380 DISTRIBUTION OF THE NEBULA. SECT. XXXVII 



be hollow shells, only emitting light from their sur- 

 faces. 



The existence of every degree of ellipticity in the 

 nebulae from long lenticular rays to the exact circular 

 form and of every shade of central condensation from 

 the slightest increase of density to apparently a solid 

 nucleus may be accounted for by supposing the general 

 constitutions of these nebulae to be that of oblate sphe- 

 roidal masses of every degree of flatness, from the 

 sphere to the disc, and of every variety in their density 

 and ellipticity toward the center. It would be errone- 

 ous, however, to imagine that the forms of these sys- 

 tems are maintained by forces identical with those 

 already described, which determine the form of a fluid 

 mass in rotation ; because, if the nebulae be only clus- 

 ters of separate stars, as in the greater number of cases 

 there is every reason to believe them to be, no pressure 

 can be propagated through them. Consequently, since 

 no general rotation of such a system as one mass can 

 be supposed, it may be conceived to be a quiescent form, 

 comprising within its limits an indefinite multitude of 

 stars, each of which may be moving in an orbit about 

 the common center of the whole, in virtue of a law of 

 internal gravitation resulting from the compound gravi- 

 tation of all its parts. Sir John Herschel has proved 

 that the existence of such a system is not inconsistent 

 with the law of gravitation under certain conditions. 



The distribution of the nebulae over the heavens is 

 even more irregular than that of the stars. In some 

 places they are so crowded together as scarcely to allow 

 one to pass through the field of the telescope before 

 another appears, while in other parts hours elapse with- 

 out a single nebula occurring. They are in general only 

 to be seen with the very best telescopes, and are most 

 abundant in a zone whose general direction is not far 

 from the hour circles O h and 12 h , and which crosses the 

 milky way nearly at right angles. Where that zone 

 crosses the constellations Virgo, Coma Berenices, and 

 the Great Bear, they are to be found in multitudes. 



Such is a brief account of the discoveries contained 

 in Sir John Herschel's paper, which, for sublimity of 

 views and patient investigation, has not been surpassed. 



