This was the celebrated discovery of the Aberration of Light, a triumph of exact observation and of clear insight. As to the exactness of Bradley's observations, it is sufficient to say that his determination of the value of the 'Constant of Aberration' gave it as 20·39´´; the value adopted to-day is 20·47´´. On the death of Halley, in 1742, Bradley was appointed to succeed him. He found the Observatory in as utterly disheartening a condition as his predecessors had done. As already mentioned, Halley had not the same qualifications as an observer that Flamsteed had. He was, further, an old man when appointed to the post, he had no assistant provided for him, and the last five years of his life his health and strength had entirely given way. Under these circumstances, it was no wonder that Bradley found the instruments of the Observatory in a deplorable state. Nevertheless, he set to work most energetically, and in the year of his appointment he made 1500 observations in the last five months of the year. He was particularly earnest in examining the condition and the errors of his instruments; and as their defects became known to him, he was more and more anxious for a better equipment. He moved the Royal Society, therefore, to apply on his behalf for the instruments he required; and a petition from that body, in 1748, obtained what in those days must be considered the generous grant of £1000, the proceeds of the sale of old Admiralty stores. The principal instruments purchased therewith were a mural quadrant and a transit instrument, both eight feet in focal length, still preserved on the walls of the transit-room. It is interesting also to note that, following in the steps of Halley, and forecasting, as it were, the magnetic observatory which Airy would found, he devoted £20 of the grant to purchasing magnetic instruments. Meantime he had continued his observations on aberration, and had discovered that the aberration theory was not sufficient entirely to account for the apparent changes in places of stars which he had discovered. A second cause was at work, a movement of the earth's axis, a 'wobble' in its inclination, technically known as Nutation, which is due to the action of the moon, and goes through its course in a period of nineteen years.