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the intuition of genius, Herschel, at an early period in his career, leaped to the conclusion that, as a planet revolves round the sun, so, in the regions of space, stars may revolve round stars, or sun round sun. It was a magnificent idea, apparently beyond proof, and would be reckoned among the useless things of science. "We have already shown," he wrote in 1803, "the possibility that two stars, whatever be their relative magnitudes, may revolve, either in circles or ellipses, round their common centre of gravity; and that, among the multitude of the stars of the heavens, there should be many sufficiently near each other to occasion this mutual revolution, must also appear highly probable." A sun of enormous size and brightness revolving round another sun as big or as bright, but it may be of a different colour, might be and really was regarded as the dream of a poet, imagining things that mathematics, with inexorable logic, gave no countenance to. But imagination sometimes realises truth long before the facts of science make it known. It was so here. "I shall therefore now proceed to give an account of a series of observations on double stars, comprehending a period of about twenty-five years, which, if I am not mistaken, will go to prove that many of