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Rh He conceived the idea of a 40-feet reflector, with a 4-feet mirror at the bottom of the tube, cast and polished by himself. His own account of the beginning of this magnificent work is this: "In the year 1783 I finished a very good 20-feet reflector with a large aperture, and mounted it upon the plan of my present telescope. After two years' observation with it, the great advantage of such apertures appeared so clearly to me, that I recurred to my former intention of increasing them still farther; and being now sufficiently provided with experience in the work I wished to undertake, the President of our Royal Society, who is always ready to promote useful undertakings, had the goodness to lay my design before the King. His Majesty was graciously pleased to approve of it, and with his usual liberality to support it with his Royal bounty." There is this to be said on the departure now made, that the great telescope, from the difficulty of handling it, cannot be considered to have altogether answered his expectations, for the 20-feet continued to be his favourite in studying the heavens. But he was full of hope. "By applying ourselves," he wrote in April 1784, "with all our powers to the improvement of telescopes, which I look upon as yet in their infant state, and turning them with assiduity to the study of the heavens, we shall in time obtain some faint knowledge of, and perhaps be able partly to delineate, The Interior Construction of the Universe."

Herschel himself devised and superintended everything about this great telescope. None but "common workmen" were employed, as was also the case with the greater reflector built by Lord Rosse, sixty years later. The woodwork of the stand, and machines for