It must not be supposed that high powers can often be used in the English climate. Herschel found that there are not above a hundred hours in the year during which the heavens can be advantageously observed with a telescope of forty feet, furnished with a magnifying power of a thousand.
It was not till twenty years of continuous labour had enabled Herschel fully to estimate the value of his discovery that he gave to the scientific world his memoir on the power of penetrating into space by telescopes. He found that with his twenty-feet telescope he could penetrate into space seventy-five times further than with the naked eye; with a twenty-five-feet telescope, he could reach ninety-six times; and with his great forty-feet instrument, 192 times the distance which could be attained by human vision unassisted. As the eye of man can barely discern stars of what is called the seventh magnitude,[1] it follows that the great telescope rendered visible stars so amazingly remote, that light, leaping over a space of a hundred and eighty-five thousand miles, three times the diameter of our globe, in a single second, would take half a million of years to travel from them to the earth. If such a star were this day extinguished, it would still continue to be seen from the earth for 500,000 years.
Such is the domain added by Herschel to astronomy.
In the early part of 1786 the Herschels removed to Slough, and Sir William determined to set to work in earnest on a telescope forty feet in length. He took the twenty-feet as a model. The latter instrument had already been so improved by constant alteration, that it was found practically serviceable for that purpose. His friend Sir William Watson undertook, through Madam Schwellenberg, who was attached to the court, to bring the undertaking under the notice of the king. In consequence of this, he soon afterwards received, through Sir J. Banks, the promise that 2,000l. would be granted to enable him to make himself an instrument. It is nowhere stated whether this sum was defrayed by the Royal Society, of which Sir Joseph was president, or by the king. The work proceeded with rapidity. Smiths toiled all the summer on the ironwork, and troops of labourers relieved each other at the grinding of the great mirror. Tools had to be forged specially for this purpose, ground to be levelled, brick foundations to be laid—in short, the gardens of the new house were the scene of great bustle and activity. The heavy castings were made in London, and brought to Windsor by water. It soon appeared that the expense of these improvements could not be defrayed out of the 200l. a year which was allowed him by the king, or even by the extra 2,000l. procured through Sir William Watson. Moreover, as we gather, the salary was not paid with very great regularity. Sir William therefore established a regular manufacture of telescopes, of which he and his sister ground all the mirrors, and superintended all the details. Though money for necessary household expenses, as well as for astronomical purposes, was thus acquired, Herschel felt keenly that he was doing an injustice to himself and to the cause of astronomy by giving up his time to the making of telescopes for other observers. Mention is made in the journals of several telescopes of remarkable dimensions. One for the king of Spain was executed at a cost of 3,150l. Miss Herschel notes that she was much hindered in her work by the packing of the Spanish telescope, which was done at the barn and rickyard of Upton, where she was then lodging, her own room being all the time filled with the optical apparatus. The Prince of Canino paid 2,310l. for a ten- and a seven-foot telescope from the same indefatigable hands. It is evident that though the pecuniary profit was great, it was dearly purchased at the expense of health and time which was entailed upon Sir William by labour so severe. There is no doubt that the exhaustion produced by grinding mirrors told seriously upon his health.
In the summer which saw them installed at Slough Miss Herschel appeared as an original discoverer. Sir William was called away to deliver a ten-foot telescope as a present from the king to the observatory of Göttingen. While he was absent, Miss Herschel resumed her "sweeping," for which her position as assistant usually left her but little time, but to which she was intensely devoted. Her diary on the 1st of August, 1786, contains the following entry:—
This evening I saw an object which I believe will prove to-morrow night to be a comet.
All next day she steadily pursued her
- ↑ Stars are known empirically as of the first, second, third, etc., magnitude. The theory is that all are of the same size, and dwindle from the first to the second magnitude, and so on in proportion to their distance. This theory is known not to be exactly correct, but it is convenient for astronomical purposes, and is therefore conventionally used.
