In July 1783 Herschel began his observations with his large twenty-foot telescope, though it was in an unfinished state; and his sister watched and waited with much apprehension when she knew him to be elevated some fifteen feet or more on a temporary crossbeam instead of a safe gallery. Here it is needful to explain, perhaps, that these huge astronomical telescopes are not used like ordinary glasses, to one end of which the observer applies his eye; the objects towards which the tube is directed being thrown upon a large mirror, which is attached to it externally at some distance from the ground. The observer, therefore, needs to be mounted on an elevated platform or gallery, from which he can conveniently inspect the mirror. One night, in a very high wind, Herschel had scarcely descended from his station before the whole apparatus came down; and his sister was in continual apprehension of some serious accident. One such, indeed, occurred, and to herself. The evening of the 31st of December had been cloudy, but as a few stars shone forth about ten o'clock, hurried preparations were made for observing. Herschel, standing at the front of the telescope, directed his sister to make a certain alteration in the lateral motion, which was done by machinery, on which the point of support of the tube and mirror rested. At each end of the machine or trough was an iron hook, such as butchers use for suspending their joints of meat; and having to run in the dark across ground covered a foot deep with melting snow, Miss Herschel fell on one of these hooks, which entered her right leg above the knee. To her brother's injunction, "Make haste!" she could answer only by a pitiful cry, "I am hooked!" He and the workmen hastened immediately to her assistance, but they could not disentangle her without leaving nearly two ounces of her flesh behind. For some weeks she was an invalid, and at one time it was feared that amputation might be necessary.
Not satisfied with the magnifying power of any of the instruments he had hitherto constructed, Herschel resolved, in 1784, to attempt a forty-foot telescope. Such a work, however, was far beyond his limited private resources; and he did not venture to undertake it until promised a royal bounty of £2000. Then he removed from Datchet to Clay Hall, Old Windsor; and again, in 1786, to Slough, where he finally settled, and succeeded in erecting a commodious and well-equipped observatory. "We may confidently assert," says Arago, "relative to the little house and garden of Slough, that it is the spot of all the world where the greatest number of discoveries have been made. The name of that village will never perish: science will transmit it religiously to our latest posterity."