200, with the same instrument which now gives me 460 so distinct that in fine weather I can wish for nothing more so. When you want to practise seeing (for believe me Sir,-to use a musical phrase-you must not expect to see at sight or a livre ouvert) apply a power something higher than what you can see well with, and go on increasing it after you have used it for some time.’165
Caroline later assembled an index of all Herschel’s remarks on practical observation. Under ‘Trials of Different Eyes and Seeings’ she listed such topics as the distortion effect of ‘looking long at an object etc’, the need to progress from lower to higher powers of magnification, the fact that ‘different eyes judge differently of [the same] colours’, that ‘eyes tire’ without the observer noticing, and that ‘we see things always smaller at first, when difficult to be seen’.166
Under another heading, ‘Airs and Situations’, she listed the particular locations and atmospheric conditions which affected a telescope. These were not always self-evident. The atmosphere itself had ‘prismatic powers’, and distortions could be produced by ‘field breezes’, viewing ‘over the roof of a house’, or standing ‘within 6 or 8 feet of a door’. Surprisingly, because of thermal ripples rising from the ground, ‘evenings tho’ apparently fine, are not always good for viewing’. By contrast, ‘moist air was favourable’, and damp or rain, even certain kinds of fog, were ‘no hindrance to seeing’. It was possible to observe in conditions of severe frost, or even falling snow, provided the mirrors were kept clear of ice.167
Caroline gave the term ‘sweeping’ a certain domestic familiarity, so that in her letters she sometimes implies she is a sort of celestial housekeeper, brushing and dusting the stars to keep them in a good state for her brother, a sort of heavenly Hausfrau. But perhaps she also had deeper feelings about the cosmos she was now discovering. It was no longer a mere hobby to please him. Once they had moved to Datchet, in the summer of 1782 Herschel began to train her more carefully in observation techniques, so she could become a genuine ‘assistant-astronomer’. By way of encouragement he built her a special lightweight sweeper, consisting of ‘a tube with two glasses’ (i.e. a traditional refractor), and instructed her ‘to sweep for comets’.
Initially she found working on her own in the dark rather daunting. ‘I see from my Journal that I began August 22nd 1782, to write down and describe all remarkable appearance I saw in my sweeps, which were horizontal. But it was not till the last two months of the same year that I felt the least encouragement to spend the star-light nights on a grass-plot covered with dew or hoar frost, without a human being near enough to be within call.’168 Besides, at this early stage Caroline knew ‘too little of the real heavens to be able to point out every object so as to find it again without losing too much time by consulting the Atlas’. As all novice astronomers find, stars move disconcertingly rapidly through a telescopic field of vision, even that of a low-powered telescope, and can easily slip away in the few moments spent consulting a star chart and then readjusting one’s eyes to night vision. (Night vision can take as long as thirty minutes to establish its full sensitivity.)
Clearly things were better for Caroline when Herschel was on hand in the garden, and not away at Windsor doing royal demonstrations. ‘All these troubles were removed when I knew my brother to be at no great distance making observations with his various instruments on double stars, planets etc, and I could have his assistance immediately when I found a nebula, or cluster of stars.’ In this first year Caroline found no comets, and only succeeded in identifying fourteen of the hundred or so known nebulae. She was too often interrupted by Herschel’s imperious shout, when he wanted her to write down some new observation made with the large twenty-foot.169
Such teamwork was essential to the sweeping procedure that the Herschels developed. As William made his observations, he would call out precise descriptions of what he saw (with special attention to double stars, nebulae or comets). He would give magnitudes, colour and approximate distances and angles (using a micrometer) from other known stars within the field of view. Standing below him in the grass, and later sitting at a folding table, Caroline would meticulously note all this data down, using pen and ink and a carefully shrouded candle lantern, and consulting their ‘zone clock’ (a clock using a time scale related to the position of the stars, rather than the sun). Alexander Aubert would later give them a magnificent Shelton clock, with compensated brass pendulum, as a contribution to their work.170
With Herschel, this was not tranquil or contemplative work, as might be supposed. Caroline would ‘run to the clocks, write down a memorandum, fetch and carry instruments, or measure the ground with poles etc etc of which something of the kind every moment would occur’.171 Sometimes she would call back questions, asking for further clarifications. Most importantly she would note the exact time of each observation, using the special zone clock, which would give a precise position as each object rotated through the meridian. By this method, at no point would William have to compromise his night vision by looking at a lit page and taking his own notes.
Herschel described their sweeping methods in a paper published in April 1786, ‘One Thousand New Nebulae’. Crucial to his technique was that he did not have to take his eye away from the lens, but could ‘shout out’ his observations while his assistant wrote them down and ‘loudly repeated’ them back to him. This had ‘the singular advantage’, as he put it, ‘that the descriptions were actually writing and repeating to me while I had the object before my eye, and could at pleasure correct them’. The distinct tone of military command was emphasised by the fact that nowhere in this paper did Herschel mention that his assistant was Caroline.172
Standing under a night sky observing the stars can be one of the most romantic and sublime of all experiences.♣ But the Herschels’ sweeps were fantastically prolonged and demanding. In clear weather, they would often go on for six or seven hours without a break. They began at eleven at night, and often did not go to bed before dawn, in a mixed state of exhaustion and euphoria. Both slept till midday, and the house had to be kept quiet most of the morning, although Caroline often seems to have been up early, drinking coffee and writing up the night’s observations in long, minute columns of figures: a sort of double book-keeping which she often referred to as ‘minding the heavens’.
Observations and note-making required dogged precision and absolute concentration. It could be chill even in summer, and in winter the frost covered the grass around them, and the wind moaned through the trees. (Nevil Maskelyne had a special woollen one-piece observation suit made for him at Greenwich, with padded panels that made him look like a premonition of the Michelin Man.) Herschel took to rubbing his face and hands with raw onions to keep out the cold. When Banks came down to join them he sometimes brought oversize shoes so he could wear half a dozen pairs of stockings inside them. Caroline layered herself in woollen petticoats. Frequently it was so cold that films of ice formed on the telescope mirrors, the ink clotted in the well, and frozen beads blunted the tip of Caroline’s quill.173
It could also be dangerous. Caroline wrote: ‘I could give a pretty long list of accidents of which my Brother as well as myself narrowly escaped of proving fatal for observing with such large machineries, where all around is in darkness [and] is not unattended with danger; especially when personal safety is the last thing with which the mind is occupied at such times.’174 The winter of 1783 was especially harsh. On one night in November that year, when William was mounted
