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The mirror by which it was first shown measured nearly fifty inches across, and weighed 2,118 pounds. It was slung in a ring, and the sheet-iron tube in which it rested was thirty-nine and a-half feet long and four feet ten inches wide. Ladders fifty feet in length gave access to a movable stage, from which the observer communicated through speaking tubes with his assistants. The whole erection stood on a revolving platform; for the modern equatorial form of mount, by which the diurnal course of the heavens is automatically followed, was not then practically available, and the necessary movements had to be imparted by hand. This involved the attendance of two workmen, but was otherwise less inconvenient than might be supposed, owing to the skill with which the required mechanism was contrived.

Herschel estimated that, with a magnifying-power of 1,000, this grand instrument could, in the climate of England, be effectively used during no more than one hundred hours of every year. A review with it of the whole heavens would hence have occupied eight centuries. In point of fact, he found the opportunities for its employment scarce. The machine took some time to get started, while the twenty-foot was ready in ten minutes. The speculum, moreover, proved unpleasantly liable to become dewed in moist weather, or frozen up in cold; and, in spite of all imaginable care, it preserved the delicacy of its polish no more than two years. An economist of minutes, such as its maker, could, then, do no otherwise than let the giant telescope lie by unless its powers were expressly needed. They were surprisingly effective. “With the forty-foot instrument,” he reported to the Royal Society in 1800, “the appearance of Sirius announced itself at a great distance like the dawn of the morning, till this brilliant star at last entered the field, with all the splendour of the rising sun, and forced me to take my eye from that beautiful sight.” Which, however, left the vision impaired in delicacy for nigh upon half-an-hour.