It would appear therefore from this hypothesis, as was first suggested by professor Kant, and afterwards by Lambert, that all the stars in the universe are col lected into nebula, and that all the insulated and scat tered stars which appear in the heavens belong to the particular nebula in which our system is placed. Dr Herschel has followed out this opinion with his usual success, and has observed the position of no fewer than 2500 distinct nebula. These nebula were generally detected in certain directions rather than others ; and the spaces preceding them were often so completely deprived of their stars, that not a single star could be seen in the field of the telescope. From this chipm stance he concludes, that the condensation of the stars into nebula of different kinds, has arisen from their mutual attraction ; and he endeavours to shew, from this principle, how the various combinations have been made. He supposes also, that several nebula are formed by the decomposition of larger nebula, that many nebula of this kind are at present detaching themselves from the nebula of the milky way, and that others have been actually detached from the sides of it. He thinks that the opening of about 4 degrees in the body of the Scor pion, arises from this cause, and that the stars which once filled this opening now form the 80th " Nebuleuse sans etuiles," in the Connoissance des Temps, which is situated on its western border.
The limits of this article will not permit us to give a complete view of Dr Herschel's interesting specula tions on the construction of the heavens. 'We shall merely mention the different arrangements into which he imagines the stars to be formed. He supposes that sonic of them arc insulated like our sun, surrounded with planets and comets, and beyond the attractive force of other stars. He considers double stars as so near each other, as to be affected with their mutual gravita tion, and as preserving their distance by their centrifu gal force, arising from a revolution round their common centre of gravity. The two stars for example, which form the double star Castor, have varied in their angu lar situation more than 45° since they were observed by Bradley in 1759, and appear to perform a retrograde revolution in 342 years, in a plane perpendicular to the direction of the sun. Dr Herschel has found them in
intermediate angular positions, at intermediate times, but never could perceive any change in their distance. The retrograde revolution of y Leonis, another double star, is supposed to be in a plane considerably inclined to the line in which we view it, and to be completed in 1200 years. The stars of E Bootes perform a direct revolution in 1681 years, in a plane oblique to the sun. The stars of c)` Serpentis, perform a retrograde revolu tion in about n75 years, and those of y Virginis in 708 years, without any change in their distance. In 1802, the large star Hercules eclipsed the smaller one, though they were separate in 1782. Other stars he supposes to be united in triple and quadruple, and still more complicated systems. Round nebula, like the milky way, which appear brightest in the middle, form a fourth class. Groupes of stars are those in which there is no apparent condensation, and clusters are distin guished from these, and from nebula, by a great cen tral compression. Another class contains nebula, of which the component stars have not been perceived. Some of these are at such an immense distance, that Dr Herschel supposes their light to have been a million of years in travelling to our system. The last class contains single stars, or rather bright spots, surrounded with a milky nebulosity, arising from some shining fluid. The bright central point he supposes to be the fluid in a condensed state ; and as some of these nebula have no central points, he imagines that new stars may ap pear to be formed by the compression of the luminous fluid. This luminous fluid he conceives to be some how or other formed by means of the light that is continually issuing from the innumerable suns which fill the im mensity of space.
A catalogue of the most important double and nebu lous stars shall be given at the end of this article. See Kant's fillgemeine Xaturgeschichte. Lambert's Photome tria,§ 1139. Pigott, Phil. Trans. 1781, p. 82. Cassini, Mem. ?ead, Par. 1789, 331. Michel, Phil. Trans. 1784, p. 35. Le Monnier, Mem. -dead. Par. 1789, p. 610. Her schel, Phil. Trans. 1782, p. 112 ; 1784, p. 437; 1785, p. 40, 213; 1786, p. 457; 1789, p. 212; 1791, p. 71; 1795, p. 46; 1802, p. 477 ; 1803, p. 339 ; 1804, p. 353. (A)