At the return of Halley’s comet in 1910 it was conjectured that it would probably be greatly disturbed by the influence of the planet Jupiter, and that of Uranus and the newly discovered planet Neptune. It was therefore possible for Dr. P. H. Cowell and Dr. A. C. D. Crommelin (both of the Royal Observatory, Greenwich) to make a prediction so exact that the comet was found within six minutes of arc in R. A., and four minutes in declination of its predicted place, as shown on the first photograph obtained. This was equivalent to an angular distance in the sky less than one quarter of the diameter of the moon. (Incidentally, a prize of 1,000 marks, which had been offered by Mr. Lindemann for the most accurate prediction of the comet’s arrival at perihelion, was won and divided between the two mathematicians.) De Pontecoulant had made calculations regarding the return of the comet many years earlier which were fairly near the truth, but one month too late. It was action of Jupiter about the 1835 perihelion that had such an effect on the 1910 return. The action of Jupiter at any return does not produce a notable effect till the following return. One of the first photographs obtained of Halley’s comet at this return was due to the foresight of Herr Max Wolf of the Heidelberg Observatory, in exposing a photographic plate for several weeks beforehand, so as to entrap the wanderer at the first opportunity. It was caught at 2 A.M. in the morning of September 12, 1909, engraving its image on the photographic plate, a welcome message announcing its advent to the astronomical world. (The first photograph obtained of Halley’s comet was taken at Helwan on August 24, but Herr Wolf was the first to identify the comet’s image on the plate. There were also many early photographs taken at Greenwich.) For thirty-two years it had remained beyond the orbit of the outermost planet Neptune, then, obedient to the attractive power of its lord and master the sun, it had started on the return trip. Despite its enormous distance from our planet, and the fact that it was beyond reach of telescope or camera, it was possible for mathematicians to trace its path with unerring accuracy. It had approached the orbit of Neptune after the year 1888, the orbit of Uranus about ten years later, crossing that of Saturn in 1908. The following year it arrived at the orbit of Jupiter, thus bringing it within the range of both the photographic plate and giant telescopes. Its actual return to perihelion in 1910 differed by two and seven-tenths days from the prediction which can be explained only by the existence of forces which are not pure gravitation, or the possibility of another planet beyond Neptune, as yet undiscovered, acting as a disturbing factor.