.NEHT.H.,,E (ante). The earliest mention of nebulae is that of the Arabian astronomer Sufi. who, in the 10th c. described the Magellanic clouds and the nebula in Andromeda. Ptolemy lint made a catalogue of " cloudystars," but the objects he mentioned are easily resolvable into star clusters with small telescopes. The nebula in Andromeda, which is visible to the naked eye, was the only one discovered before the invention of the tele scope. although a few others are now known which are faintly visible. In 1656 Ilnyghens discovered the great nebula in the constellation Orion, publishing an account of it along with the discovery of the rings of Saturn in 1659. In 1714 Halley described six nebulfe. In 17:i5 La Caille sent a list of 42 nebula which he had discovered at the cape of Good Hope. to the academy of sciences at Paris. iMessiers published a list in 1783 and 1784, containing an account of 103 nebulae, the greatest number described by one observer before the observations of sir William Herschel, who, in 1781, with a five-foot reflector of his own construction began his investigation of the heavens, his first performance being the discovery of Uranus. By the use of this and other telescopes he was enabled in 1786 to communicate to the royal soeietv a catalogue of 1000 new nebulm and clusters; in 1787 a second catalogue of another 1000, and in 1802 a third, containing 500 more. Sir john Herschel, between 1825 and 1830 reviewed a portion of his father's work, add ing a list of 500 nebula; and clusters of his own discovery, making in all a list of 2,306 nebulae and clusters in the northern hemisphere, which lie sent to the royal society. In 1833 he took his instruments to the cape of Good Hope, the result of which was a catalogue of 1708 nebula and clusters in the southern heavens. The whole number of nebulae and clusters now known is over 5,000. The most complete catalogue is that of sir John Herschel, published in 1864, which contains all- that were accessible to him up to 1863. The number there recorded is 5,079. The nebukz have presented a subject about which there has always been doubt, in regard to their position as well as constitution. After many of them had been resolved into starry clusters it was thought they were all gal 'axles similar to our own solar system, and sir William Herschel adopted this view in regard to certain nebula' which he supposed were external to our stellar system, but after wards in developing, his nebular hypothesis, he found it difficult to distinguish between the external nebulae and those which are generally thought to he parts of OM' own sidereal system. Proctor expresses the opinion " that our sidereal system extends far beyond the limits which have ordinarily heen assigned to it, and that there are no nebulae wiuch can be regarded as external' to-it;" Herschel eonelnded-that there werenebulte which arc not resolvable into stars, and that they consisted of gaseous matter. The forms of nebula; are various, and they change in appearance with different powers of the telescope. The spina nebulae, which were first made known by lord Bosse are examples of this kind, their spiral form having previously not been suspected. Sonic planetary nebular resemble planets, and when viewed with telescopes of high power present a complicated appear ance. Some have the appearance of a ring and are called annular nebular; there is a beautiful example in the constellation Lyra in which a central mass is surrounded by a ring. Some are in pairs, like double stars. Small nebular generally have the appear
ance of a bright nucleus surrounded by a veil; some of them are called stellar nebular. Others are very irregular, and have long filmy arras. Those in Orion and Andromeda arc examples, as also the great nebula in Argo which was carefully described by sir John Herschel in his Cape observations. Those nebuke which are resolvable into stars give spectra which resemble the spectra of stars, while those which are obtained from the light of unresolvable nebular give a spectrum of three, sometimes four, bright liues, one of which corresponds to a line in the spectrum of hydrogen, and another to a line in that of nitrogen. The planetary nebulte also give similar gaseous spectra. Of 70 nebulte examined by Huggins about one-third gave gaseous spectra. The nebular hypothesis in regard to the formation of the universe, although previously proposet1 by Swedenborg, Buffou, and Kant, was first systematized by Herschel and Laplace, and has been since modified. Laplace's earlier ideas were embraced in a consideration as to the manner of the formation of our solar system. He conceived that a mass of highly heated vaporous matter occupied a space larger than the orbit of the furthest p:anet. In consequence of molecular and gravitating forces it acquired rotation, by which it threw off rings of matter which afterwards broke up into planets and thei r satellites. Ifersehas theory causecl a modification of Laplace's who adopted flie idea that primor.lhil nebulous matter still existed which was being formed into nebulae and clusters. The general idea now entertained is that this primordial matter has accomplished the work of world formation by the action of gravity aided by molecular forces. It is assumed by some that on physical principles primordial matter widely distributed through space would pass through the following changes. Gravitation would cause the mass to contract, and become more dense; this followed by atomic repulsion, which acting against gravitation would produce heat. After a certain degree of condensation had taken place molecular combination would result, which would again cause a great evolation of heat; this would be followed by radiation and precipitation of binary atoms as flocculi floating in the rarer medium: these flocculi will tend to move toward a common center, but as the mass is irregular the motion will really be to one side of the center. This will result iu a spiral movement. Mutual attraction will produce groups of flocculi, moving around local centers of gravity. There will be here and there detached portions which will not coalesce with the huger internal masses, but will slowly follow without overtaking them, thus accounting for the formation of comets. Many dynamic principles are involved in such motions and changes of matter which have received the attention of scientists, of which may be mentioned the investigation of molecular vortices by Rankine, and of vortex rings by Helmholtz and Thomson, and the preservation and disruption of revolving rings by Maxwell, Peirce, and Hind in memoirs on the rings of Saturn, that although grand and instructive views have been obtained of the regions of space, the brilliant investigations of science have as yet afforded no positive knowledge of its Midi Ste depths, or of its genesis.