is seen a mass both 

 more luminous and more resolvable than the rest. As- 

 Bume that, in process of time, all the spiral streaks of 

 luminous matter which converge to this centre are drawn 

 into it, as they must be ; assume further, that the flocculi 

 or other discrete bodies constituting these luminous streaks 

 aggregate into larger masses at the same time that they 

 approach the central group, and that the masses forming 

 this central group also aggregate into larger masses (both 

 which are necessary assumj^tions) ; and there will finally 

 result a more or less globular group of such larger masses, 

 which will be resolvable with comparative ease. And, as 

 the coalescence and concentration go on, the constituent 

 masses will gradually become fewer, larger, brighter, and 

 more densely collected around the common centre of gravi- 

 ty. See now how completely this inference agrees with 

 observation. " The circular form is that which most com- 

 monly characterizes resolvable nebulae," writes Arago. 

 ^' Resolvable nebulae," says Sir John Herschel, " are almost 

 universally round or oval." Moreover, the centre of each 

 group habitually displays a closer clustering of the consti- 

 tuent masses than elsewhere ; and it is shown that, under 

 the law of gravitation, which we know extends to the stars, 

 this distribution is not one of equihbrium, but implies pro- 

 gressing concentration. While, just as we inferred that, 

 according to circumstances, the extent to which aggrega- 

 tion has been carried must vary ; so we find that, in fact, 

 there are regular nebulae of all degrees of resolvability, 

 from those consisting of innumerable minute discrete 

 masses, to those in which there are a few large bodies 

 worthy to be called stars. 



On the one hand, then, we see that the notion, of 



