But not content with augmenting our own, it is to Herschel we owe the discovery of other systems. That the fixed stars were each the centre of a number of planets was suspected, perhaps rather prematurely, before his observations were made known. But the first positive addition to our knowledge of _systems_, that is of bodies which move in any degree of connexion with each other, is to be found in his paper read to the Royal Society, June 9, 1803, announcing that Castor, γ Leonis, ε Bootis, ζ Herculis, δ Serpentis, γ Virginis, were most probably _binary_[4] stars. The existence of such systems has been confirmed by Sir J. Herschel and Professor Struve, and the duration of the periods given by Herschel has been sufficiently confirmed to make the exactness of his observations remarkable. But to new planets, and new systems, Herschel added new universes; or, more properly speaking, showed that the universe consisted of portions, each conveying as large an idea of extent and number, as the whole of what was previously called _the universe_. His great telescope furnished sufficient facts, and his mind was not slow to draw a conjectural inference, which must be classed among the happiest efforts of reasoning speculation. The resolution of the milky way into stars proved that we are situated in a stratum of such bodies much thicker in some directions than others: this led to the inference that some or all of the nebulæ with which the sky is crowded might be similar enormous groups of stars; and the resolution of some of the nebulæ into detached portions was a first step towards the demonstration of the conjecture. Footnote 4: _Double_ stars, those which are so near to each other as to appear one to the naked eye: _binary_ systems, double stars which revolve round each other. There is enough yet unmentioned,—in the discovery of the time of rotation of Saturn—that of Jupiter’s satellites—that of the refrangibility of heat—the experiments on colours—the enormous collection of nebulæ—the experimental determination of the magnitude of stars—the researches and conjectures on the physical constitution of the sun—those on the qualities of telescopes, &c. &c.,—to form by itself no ordinary title to the recollection of posterity. But we must refer to Sir J. Herschel’s Astronomy, in which will be found such an account of them as the plan of the work permitted, by one who has shown himself as indisposed to exaggerate, as interested to explain. In the labours of his observatory Herschel was assisted by his sister, Miss Caroline Herschel, with whose help he published, in 1798, his catalogue of Flamsteed’s stars. This lady, whose exertions, both as an observer and calculator, are well known to astronomers, is still living, at a very advanced age, in Hanover.