IX. Milky nebulosity. This class consisted partly of aggregate collections of stars which were nebulous only in appearance, and partly of objects of a purely nebulous substance, like the nebula of Orion.
X. Nebulous stars.—Theso were stars surrounded by a nebulous atmosphere.
XI. Planetary nebultr.—This class consisted of nebulae of a perfectly round form, exhibiting a resemblance to the planetary bodies of the solar system, only they shone by a very pale light.
X11. Planetary nebnlee with ecntres.—The aspect exhibited by these objects suggested the idea of a gradual condensation of the nebula into a single luminous point.
In 1811, Herschel communicated to the Royal Society a paper, in which he explained his views of the transformation of nebulas into stars. He endeavoured to prove that the nebulous substance, which his observations indicated to be extensively diffused throughout the sidereal universe, had a tendency to form into distinct aggregations of matter, in consequence of the mutual attraction of its particles, and that the resulting mass in each case by a process of gradual conden sation finally acquired the consistency of a solid body. Vlore recent observations of nebulae have not served to confirm this bold hypothesis.
In 1833, the younger Herschel communicated to the Royal Society a parer, containing the results of a re-observation of the nebulic of the northern hemisphere. These results are embodied in a catalogue con
taining 2306 nebulie and clusters, of which about 500 were discovered by himself. In 1828. Mr. Dunlop communicated to the Royal Society a catalogue of 629 nebuhe and clusters observed in the southern hemisphere; and in 1847, the younger Herschel published the results of his astronomical observations at the Cape of Good Hope which, besides various other objects of importance, contained a catalogue of 1708 nebuke and clusters observed in the southern hemisphere.
The most recent contribution to nebular astronomy is due to Lord Rowse, who in 1850 communicated to the Royal Society a paper, con taining the results of observations made by him, chiefly with his celebrated six foot reflector. One of the most remarkable facts die closed by these observations consists in the spiral arrangement of several of the objects. It appears also from them, that many nebulae which had been previously considered as irrcsolvible objects, consist of agglomerations of stars, and the conclusion naturally suggested by such observations is, that all nebula' are in reality nothing more than aggregate masses of stars, which merely require telescopes of adequate power to reveal their true structure.