The spectroscope has done no less than photography to reanimate the study of astronomy. The fact is that, with these two helping means of observation, it has become possible for the ordinary man to witness and appreciate some of the discoveries of astronomers, though the true and accurate handling of all that is revealed concerning the stars is essentially a matter of measurement, and therefore only to be dealt with strictly by mathematicians. The desire to obtain ever more and more accurate measurement of the movement and the size of the heavenly bodies is the mainspring of all astronomical discovery, and, indeed, the attempt to gain more and more detailed measurement of the factors at work is the motive—more or less immediate—of all accurate investigation of nature. Recently the astronomers of the Royal Observatory at Greenwich have photographed the new comet (the third of 1907) in a way in which no comet has ever been photographed before. On many consecutive nights for several weeks they were at work photographing it on the dry plate, at intervals of two or three hours, and the pictures obtained (which I have seen at the rooms of the Royal Astronomical Society) show the most wonderful changes of form of its tail, so that they look more like the record of the changes of some living creature than those of a heavenly body. Already, in October 1909, Halley’s comet, which has been anxiously awaited, has been seen, though it is not expected to be bright and visible to all until May 1910. Comets are among the exceptional delights of the astronomer—that is to say, big comets, for two or three small comets visible only by a telescope or by photography turn up every year. Some comets are expected visitors, others make their appearance quite casually, some because they apparently have no regular period, some because that period is as yet undiscovered. Edmund Halley was the first to discover the law of movement of a comet and to predict the return in 1758 of that seen in 1682. He did not live to witness the verification of his prediction. This comet, now called Halley’s comet, was, he conjectured, the same which had appeared in 1531 and in 1607. His prediction of its return proved to be a year out (owing to perturbations caused by Neptune and Uranus, two planets undiscovered in his day), but it appeared in 1759, and went round once again and reappeared in 1835, and now is eagerly expected by astronomers to appear in full brilliancy in 1910. Its period is about seventy-five or seventy-six years. XXIV COMETS