“On the Power of Penetrating into Space”: The Telescopes of William Herschel

Herschel to Aubert , 9 January 1782, Royal Astronomical Society MS Herschel W.1/1, pp. 21–22; Lubbock C. A. , The Herschel chronicle (Cambridge, 1933), hereafter cited as Chronicle, 102–3.

RAS MS Herschel W.1/5.1; Chronicle, 64–65. Herschel would have derived the notion of light-gathering power from Robert Smith's Opticks, see Hoskin M. A. , William Herschel and the construction of the heavens (London, 1963), 21.

RAS MSS Herschel W.5/12.1; 2/1.1, f. 1; 1/1, p. 58; The scientific papers of Sir William Herschel, ed. Dreyer J. L. E. (London, 1912), hereafter cited as Papers, i, xxiv-xxv.

RAS MS Herschel W.2/1.1, f. 1; Papers, ii, 490.

RAS MSS Herschel W.5/12.1, pp. 2–5; 2/1.1; 1/1, p. 16; Papers, i, xxv, 19. To place Herschel's instruments in context note that Short had advertised reflectors of up to 12ft focus and 18 inches aperture. See King H. C. , The history of the telescope (London, 1955), chap. 5.

Chronicle, 136.

Papers, i, 485.

RAS MSS Herschel W.5/12, 5/13, 5/14.

RAS MS Herschel W.5/12.1, pp. 7–41; Papers, i, xxvi.

RAS MS Herschel W.2/1.1, f. 81; Papers, i, 371.

RAS MS Herschel W.5/12.1, p. 42.

Ibid.

Ibid., p. 44; RAS MS Herschel W.2/1.1, f. 88v.

RAS MS Herschel W.5/12.1, p. 48.

Herschel to Watson , 7 January 1782, RAS MS Herschel W.1/1, pp. 17–18; Chronicle, 101; cf. ibid., 104–5.

RAS MS Herschel W.1/1, pp. 70–72; Papers, ii, 240–1. Naturally this depended on the nature of the fog; Herschel noted on 31 December 1799, “… I have formerly been used to see remarkably well in misty and foggy weather, and suspect rather that the present mist is the smoke of London, the wind being in a proper quarter to bring it hither”, RAS MS Herschel W.2/2.5, f. 58v.

RAS MS Herschel W.2/1.3, f. 15; Papers, ii,.

RAS MS Herschel W.7/8, p. 33; Papers, i, xxiii.

RAS MS Herschel W.2/1.2, ff. 8–10.

RAS MS Herschel W.1/1, p. 38; Papers, i, 99.

RAS MSS Herschel W.5/5, no. 1–4.

RAS MS Herschel W.1/1, p. 84. For the dating, see RAS MS Herschel W.1/13.W.28 and note that one of the drawings (see Figure 3) is dated 1783.

RAS MS Herschel W.1/13.W.28 (I owe this reference to M. A. Hoskin). It is not a drawing of Watson's 7ft telescope, preserved at the Science Museum, London.

Papers, i, 485.

Ibid., 9.

RAS MS Herschel W.5/5, no. 6. Since we are concerned with the history and prehistory of the instruments of high space-penetrating power, later smaller telescopes–-Gregorians and Newtonian “sweepers”–-will not be considered.

Chronicle, facing p. 79; King H. C. , op. cit. (ref. 5), 123. It is correctly identified in Maurer A. , “William Herschel's Astronomical Telescopes”, Journal of the British Astronomical Association, lxxxi (1971), 284–91, p. 286.

RAS MS Herschel W.5/12.1, p. 56; Papers, i, xxvii.

Chronicle, 87.

RAS MS Herschel W.5/12.1, pp. 48ff; Papers, i, xxvi–xxviii; Chronicle, 88–90.

RAS MS Herschel W.5/12.1, p. 56; Papers, i, 486.

RAS MS Herschel W.5/12.1, p. 56.

Ibid.

Herschel expected to be able to “observe the heavens in the meridian from about 10 degrees above the horizon up to the zenith”, ibid.

RAS MS Herschel W.2/1.1, f. 42v.

RAS MSS Herschel W.2/1.5, f. 16v; 1/1, p. 69; Papers, ii, 242; Chronicle, 135–6.

RAS MS Herschel W.2/1.5, f. 41v.

Ibid., f. 38v.

Hoskin , William Herschel and the construction of the heavens, chap. 3.

RAS MS Herschel W.2/1.6, f. 29v.

Ibid.; Papers, ii, 653.

RAS MS Herschel W.2/1.6, f. 43v; Papers, ii, 657.

RAS MS Herschel W.2/1.6, f. 44; Papers, ii, 657.

Herschel began his account of the proposed 30ft, in January 1781, by saying: “Having long ago intended to make a very large reflector, as soon as …”, RAS MS Herschel W.5/12.1, p. 48. Cf. a letter of February 1783 (RAS MS Herschel W.1/1, p. 63) where he describes himself as “determined to carry improvements in telescopes to their utmost extent”.

Chronicle, 116.

RAS MS Herschel W.1/1, pp. 37–38; Papers, i, 99.

Ibid., 157.

RAS MS Herschel W.5/12.1, pp. 1–2; Papers, i, xxiv.

Ibid., 485.

RAS MSS Herschel W.5/12.1, pp. 61–62; 2/1.7, f. 14; 4/1.5, p. 446.

RAS MSS Herschel W.5/12.1, p. 62; 2/1.7, f. 48v; 4/1.6, pp. 514, 517, 523.

RAS MSS Herschel W.2/1.7, f. 15; 4/1.5, pp. 448–9. Herschel had finished his “third review” only on 26 September, ibid., 440.

Papers, i, 162–4.

RAS MS Herschel W.5/15. There is another print at the Science Museum, London. Like those at the RAS it came directly from Herschel's descendants. There is also a print at the National Maritime Museum, Greenwich.

Watson to Herschel, 9 November 1783, RAS MS Herschel W.1/13.W.29. Watson wrote on 23 November: “The account you gave me of your wonderful machine gave me infinite satisfaction …”, ibid., W.30.

RAS MS Herschel W.2/1.8, f. 18v.

RAS MSS Herschel W.2/1.9, f. 4; 4/1.7, pp. 614–15; Papers, i, 260.

RAS MSS Herschel W.2/1.7, ff. 24v, 49; 4/1.5, p. 462.

RAS MS Herschel W.2/3.6, sws (“sweeps”) 600, 608, 609; Papers, i, 294, 312.

RAS MS Herschel W.1/1, pp. 198–9; Chronicle, 214. In fact he commonly used the right eye, though occasionally he experimented with binocular vision, RAS MS Herschel W.2/3.7, sws 769, 814; Papers, i, xliii.

There is no detailed account of the 20ft. There is, however, a general outline at Papers, i, 260–4, and much information may be gathered from the various observation journals. There is also a copy of the instructions Herschel sent with the St Petersburg 20ft, RAS MS Herschel W.5/9.

RAS MS Herschel W.2/3.5, sws 537–8; note also 2/3.4, sw. 403.

Such as the 40ft, the 25ft and the rebuilt 20ft. Caroline seems to be referring to the 20ft when she says that Piazzi once tripped over the projecting rack-bar, Chronicle, 138; the 40ft had two such bars. Note the other known illustration of the 20ft, see Papers, i, facing xxxvii, and contrast this with Figure 6.

Papers, i, 261.

Note King, op. cit. (ref. 5), 128.

Papers, i, 261; italics added.

Chronicle, 137.

Papers, i, 157.

Ibid., 261.

RAS MS Herschel W.2/1.10, f. 20.

Ibid., ff. 20v–27. Herschel wrote to his brother Alexander, 10 March 1785, “I have made great improvements in the motions of my 20 feet telescope, and can now in less than a quarter of an hour, by myself set my instrument upon any part of the heavens so as to be ready for sweeping in such a part; except just in the zenith”, RAS MS Herschel W.1/9·2.

Including the rebuilt large 20ft, see below.

Papers, i, 261; RAS MS Herschel W.4/1.5, pp. 448–50.

Papers, i, 261.

Ibid., 261–2; RAS MSS Herschel W.2/3.1, sw. 46; 4/1.5, p. 467.

Papers, i, 260.

Ibid., 262; RAS MSS Herschel W.2/3.1, sw. 136; 2/3.4, sw. 426; 4/1.6, p. 512.

RAS MS Herschel W.5/9, pp. 1–2.

Cf. ibid., pp. 10–11.

For the right ascension apparatus, see RAS MSS Herschel W.2/1.9, ff. 32v, 36v; 2/1.10, f. 58; 2/3.2, sw. 244; 2/3.3, sw. 330; 2/3.4, sw. 418; 2/3.5, sw. 549; 4/1.6, p. 494; Papers, i, pp. 262–4.

RAS MS Herschel W.2/3.4, sw. 413.

RAS MSS Herschel W.2/3.1, sw. 68; 4/1.5, p. 480; Papers, i, 262. It is not clear just where this scale was placed.

RAS MSS Herschel W.2/1.10, f. 58; 2/3.1, sw. 136; 2/3.3, sw. 330; 4/1.6, pp. 512–13; Papers, i, 263.

RAS MSS Herschel W.2/3.1, sw. 91; 4/1.6, p. 494; Papers, i, 262.

RAS MS Herschel W.2/3.2, sw. 245; Papers, i, 262.

RAS MSS Herschel W.2/1.9, f. 19v; 2/1.10, f. 28v; 2/3.2, sw. 232; 2/3.3, sw. 288; 2/3.4, sw. 401; 4/1.7, p. 645.

RAS MSS Herschel W.2/3.4, sws 434, 440, 442, 451, 453, 466; 2/3.5, sw. 490; Papers, i, 263.

The account of the polar distance machine is in this case a little conjectural as, unlike the 40ft, Herschel's own 20ft is not well documented. In the St Petersburg 20ft the polar distance line passed over a pulley at the tube, doubled back to the polar distance machine and eventually ended at a weight hanging beneath the ladder, which helped to maintain the correct tension, RAS MS Herschel W.5/9, pp. 14, 21–22.

RAS MS Herschel W.2/3.5, sws 532–3. For methods of counterbalancing the tube, see Papers, i, 526–7; British Museum Microfilm M/588 (1) (see ref. 178).

Papers, i, 247–8.

Ibid., 248. This implied a telescope with an aperture approximately 4½ft.

Ibid., 255.

RAS MS Herschel W.1/5.1. Compare what Herschel says at Papers, i, 485–6.

RAS MS Herschel W.5/12.1, pp. 68–69.

Ibid., 69–70.

Papers, i, 486; RAS MSS Herschel W.1/1, p. 143; 7/8, p. 37. Caroline said that the move was on 3 April (Chronicle, 144), whereas William dated it to Lady Day, RAS MS Herschel W.7/8, p. 37. Caroline's autobiography must be used with some caution. Her inherently unlikely, but frequently repeated, claim that “The last night at Clay Hall was spent in Sweeping till daylight, and the next the Telescope stud ready for observation at Slough” (Chronicle, 144) is proved wrong by the observing journals. Note also ref. 146.

Chronicle, 149, 152, 156; RAS MSS Herschel W.5/12.1, p. 73; 1/1, p. 153.

RAS MSS Herschel W.5/12.1, p. 74; 2/3.6, sw. 702.

Papers, i, 487.

RAS MS Herschel W.2/1.12, f. 13v.

RAS MS Herschel W.5/12.1, p. 90; 1/9.3; Papers, i, 486.

RAS MS Herschel W.5/12.1, pp. 71–73.

RAS MSS Herschel W.5/12.1, p. 92; 5/14.1, f. 19.

RAS MSS Herschel W.2/4, f. 1; 5/12.1, p. 95; 1/1, pp. 169–70.

The mirror had been tried on the night of 24 October. On 26 October Herschel wrote to Charles Blagden: “The focal length of my second 40ft. Speculum is about 14 or 15 inches longer than I wish to have it, for which reason I have left it in an unfinished state…. As soon as I shall have compleated a very considerable intended improvement in the polishing apparatus by which I hope to gain a full directing power (which I could hardly ever have over 20 or 21 men which I was forced to employ in the work) I shall proceed to shorten the focal length and compleat the Speculum”, RAS MS Herschel W.1/1, pp. 169–70.

RAS MS Herschel W.5/12.1, p. 95. He wrote to Blagden on 18 January: “The polishing apparatus of the 20 feet Telescope answers my utmost expectation or rather exceeds it. I am still pursuing my experiments with it, as I would compleatly remove every difficulty in this before I try the 40 feet in the same manner”, RAS MS Herschel W.1/1, p. 172.

RAS MSS Herschel W.2/1.12, f. 21v; 2/3.7, sw. 904; 5/12.1, pp. 95–106, 113–14.

Ibid., 116–17.

RAS MSS Herschel W.2/4, f. lv; 5/12.1, pp. 119–24; 1/1, p. 178.

RAS MS Herschel W.5/12.3, expt 3.

RAS MS Herschel W.5/14.1, f. 35v.

Ibid., f. 18.

RAS MS Herschel W.5/2.4.

Caroline Herschel recorded that in 1822 William gave a visitor “a Plate of the 40 feet telescope”, Chronicle, 360. Another copy of the original 40ft engraving is preserved at the Cambridge University Institute of Astronomy; it was presented to the University Observatory by Herschel John c. 1830. It is likely that further copies of this engraving, and of that of the large 20ft, will be found. The illustration of the 40ft in Chronicle, facing p. 167, has been taken from the original engraving.

Papers, i, 485–527. Even so, it is surprising how easily misconceptions seem to arise. The paragraph on the motions of the 40ft at King , op. cit. (ref. 5), pp. 130–2, is very inaccurate. For example, the measure of polar distance did not derive from the winch that raised the tube, Herschel could not control a fine vertical motion from the observing seat, and the polar distance clock had no bells.

Herschel had originally planned an octagonal tube, RAS MS Herschel W.1/5.1.

Papers, ii, 47; RAS MS Herschel W.5/12.1, p. 75.

Ibid.

RAS MS Herschel W.1/13.A.26; Chronicle, 180–1.

See RAS MS Herschel W.5/9, p. 15; British Museum Microfilm M/588(1).

RAS MS Herschel W.2/4, f. 1.

Papers, i, 511.

Ibid., li–liv.

Ibid., ii, 536, 543–4.

Ibid., 543.

Ibid., 51–52, 589.

RAS MS Herschel W.2/8.1; 2/4, ff. 3–4; Papers, ii, 589.

RAS MS Herschel W.2/1.12, f. 41v.

Ibid., f. 48v.

Ibid., f. 54.

RAS MS Herschel W.5/12.1, p. 128.

RAS MS Herschel W.5/12.4, expts 192, 194; for 1807–8, see ibid., expts 158–85, passim; Chronicle, 323. It is stated at ibid., 342, that polishing continued until 1814, but I have found no other evidence for this. Herschel wrote on 31 October 1810: “… The mirror is already much injured, though it is not much above 2½ years since it was last polished. A mirror of this size cannot last above 3 or 4 years without being repolished. There ought to be two of them as was intended, but my thin one will admit no perfection in figure …”, RAS MS Herschel W.2/2.8, f. 3.

See for example, RAS MSS Herschel W.1/1, p. 271; 2/2.8, f. 3; 5/12.4, expt 192.

RAS MSS Herschel W.1/1, pp. 210–11; 1/13.M.92. In fact two smaller telescopes were made (Papers, i, l–li) and are preserved at Madrid.

See ref. 134; also RAS MS Herschel W.1/1, p. 214; Chronicle, 261.

RAS MS Herschel W.1/1, pp. 202–5. Note, in connection with the engravings published in 1794, that along with this letter, concerning a telescope for St Petersburg, Herschel sends “drawings” of the 20ft and 40ft. An unusual commercially-made telescope, which is not considered here, was a 14ft Newtonian, aperture 13·6 inches, made for the Glasgow Observatory in 1810–11, RAS MSS Herschel W.1/8.37; 5/12.4, expts 212–51; 5/14.1, ff. 4, 6, 7; 2/2.8, f. 4v.

RAS MS Herschel W.5/14.1, f. 11; 5/12.3, expts 328–72, 439–503, passim.

Further polishing took place in August 1799 and December 1800–March 1801, ibid., expts 558–9, 579–86.

RAS MS Herschel W.2/1.12, f. 21v.

Ibid., ff. 31, 32, 33v; RAS MS Herschel W.5/12.3, expt 448.

RAS MS Herschel W.2/1.12, f. 31v.

Papers, ii, 46, 560.

Ibid., 46.

Ibid., 547.

Herschel sometimes complained that the large instruments gave too much light for viewing planets comfortably, e.g., Papers, i, 444. There is an observation of Saturn with the 25ft recorded in April 1801, RAS MS Herschel W.2/2.6, f. 19: “It shews saturn very well”.

Tinoco J. , Apuntes para la historia del Observatorio de Madrid (Madrid, 1951), 25–26. According to Caroline Herschel, the telescope was not shipped until October 1805, Chronicle, 319, but the polishing record is more consistent with the earlier date, see ref. 138.

Revista de la Sociedad Astronómica de España y América, xx (1930), 99; Garrorena P. Carrasco , “El Gran Telescopio de Herschel-Mendoza, adquirido para Madrid a fines del siglo xviii”, Revista de la Real Academia de Ciencias Exactas, Fisicas y Naturales de Madrid, xxvi (1931), 51–59; Tinoco , op. cit. (ref. 146).

See Garrorena Carrasco , op. cit. (ref. 147).

Compare RAS MS Herschel W.5/8, f. 4.

RAS MSS Herschel W.1/13.M. 93–96.

RAS MS Herschel W.5/11.2.

Further information comes from copies of the lists of the contents of the packing-cases and the directions for using the telescope, RAS MSS Herschel W.5/11.1–3.

Papers, i, 491, 498.

RAS MS Herschel W.5/12.3, expt 547.

RAS MS Herschel W.5/14.1, f. 4v.

RAS MS Herschel W.5/8, f. 8.

RAS MSS Herschel W.1/13.T.10–11.

Ibid., T.10; RAS MSS Herschel W.5/12.3, expt 547; 5/14.1, f. 13v.

RAS MS Herschel W.5/12.3, expt 563.

Ibid., expts 547–51, 560, 562–3.

Herschel wrote in 1785 of the proposed 30ft speculum of 1781: “The mould in which it was to be cast is still, with the furnace I build [sic.] for casting, in my brothers house at Bath”, RAS MS Herschel W.1/5.1.

RAS MS Herschel W.2/2.5, f. 52v.

Details of this telescope can be found at RAS MS Herschel W.5/8. King , op. cit. (ref. 5), 91, is mistaken in thinking that Michell's telescope was a Gregorian, see RAS MSS Herschel W.1/13.M.99; 1/13.T.10; 7/14.

Papers, ii, 537.

RAS MS Herschel W.2/2.7, f. 28v.

Ibid., f. 33.

RAS MS Herschel W.5/12.4, expt 450.

RAS MSS Herschel W.1/4; 1/1, p. 290. The final careful polishing was carried on from October 1814 till March 1815, and again from October 1815 till April 1816, RAS MS Herschel W.5/12.4, expts 402–23, 430–62.

RAS MS Herschel W.1/4.6 (note that he sent Bonaparte “two engravings of the 20 feet telescope”); 5/8, f. 4.

Papers, ii, 536–7.

RAS MS Herschel W.2/2.8, f. 20. Both Dreyer at Papers, i, liii, and Chronicle, 342, say that these observations were made in August 1815, but this is incorrect.

RAS MS Herschel W.7/11, pp. 10–11; Papers, i, lv.

RAS MSS Herschel W.5/12.4, expts 467–9, 474–96; 1/11.4.

Herschel J. F. W. , “Account of some Observations made with a 20-feet Reflecting Telescope”, Memoirs of the Royal Astronomical Society, ii (1826), 459–97, p. 459; Herschel J. F. W. , Results of astronomical observations made during the years 1834–8 at the Cape of Good Hope (London, 1847), ix; British Museum Microfilm M/588(1). This collaboration between father and son must have been a very valuable experience for John Herschel. It is interesting in this connection to note that William Herschel's final summary of his polishing experiments, entitled “Results of Experiments on the Construction of Mirrors for Telescopes” (RAS MS Herschel W.5/14), was accompanied by 59 machine drawings, all executed by John.

Herschel J. F. W. 1847, op. cit. (ref. 174), ix; BM Microfilm M/588(1).

Herschel J. F. W. 1826, op. cit. (ref. 174), 459.

RAS MSS Herschel J.10/1; 10/4; Herschel J. F. W. 1847, op. cit. (ref. 174), xx.

An incomplete version of this account is included in BM Microfilm M/588(1).

Ibid. It is not quite clear how the tube was supported in the earlier large 20ft.

Herschel J. F. W. 1826, op. cit. (ref. 174), 461. King , op. cit. (ref. 5), 202, credits John Herschel with having carried out his Cape sweeps unassisted. However, this is not true as he had an attendant, one John Stone, who was also “a ready mechanic”, Herschel J. F. W. 1847, op. cit. (ref. 174), ix; BM Microfilm M/588(1).

Herschel J. F. W. 1826, op. cit. (ref. 174), 460; BM Microfilm M/588(1).

Herschel J. F. W. 1826, op. cit. (ref. 174), 461–2; BM Microfilm M/588(1).

RAS MS Herschel W.1/5.1.

Papers, ii, 589.

Ibid., 607–13.

This was pointed out by Dreyer , Papers, i, xlviii. It is not clear why the results of Herschel's polishing experiments were never published (see ref. 8). By January 1805 Herschel had written “a work on the subject of making mirrors for telescopes of all sizes, in which the method of giving them not only the parabolic form but any other of the conic sections that may be required, is explained with perfect clearness and supported by several thousands of facts”, and was beginning to arrange its publication by the Royal Society, see RAS MSS Herschel W.1/13.B.126; 1/1, p. 257.

BM Microfilm M/588(1).

The scientific papers of William Parsons, third Earl of Rosse, 1800–1867, ed Parsons C. (London, 1926), 14.

In a letter published in The Times, 6 October 1838.

See for example Parsons , op. cit. (ref. 188), 9.

Note ibid., 101.

Ibid., 16.

Ibid., 80.