ADMISSION FREE BROAD SHEET communicates the work of the Museum of the History of Science, Oxford. It is posted on the Museum’s website, sold in the shop, and distributed to members of the mailing list, see www.mhs.ox.ac.uk. £1.00 Broad Sheet is produced by the richard rowley 2008 Museum of the History of Science, Oxford Broad Street, Oxford OX1 3AZ Tel +44(0)1865 277280 Fax +44(0)1865 277288 Web: www.mhs.ox.ac.uk Email: [email protected] The ENGLISH TELESCOPE from The ENGLISH TELESCOPE from NEWTON to HeRSCHEL NEWTON to HERSCHEL The Anniversary of the Telescope a SPECIAL 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 OCTOBER 2008 - 22nd MARCH 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.