Page:Sir William Herschel, his life and works (1881).djvu/233

Rh had such an origin. When nebulæ appear to us as round masses, they are in reality globular in form, and this form is at once the effect and the proof of a gravitating cause.

The central brightness of nebulæ points out the seat of the attraction; and the completeness of the approximation to a spherical form points out the length of time that the gravitating forces have been at work. Those nebulæ (and clusters) which are most perfect in the globular form, have been longest exposed to central forces. The planetary nebulæ are the oldest in our system. They must have a rotatory motion on their axes.

By progressive condensation planetary nebulæ may be successively converted into bright stellar nebulæ, or into nebulous stars, and these again, by the effects of the same cause, into insulated or double stars. This chain of theorems, laid down in the memoir of 1811, is enforced in 1814 with examples which show how the nebulous appearance may grow into the sidereal. selects from the hundreds of instances in his note-books, nebulæ in every stage of progress, and traces the