In the pursuit of this course of argument we are led to inquire whether there exists in the universe any matter from which planetary bodies could be formed, and how far their formation from such matter can be explained by the operation of known material laws.
Before the telescope revealed the hidden wonders of the skies, and brought its rich fruits into our garner of knowledge concerning the nature of the universe, the philosophic minds of some early astronomers, Kepler and Tycho Brahe to wit, entertained the idea that the sun and the stars—the suns of distant systems—were formed by the condensation of celestial vapours into spherical bodies; Kepler basing his opinion on the phenomena of the sudden shining forth of new stars on the margin of the Milky Way. But it was when the telescope pierced into the depths of celestial space, and brought to light the host of those marvellous objects, the nebulæ, that the strongest evidence was afforded of the probable validity of these suppositions. The mention of “nebulous stars” made by the earlier astronomers refers only to clusters of telescopic stars which the naked eye perceives as small patches of nebulous light; and it does not appear that even the nebula in Andromeda, although so plainly discernible as to be often now-a-days mistaken by the uninitiated for a comet, was known, until it was discovered by means of a telescope, in 1612, by Simon Marius, who described it as resembling a candle shining through semi-transparent horn, as in a lantern, and without any appearance of stars. Forty years after this date Huygens discovered the splendid nebula in the sword handle of Orion, and in 1665 another was detected by Hevelius. In 1667 Halley (afterwards Astronomer Royal) discovered a fourth; a fifth was found by Kirsch in 1681, and a sixth by Halley again in 1714. Half a century after this the labours of Messier expanded the list of known nebulæ and clusters to 103, a catalogue of which appeared in the “Connaissance du Temps” (the French “Nautical Almanac”) for the years 1783-1784. But this branch of celestial discovery achieved its most brilliant results when the rare penetration, the indomitable perseverance, and the powerful instruments of the elder Herschel were brought to bear upon it. In the year 1779 this great astronomer began to search after nebulæ with a seven-inch reflector, which he subsequently superseded by the great one of forty feet focus and four feet aperture. In 1786 he published his first catalogue of 1000 nebulæ; three years later he astonished the learned world by a second catalogue containing 1000 more, and in 1802 a third came forth comprising other 500, making 2500 in all! This number has been so far increased by the labours of more recent astronomers that the last complete catalogue, that of Sir John Herschel, published a few years ago, contains the places of 5063 nebulæ and clusters.