It was whilst he was Savilian Professor that Bradley made that great discovery which will always be associated with his name. Though professor at Oxford, he had continued to assist his uncle, Mr. Pound, at his observations at Wanstead, and after the death of the latter he still lived there as much as possible, and continued his astronomical work. But in 1725 he was invited by Mr. Samuel Molyneux, who had set up a twenty-four-foot telescope made by Graham as a zenith tube at his house on Kew Green, to verify some observations which he was making. These were of the star Gamma Draconis, a star which passes through the zenith of London, and which, therefore, had been much observed both by Flamsteed and Hooke, inasmuch as by fixing a telescope in an absolutely vertical position--a position which could be easily verified--it was easy to ascertain if there was any minute change in the apparent position of the star. Dr. Hooke had declared that there was such a change, a change due to the motion of the earth in its orbit, which would prove that the star was not an infinite distance from the earth, the seeming change of its place in the sky corresponding to the change in the place of the earth from which the observer was viewing it. Bradley found at once that there was such a change--a marked one. It amounted to as much as 1´´ of arc in three days; but it was not in the direction in which the parallax of the star would have moved it, but in the opposite. Whether, therefore, the star was near enough to show any parallax or not, some other cause was giving rise to an apparent displacement of the star, which entirely masked and overcame the effect of parallax.