While Halley’s comet, at its return in 1910, was undoubtedly a marvelous object as seen in the clear skies of America and in southern climes, yet it was more or less of a disappointment to watchers of the sky in England, because the view was impaired by twilight and low altitude. Nor did it come up to the expectations of those whose hopes had been aroused by the fine series of ever-varying appearances, recorded by the camera in connection with the Morehouse comet, referred to in the last chapter. Nevertheless, according to Professor Barnard, expert in photography of celestial objects, had it not been for the remarkable phenomena recorded by the camera in connection with the Brooks comet of 1893 (see photograph), and the Morehouse comet of 1908, the numerous photographs obtained of Halley’s comet would have placed it in the first rank among the records of these bodies. Yet while it lacked much of interest as seen with the eye of the sensitive plate, it left a lasting impression on the human eye, adding renewed interest to its long life history of more than two thousand years. The train of the comet reached the prodigious length of 140°, owing to its being so near the earth, and its great curvature was shown by the fact that it remained visible in the morning sky for two days after the head had become visible in the evening sky. [Illustration: HALLEY’S COMET From photograph taken at Union Observatory, Johannesburg, May 5, 1910. Exposure 60 minutes. ] Halley, by whose name the comet is known, was the first definitely to establish the fact, suspected before, that certain comets are regular visitors to the domain of the sun, returning at stated intervals. For this reason they are termed periodic comets. After Halley had calculated the paths of twenty-four comets, he found that three were moving in orbits almost identical. From this he assumed that the three comets must be one and the same, just as, when a train passes through a station at regular stated intervals, one is led to infer that it must be the same train. Naturally allowances must be made for delays due to fog or stormy weather, but these factors are taken into account should the train arrive after scheduled time. In the case of a comet it may be delayed by means of the disturbing effects of the giant planets Jupiter, Saturn, Uranus, and Neptune, but in Halley’s day the presence of the last two planets in the solar system was as yet unknown (Uranus was discovered in 1781, and Neptune in 1846). Therefore the following prediction made by Halley, when he was convinced that the paths of the comets which appeared in 1531, 1607, and 1682 were identical, is all the more wonderful, since only an approximate allowance had been made for these disturbing factors. Referring to the comet of 1682, he said: “If it should return according to our predictions about the year 1758, impartial posterity will not refuse to acknowledge that this was first discovered by an Englishman.”