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The summer of 1788 was spent in getting ready the finest telescope Herschel had yet employed. It was called the “large twenty-foot” because of the size of its speculum, which was nearly nineteen inches in diameter; and with its potent help he executed his fourth and last celestial survey. His impatience to begin led him into perilous situations.

“My brother,” says Miss Herschel, “began his series of sweeps when the instrument was yet in a very unfinished state; and my feelings were not very comfortable when every moment I was alarmed by a crack or fall, knowing him to be elevated fifteen feet or more on a temporary cross-beam instead of a safe gallery. The ladders had not even their braces at the bottom; and one night, in a very high wind, he had hardly touched the ground before the whole apparatus came down. Some labouring men were called up to help in extricating the mirror, which was fortunately uninjured, but much work was cut out for carpenters next day.”

In the following March, he himself wrote to Patrick Wilson, of Glasgow, son of Dr. Alexander Wilson, the well-known professor of astronomy:—“I have finished a second speculum to my new twenty-foot, very much superior to the first, and am now reviewing the heavens with it. This will be a work of some years; but it is to me so far from laborious that it is attended with the utmost delight.” He, nevertheless, looked upon telescopes as “yet in their infant state.”