Herschel’s salary was undeniably small. It gave him the means of living, but not of observing, as he proposed to observe. If the improvement of telescopes were to be “carried to its utmost limit,” additional funds must be raised. Without an ample supply of the “sinews of war,” fresh campaigns of exploration were out of the question. There was one obvious way in which they could be provided. Herschel’s fame as an optician was spread throughout Europe. His telescopes were wanted everywhere, but could be had from himself alone; for the methods by which he wrought specula to a perfect figure are even now undivulged. They constituted, therefore, a source of profit upon which he could draw to almost any extent. He applied himself, accordingly, to make telescopes for sale. They brought in large sums. Six hundred guineas a-piece were paid to him by the King for four ten-foot reflectors; he received at a later date £3,150 for a twenty-five foot, sent to Spain; and in 1814 £2,310 from Lucien Bonaparte for two smaller instruments. The regular scale of prices (later considerably reduced) began with 200 guineas for a seven-foot, and mounted to 2,500 for a twenty-foot; and the commissions executed were innumerable.
But Herschel did not come into the world to drive a lucrative trade. It was undertaken, not for itself, but for what was to come of it; yet there was danger lest the end should be indefinitely postponed in the endeavour to secure the means.
“It seemed to be supposed,” Miss Herschel remarked, “that enough had been done when my brother was enabled to leave his profession that he might have time to make and sell telescopes. But all this was only retarding the work of a thirty or forty-foot instrument, which it was his chief object to obtain as soon as possible; for he was then on the wrong side of forty-five, and felt how great an injustice he would be doing to himself and the cause of astronomy by giving up his time to making telescopes for other observers.”
This he was, fortunately, not long obliged to do. A royal grant of £2,000 for the construction of the designed giant telescope, followed by another of equal amount, together with an annual allowance of £200 for its repairs, removed the last obstacle to his success. The wide distribution of first-class instruments might, indeed, have been thought to promise more for the advancement of astronomy than the labours of a single individual. No mistake could be greater. Not an observation worth mentioning was made with any of the numerous instruments sent out from Datchet or Slough, save only those acquired by Schröter and Pond. The rest either rusted idly, or were employed ineffectually, aptly illustrating the saying that “the man at the eye-end” is the truly essential part of a telescope.
No one knew this better than Herschel. Every serene dark night was to him a precious opportunity availed of to the last minute. The thermometer might descend below zero, ink might freeze, mirrors might crack; but, provided the stars shone, he and his sister worked on from dusk to dawn. In this way, his “third review,” begun at Bath, was finished in the spring of 1783. The swiftness with which it was conducted implied no want of thoroughness. “Many a night,” he states, “in the course of eleven or twelve hours of observation, I have carefully and singly examined not less than 400 celestial objects, besides taking measures, and sometimes viewing a particular star for half an hour together, with all the various powers.”
The assiduity appears well-nigh incredible with which he gathered in an abundant harvest of nebulæ and double stars; his elaborate papers, brimful of invention and experience, being written by day, or during nights unpropitious for star-gazing. On one occasion he is said to have worked without intermission at the telescope and the desk for seventy-two hours, and then slept unbrokenly for twenty-six hours. His instruments were never allowed to remain disabled. They were kept, like himself, on the alert. Relays of specula were provided, and one was in no case removed from the tube for re-polishing, unless another was ready to take its place. Even the meetings of the Royal Society were attended only when moonlight effaced the delicate objects of his particular search.
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.”
The ruinous mansion at Datchet having become uninhabitable, even by astronomers, their establishment was shifted, in June, 1785, to Clay Hall, near Old Windsor. Here the long-thought-of forty-foot was begun, but was not destined to be finished. A litigious landlady intervened. The next move, however, proved to be the last. It was to a commodious residence at Slough, now called “Observatory House”—“le lieu du monde,” wrote Arago, “où il a été fait le plus de découvertes.” Thither, without the loss of an hour, in April, 1786, the machinery and apparatus collected at Clay Hall were transported. Yet, “amidst all this hurrying business,” Caroline remembered “that every moment after daylight was allotted to observing. The last night at Clay Hall was spent in sweeping till daylight, and by the next evening the telescope stood ready for observation at Slough.”