The English Telescope from Newton to Herschel

a d m i s s i o n f r e e

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The English Telescope from The English Telescope from Newton to Herschel Newton to Herschel The Anniversary of the Telescope a s p e c i a l exhibition n October 1608 a patent appli- In May 1609 Galileo Galilei, a math- cian Thomas Harriot, who had been a cation was considered by the ematician in Padua, Italy, heard of the student at Oxford, began making tel- invention by ‘a certain Fleming’ of ‘a escopic drawings of the moon. Though 16th o c t o b e r 2008 - 22nd m a r c h 2009 I States General of the Nether- lands from Hans Lipperhey of Middel- spyglass by means of which visible he knew nothing of Harriot’s work, burg, ‘who claims to have a certain objects, though very distant from the Galileo also turned his telescope to his exhibition is the Muse- this period. The English made impor- In the surprising world of the eight- device by means of which all things at eye of the observer, were distinctly the night sky in the autumn and com- um’s contribution to the an- tant contributions to the instrument’s eenth-century telescope, professors a very great distance can be seen as seen as if nearby.’ He began to work menced a series of lunar observations T niversary of the telescope. development, including the invention might work at grinding mirrors, instru- if they were nearby, by looking through on improving the telescope, which was on 30 November. Our collection is strongest in the time and improvement of forms of reflect- ment makers could write textbooks, glasses which he claims to be a new expected to have military applications between the working lives of two ma- ing telescope and the invention of the a silk-weaver might challenge the invention.’ This reference, in a letter on land and at sea. Four hundred years later, 2008-9 is be- jor figures in the history of astronomy, achromatic lens. These events are en- authority of Newton, and a musician of introduction from the government of ing celebrated as the anniversary of Isaac Newton and William Herschel, a twined with engaging human stories could use home-made instruments to Zeeland dated 25 September 1608, is At this stage there is no record of the the invention of the telescope and its period from the late seventeenth to the that reflect aspects of eighteenth-cen- change the very nature of astronomy. the first secure record we have of the telescope being considered of value application to astronomy. 2009 has early nineteenth century. tury life and society. So the exhibition existence of a telescope. for studying the heavens but in early been adopted by UNESCO as the Inter- is a series of episodes about more than August 1609, the English mathemati- national Year of Astronomy. There are other reasons for choosing the technical development of a scien- tific instrument.

Optical and Mechanical Gentlemen

Refracting telescope by Will Longland, London, c.1695 n 1738 Robert Smith, Professor The precedent of Newton himself mak- In fact there is more evidence of com- Refracting telescope by of Astronomy at the University of ing mirrors for his telescopes and de- mercial experimentation than is gen- John Yarwell, London, c.1685, Cambridge, published his influen- scribing his practical methods in his erally acknowledged. While Newton with a detail showing his signature I tial Compleat System of Opticks. Along Opticks was important for making this made anonymous references to ‘one with mathematics and theoretical op- mechanical work popular among gen- of our London Artists’ or to ‘an Artificer tics, it offered a thorough account of all tlemen. Such activities also became at London’, we know that makers such the practical aspects of making mirrors part of a growing passion for using re- as Francis Hauksbee, John Rowley, Ed- for telescopes. The list of subscribers flecting telescopes in the popular study ward Scarlett and George Hearne were includes many clergymen, lawyers, of astronomy within an increasingly involved in early attempts to make re- physicians and academics, as well as fashionable interest in natural philoso- flectors. a number of well-known instrument phy. makers. Gregorian reflectors quickly became In both Newton’s Opticks and Smith’s much more popular than Newtonian. In Smith presents the mechanical work of Opticks there is discussion of the in- the arrangement due to the mathema- making mirrors as a polite occupation, clination and ability of the commercial tician James Gregory light is received appropriate to the rational gentleman. makers to contribute to the develop- by a large (primary) concave mirror at The leading exponents, who make ment of these telescopes. The authors’ the near end of the tube and reflected metal castings and grind and polish comments are generally negative. to a small (secondary) concave mirror Long and Short Telescopes them by hand, are John Hadley, Vice- Commercial opticians worked on glass at the far end from the observer, where President of the Royal Society, James lenses and it seemed doubtful whether it is reflected back to the primary, pass- Bradley, Savilian Professor at Oxford, their figuring and polishing skills could ing through a hole in the centre into the alileo’s observations of the to use an object-grass (the lens that tion, so the dispersion of light into its and Samuel Molyneux, a member of be transferred to metal mirrors, while eyepiece. Hence the characteristic hole heavens brought a rich har- first receives the light) with only a different colours could be avoided: parliament and privy counsellor. Smith at first the market for such telescopes in the Gregorian primary mirror. vest of discoveries and in the slight curvature. This meant that the ‘Seeing therefore the Improvement of G himself, a Professor in Cambridge, must have seemed uncertain. seventeenth century it was commonly deviation of the light would be slight Telescopes of given lengths by Refrac- would shortly become Master of Trin- thought that there was little left to and the telescope tube would have to tions is desperate; I contrived hereto- ity College and Vice-Chancellor of the find. One remaining problem was the be very long to allow room for an im- fore a Perspective by Reflexion, using University. puzzling appearances of the planet age to be formed. The best telescopes instead of an Object-glass a concave Saturn, solved by Christiaan Huygens’s became very long and inconvenient to of Metal.’ announcement of a ring structure in use. 1659. It was important for the subsequent Detail of a presentation inscription According to Newton’s account of the history of the telescope that in his on a Gregorian reflecting telescope Technical problems derived from two refraction or bending of light and its book Opticks, Newton describes in de- by James Short, London, 1755 deficiencies in the lenses used at the dispersion into the colours of the spec- tail how he ground and polished metal time. ‘Spherical aberration’ refers to trum, the colours were not created by mirrors with his own hands. He also the fact that lenses whose surfaces are the process of refraction. The different- describes his attempts to improve sil- sections of spheres (the only sort that ly coloured rays were already present vered glass mirrors for telescopes. could be ground and polished by spec- in the light, which was a mixture of tacle makers) do not bring all parallel these different rays, having different When he introduces his arrangement of rays of light to the same point, and so refractive properties. Refraction sim- mirrors and lenses, in the construction the image will be out of focus. ‘Chro- ply separated them out. Since lenses that came to be called ‘Newtonian’, he matic aberration’ refers to the different worked by refracting light, this disper- describes its purpose as ‘To shorten behaviour of the different colours in sion into colours was unavoidable, so Telescopes’. The great length of good light, such that, even with an optimally- Newton concluded that a refracting refractors had become a serious prob- shaped, non-spherical lens, rays of dif- telescope could never be perfected, so lem. Robert Hooke also had experi- ferent colours will not return to create as to give a truly sharp image. mented with arrangements of lenses a sharp image after refraction. and mirrors to shorten telescopes and Newton proposed instead that tel- had published a number of designs. It was found in practice that the only escopes should be made with mirrors. way to deal with these problems was They worked by reflection, not refrac- James Short and John Dollond

he fortunes of two telescope making enterprises dominate the middle decades of the eighteenth cen- tury. One was exclusively concerned with reflectors, The eyepiece of a Gregorian reflecting telescope T the other with refractors. Both had direct connections with by Benjamin Martin, London, c.1770 the technical legacy of Newton.

James Short was born in Edinburgh in 1710 and at the Uni- versity there he came under the influence and patronage of the mathematician Colin MacLaurin, who was a friend and follower of Newton, and who encouraged Short’s telescope making and promoted his reputation. Thanks to MacLaurin, Popular and Professional Astronomy Short was singled out for praise in Smith’s Opticks. He made only reflecting telescopes. A print of John Dolland, with prisms and Newton’s Opticks hile gentlemen were en- The Gregorian reflector was the ideal the period. Observatories were estab- couraged by Robert Smith telescope for the amateur natural phi- lished across Europe by state and city Short moved from Edinburgh to London in 1738 and his busi- W to grind and polish mir- losopher interested in the heavens. Mir- governments, church authorities, sci- ness was very successful: he made about 1370 telescopes rors, more entrepreneurial promoters rors could be made with much greater entific societies and wealthy individu- and was worth some £20,000 at his death in 1768. He be- of popular natural philosophy sought to diameters than lenses. A telescope als, and they were generally equipped came a fellow of the Royal Society and was close to being ap- broaden their audience and their mar- with a larger ‘aperture’ would collect with measuring instruments by London pointed Astronomer Royal, believing he had been denied the ket. The common signal they adopted more light and an observer could enjoy makers, such as John Bird, Jonathan appointment on account of his support for the chronometer to announce that they offered a tel- greater detail in the moon or Saturn, Sisson, Jesse Ramsden or Edward maker John Harrison. The Museum has the largest collection escopic astronomy that would be en- or wonder at the variety of nebulae or Troughton. of telescopes by James Short: nine are included in the exhibi- tertaining as well as educational - and clusters of stars. That they had little tion, while a tenth - an example of the largest in Short’s range even devotional - was to describe it as potential for measurement was of no There was a clear distinction between - can be seen outside the entrance to the gallery. suitable for ladies as well as gentle- importance in this context. The Grego- the measuring activity in observatories, men. rian had the intuitive advantage over conducted by ‘astronomers’, and the John Dollond began his optical researches with complete the Newtonian that the observer was qualitative observing open to anyone confidence in Newton’s conclusions but he came to doubt It was often the instrument makers looking towards the object of interest. curious to view interesting objects in the reliability of the assumption that the dispersion of light Portrait of James Short, oil on canvas, who promoted such initiatives, through the night sky. The telescopic needs of c.1758, attributed to Benjamin Wilson (its separation into component colours) for a given degree of writing or publishing popular text- Gregorians are found in great num- the former were met by a refractor of refraction or bending was the same for all types of glass. Dol- books, giving or housing subscription bers in the eighteenth century, signed relatively modest aperture, equipped as John Dollond was a silk-weaver in a Huguenot family in Spi- lond’s subsequent experiments showed that this was not the lecture series with entertaining dem- by a great many London makers. It is a sight, whereas a reflector suited the talfields, whose interest in practical optics led him to encour- case and that high-lead ‘flint’ glass had a significantly great- onstrations, and devising instruments far from certain, however, that many latter, since the larger aperture possi- age his son Peter to become an optical instrument maker and er dispersive power than the more common optical ‘crown’ that were easy to use, appropriate to were the actual makers, as there was ble with a mirror was good for admiring eventually to join Peter in the partnership of ‘J. Dollond and glass. This meant that by combining their effects light could a domestic interior, and accommodated a great deal of sub-contracting to ac- detail in the solar system or wondering Son’. He came to challenge Newton’s insistence that refract- be bent without being split into colours, and an ‘achromatic’ to a range of incomes. quire parts, as well as retailing instru- at fainter and farther objects, such as ing telescopes inevitably produced blurred images with col- combination of two lenses could produce overall deviation ments in fact made by others. nebulae. oured fringes, and his portrait shows him holding Newton’s without dispersion. Newton had been wrong. James Ferguson published An easy Opticks, with a place marked, no doubt indicating the location introduction to astronomy, for young One place where refracting telescopes When the great cataloguer of nebula, of Newton’s error. Dollond explained his technique in a paper to the Royal Soci- gentlemen and ladies, while Benjamin are found in eighteenth-century as- William Herschel, detected an un- ety in 1758 and was awarded the prestigious Copley Medal. Martin offered The young gentleman tronomy is as telescopic sights on the known planet while using a reflecting James Short advertised himself as ‘optician solely for reflect- In the same year he also took out a patent on his new lens- and lady’s philosophy. Experimental measuring instruments used in ob- telescope to search for double stars, ing telescopes’. In the world of instrument making it was es in partnership with the optical instrument maker Francis natural philosophy, with astronomy servatories. After initial controversy the famous astronomer Jérôme La- very unusual to be so specialised but with so much prestige Watkins, who contributed his financial support. Dollond was as a prominent component, became a over whether such sights really would lande reported that this extraordinary attached to making reflectors, Short’s stipulation may have elected a fellow of the Royal Society in May 1761 but died fashionable pastime. improve accuracy, they became univer- and unprecedented feat had been been an assertion of status as much as a business title. He suddenly in November and his share in the patent passed to sally adopted in the great expansion of achieved, not by an astronomer at all, himself probably made only the mirrors of his telescopes and his son Peter. observatory work that characterised but by a musician. not the tubes and stands.

The Herschels and their Future Peter Dollond illiam Herschel came from Germany to England the world of professional astronomers and his agenda was in 1757, driven by adverse fortune in the Seven quite different from theirs. He quickly began to increase the Against Allcomers W Years War and seeking a living as a musician. size of his reflecting telescopes so as to enhance his view of He was successful as an organist, impresario, teacher and the wonders he was discovering in the heavens. composer, and was appointed organist to the Octagon Chapel in Bath in 1766. Here he was joined by his sister Caroline, Herschel might have remained an enthusiastic amateur and ohn Dollond’s motives for patent- – indeed he was probably the most who was a talented singer. his ambitions for stellar astronomy ignored but for his extraor- ing the achromatic lens are not effective critics of Dollond’s position. dinary discovery of a planet in the solar system, the first such entirely clear but his son Peter J Herschel was drawn into astronomy discovery in the entire history of written astronomy. On 13 quickly set about exploiting the Peter, ever anxious to defend his fa- in a manner typical of the amateur March 1781 Herschel noted ‘a curious Nebulous Star or per- commercial advantage it gave him ther’s reputation, in 1789 presented in the eighteenth century: he read haps a Comet’; this turned out to be the planet subsequently over other instrument makers. a paper on the subject to the Royal Society. It was read to a meeting James Ferguson on Newtonian named ‘Uranus’. Herschel was made a fellow of the Royal astronomy and then Robert Society, awarded the Society’s Copley Medal, and granted Peter bought the share owned by but not published, whereupon Dol- Smith’s Opticks with its a pension by George III. He abandoned music for full-time his father’s partner Francis Watkins lond himself arranged its publication. instructions for making astronomy. However outlandish his future plans for building in 1763 and in the same year took Ramsden responded with an account, mirrors, he joined the telescopes, however imaginative and even embarrassing his legal action against Watkins for in- also read before the Society, where Bath Philosophical speculations, he could not be ignored. fringement, and in the following year he offered his personal experience of Society and he be- against the widow of the optician the matter and attributed the inven- gan to make and An essential ingredient in Herschel’s success was the manual James Ayscough. Disgruntled and tion of the achromatic lens, not to use his own tel- skill he developed for grinding and polishing metal mirrors. Refracting telescope by J. Dollond alarmed at this, 35 London opticians his father-in-law John Dollond, but & Son, London, c.1760; a very rare escopes. Her- Herschel had begun polishing in 1773 but in March 1778 he petitioned the Privy Council for the to Chester Moor Hall, who had been example of a large telescope by the schel stood embarked on a systematic series of experiments on casting, original Dollond partnership patent to be revoked on the grounds championed previously by the London o u t s i d e grinding and polishing, while carefully recording his proce- that the achromatic lens had been opticians in their dispute with Peter. dures and results. Part of his manuscript record of his polish- invented previously by someone else, ing experiments is in the exhibition. an amateur optician called Chester The exhibition contains two key doc- Moor Hall, and had been widely sold uments in the controversy: Peter Dol- Although the reflecting telescope was introduced by Newton before the patent application. The lond’s printed defence of his father because he believed that the refractor could not be improved, petition was not successful and Dol- and his draft response to Ramsden in the discovery of Newton’s mistake and the recovery of the lond continued with successful court manuscript. It was central to Peter’s refractor’s fortunes in astronomy did not spell the end of the actions against other makers to se- cause that his father had not simply reflector. Herschel had mapped out a programme for inves- cure his position. In a judgement in been a jobbing optician, but a natural tigating the universe at large in parallel with the instru- Dollond’s favour Lord Mansfield fa- philosopher: mental ambitions that would make it possible. The pros- mously declared that it was ‘not the pect was a compelling one outside the more narrow person who locked up his invention In the beginning of the year 1757 Mr agenda of professional astronomy and it survived in his scrutoire that ought to profit Dollond having tried the … 8th Expt. Herschel into the nineteenth century and beyond. by such an invention, but he who of the 2d part of the first book of Newton’s Opticks and by that means brought it forth for the benefit of the having discovered the new principle Although it was an ‘amateur’ vision in the eight- public.’ A quadrant of 8ft radius by John which was that the dissipation of eenth century, Herschel had imagined giant reflec- Bird, 1753, with a telescopic sight the different coloured rays was not tors looking deep into space and back in time, de- One maker stood outside the fray for in the same proportion to the mean tecting unknown galaxies, discovering the physical the time being: Jesse Ramsden mar- refraction, in water as in glass ... arrangement of the entire universe and tracing its ried Peter Dollond’s sister Sarah in evolution over time. It was a vision that seemed 1766 and it is thought that he ben- The key point was still Newton’s error deranged to some of his contemporaries and Her- efitted by his family connection to the in the Opticks. schel himself ‘fit for Bedlam’, but it closely resem- patent holder, but after his separation bles the programme of modern astronomy. from Sarah and the expiry of the pat- ent, Ramsden too attacked Dollond Hand-held Gregorian reflecting telescope by Benjamin Martin, London, c.1770 William Herschel’s largest telescope, of 40ft focal length and a mirror of 4ft diameter. Image courtesy of the Royal Astronomical Society