William rented a modest house ten minutes’ walk from the Pump Room, in the upper part of Bath, at Rivers Street. He continued composing for the oboe, taught guitar, harpsichord and violin, conducted oratorios and gave singing lessons. In June 1767 he was joined by Jacob for a visit, and took up his appointment as organist and choirmaster at the Octagon Chapel, which was opened on 4 October.61
It was during this hectic period that his other secret passion exerted itself. In February 1766 the twenty-seven-year-old William Herschel started his first Astronomical Observation Journal. He recorded an eclipse of the moon, and the hazy appearance of Venus.62 Hard as he worked as a musician, he was now steadily training himself as an astronomer. He devoured books on astronomical calculation, Flamsteed’s star tables and Thomas Wright’s cosmological speculations. He attended James Ferguson’s astronomy lectures at the Pump Room in 1767, and at last met this early astronomical hero of his.63 He spent hours star-gazing in the little Rivers Street garden at night. Even when teaching his music pupils in the evening, it was said that he sometimes broke off and took them outside to look at the moon. He began to build up a small arsenal of second-hand refractor telescopes, and carefully examined their construction. He was considering what his father Isaac used to call ‘one of his contrivances’.
The refractor is the classic type of straight-through telescope originally developed by Galileo, and refined by Kepler and the great seventeenth-century Dutch astronomer Christiaan Huygens. It has magnifying lenses at each end of the tube, one fixed and the other adjustable (the eyepiece), advancing or retreating to focus the image. In extendible or retractable form, it was often used by soldiers or sailors on active service, until the arrival of the binoculars. It was just such a refractor telescope that Nelson would-or would not-put to his blind eye at the battle of Copenhagen in 1798. The snapping closed of the retractable mechanism became a gesture of decision and command.
Herschel found that most refractor telescopes were satisfactory for simple low-magnification viewing of the moon or the planets. But astronomical versions were absurdly cumbersome (some up to twenty-five foot long), and almost useless for high-magnification observation of the stars. The curve or bulge in the magnifying lens acted like a prism, and broke up the white stellar light into distorting rainbow-coloured fringes at the edges. (This became known as ‘chromatic aberration’. A shortsighted person can see these rainbow aberrations of starlight with the naked eye, because his pupil is also distorted at the edges.) Newton, observing this in his famous prism experiments at Cambridge, had invented an entirely different type of telescope, the reflector. But his, which he donated to the Royal Society, was only six inches long, with a magnifying power of forty.64
Confined to refractors, most eighteenth-century British astronomers had paid little attention to stellar astronomy, except where it served for navigation purposes. (The John Dolland achromatic telescope, which corrected some prismatic distortion, was only invented in 1758, and did not come into general use-as improved by his son Peter Dolland-until the turn of the century.65) The newly appointed Astronomer Royal, Nevil Maskelyne, based at the Greenwich Observatory, was largely concerned at this time with observing lunar eclipses, planetary transits and passing comets. His special interests lay in establishing tables for use at sea as a mariner’s almanac, and in the calculation of longitude. He noted that since his seventeenth-century predecessor at Greenwich, John Flamsteed, had thoroughly mapped the heavens, he himself kept only thirty-one stars under regular observation.66
Since his long nights of riding over the moors, Herschel’s interests had roamed far beyond the safe family of the solar system, with its restricted circuit of sun, moon and six known planets. He had the courage, the wonder and the imagination of a refugee. His whole instinct was to explore, to push out, to go beyond the boundaries. Gradually he began to think about the possibilities of Newtonian reflector telescopes. Newtonians were based on a different principle from the traditional refractors. They produced increased ‘light-gathering’, rather than simple magnification. As their name implies, the primary component of a reflector telescope is a large mirror, or speculum, highly polished and subtly curved inwards (concave) so as to gather and concentrate starlight at a much greater intensity than the lens of the naked eye. This concentrated light is then viewed through a simple adjustable eyepiece inserted into the side of the tube, the whole set-up producing wonderfully bright images and little chromatic aberration.
Instead of conventional magnification, Herschel began to think in terms of something he called ‘space-penetrating power’. This was a concept he had partly developed from Robert Smith’s Opticks.67 Conventional eighteenth-century astronomers still studied the night sky as if it were a flat surface, or rather the interior surface of a decorated dome, inlaid with constellations. Flamsteed’s beautiful Celestial Atlas, first published as a large decorative folio in 1729, presented the sky like this. Its second edition of 1776 still remained the standard European book of reference for stellar identification.
Each constellation was given a double-page spread, showing the mythological figures that gave them their names drawn in flat engraved outlines, as well as the known stars belonging to the group. The brighter stars were identified by their home constellation and a Greek letter of the alphabet. So Alpha Orionis, also known by its Arabic name Betelgeuse, was the bright star on the shoulder of Orion the Hunter; and Zeta Tauri (which would later catch Herschel’s attention) was a third-magnitude star in Taurus the Bull.♣
But Herschel began to conceive of deep space. He began to imagine a telescope which might plunge deep down into the sky and explore it like a great unplumbed ocean of stars. This was something a reflector telescope might be able to do supremely well, if its concave mirror were sufficiently large. But because even small astronomical mirrors were expensive, and large ones had not yet been developed (even by London lens-makers like Dolland), Herschel realised that he would have to make them himself. Moreover, to achieve the exquisitely fine reflective surface he required, they would have to be cast in metal, not glass.
Meanwhile the other Herschel brothers began to shuttle between Bath and Hanover. Jacob came over for a brief visit in summer 1767, following Isaac’s death, but after giving virtuoso performances in the Pump Room he preferred to return to his high life in Hanover. Young Dietrich, now aged fifteen, came the following summer, and was given a fine holiday. Finally Alexander came and settled in 1770.68 William moved to a larger house at 7 New King Street, and Alexander was given the attic rooms, while William took over the first floor and had the reception rooms redecorated and furnished with a new harpsichord for his singing and music lessons.
All the time he was evidently worrying about Caroline, and finally in the spring of 1772, after long discussion with Alexander, he wrote to Hanover to ask if Caroline (then aged twenty-one, and having reached her majority) would like to join them at Bath. Knowing the opposition his proposal would face from their mother and Jacob, William put his suggestion in the most plain and practical terms, as Caroline recalled. She should make a trial as to whether ‘by his instructions I might not become a useful singer for his winter concerts and oratorios’. She could also become her brothers’ housekeeper. If after two years this ‘did not answer our expectations’, William would send her back. Significantly, he mentioned not a word of astronomy.69
Caroline longed to accept. But her mother fiercely objected, and so of course did Jacob. ‘I had set my heart upon this change in my situation, [but] Jacob began to turn the whole scheme
