An investigation of the eighteenth-century achromatic telescope

Summary

The optical quality and properties of over 200 telescopes residing in museums and private collections have been measured and tested with the goal of obtaining new information about the early development of the achromatic lens (1757–1770). Quantitative measurements of the chromatic and spherical aberration of telescope objective lenses were made and are discussed within the context of John and Peter Dollond's description of their efforts to overcome these two optical defects inherent in any single lens. Their work was chronicled in the Philosophical Transactions of 1758 and 1765. An important finding of this investigation is that a particular form of the achromatic lens that has been suggested as being a rare and early form by some investigators, and presumably introduced by Chester Moor Hall, was in fact found to be numerous and present in most museums and many private collections. It is shown that the colour correction of an early eighteenth-century doublet telescope objective is very sensitive to the ratio of the flint to crown glass focal lengths, as well as the variations of the indices of refraction with colour. This sensitivity would preclude any simple recipe for constructing an achromatic lens. The third order theory of spherical aberration of Nevil Maskelyne is reexamined, in which he pointed out that John Dollond developed a theoretical formula equivalent to what today is called geometric third order spherical aberration theory.

14. Acknowledgements

I first and foremost acknowledge Dr. G.L'E Turner for his initial encouragement, when visiting Oxford in 1982. I thank him for his support during this investigation, as well as for his solid advice and important insights into the directions to pursue. His many works on collections of instruments and what one can learn from them have been an inspiration.

Secondly, I would like to acknowledge the continued interest and unflagging support of my colleague, Professor M.E. Rudd. He was the perfect sounding board for many ideas that appear in this article. Without his interaction many of the ideas and suggestions would not gotten beyond the ‘what if’ stage. Without his effort in digitizing approximately 200 35-mm slides, this manuscript would not have come to fruition. He also did a superb job of proofreading this manuscript.

My wife Janet has been unselfish of her time in reading this manuscript, correcting my many blunders along the way, and for that I thank her.

I wish to also thank Mr. Peter Louwmann for his hospitality while studying his marvelous telescope collection. I thank also Mr. Stuart Talbot for introducing me to many telescopes that eventually became part of my collection. Many of these telescopes were used to test out different measurement procedures.

Lisa Harlow helped with the majority of the data reduction and the graphics of the colour curves and without her help this project might not have come to fruition. I thank Professor Glenn Jaecks with his timely and critical computer analysis that shed light on the colour curve behavior.

To be thanked also are the many museum assistants who brought arms full of telescopes to measure and who provided me with a space to carry out the measurements. The museums visited were:

1. The Museum of the History of Science, Oxford

2. The Science Museum, London

3. The Whipple Museum, Cambridge

4. The Royal Museum of Scotland

5. The Deutches Museum, Munich

6. The Museum Boerhave, Leiden

7. Teyler Museum, Haarlam

8. The Science Museum, Utrecht

The National Science Foundation provided funds to travel to and throughout the UK as well as to Germany and The Netherlands.

Notes

¹W.H.S. Chance, ‘The Optical Glasswerks at Benediktbeuren’, The Proceedings of the Physical Society, 49, part 5 (1937), 432–443.

²John Dollond, ‘An Account of Some Experiments concerning the Different Refrangibility of Light’ With a Letter form James Short, M.A.FRS. Philosophical Transactions, 50 (1757–1758), 733–743.

³L. Euler, ‘Sur la perfection des verres objectifs de lunettes’, Histoire de l'Academie Royal des Sciences at Belles-Lettres [de Berlin] avec les memoires, 1741, Vol. 3 p. M279.

⁴Henry C. King, The History of the Telescope (Sky Publishing Corporation, Cambridge, MA, 1955), Hall reference, pp. 144–146, 154–155.

⁵John A.Church, ‘Astronomical Computing: Designing an Aplanatic Doublet’, Sky and Telescope, November (1984), 257–261.

⁶Humphrey Lloyd, A Treatise on Light and Vision (London, Printed for Longman, Rees, Orme, Brown, and Green, 1831).

⁷King op. cit., pp.158–159.

⁸Peter Dollond, Philosophical Transactions of the Royal Society, 55 (1765), 54–56; ‘An Account of an Improvement made by Peter Dollond in his New Telescopes: In a Letter to James Short M.A.F.R.S. with a Letter of Mr. Short's to the Rev. Thomas Birch, D.D. Secet. R.S.

⁹J. Dollond op. cit.

¹⁰M. Eugene Rudd, ‘Chromatic Aberration of Eyepieces in Early Telescopes’, Annals of Science, 64(1) (January 2007), 1–18.

¹¹Thomas H. Court and Moritz von Rohr, ‘A History of the Develeopment of the Telescope from about 1675 to 1850 based on Documents in the Court Collection’, Transactions oft the Optical Society, XXX(5) (1928–1929), 277–294.

¹²H. Boegehold, ‘Zur Vor-und Fruhgeschichte der achromatischen Fernrohrobjecktive’, Forschugen Zur Geschichte der Optik, pp. 81–114, August 1948.

¹³Duane H. Jaecks, ‘Developments in 18^th Century Optics and Early Instrumentation’, in The History and Preservation of Chemical Instrumentation, edited by J.T. Stock and M.V. Orna (D. Reidel Publishing Company, 1986), pp. 51–65.

¹⁴. R. Willach, ‘New Light on the Invention of the Achromatic Telescope Objective’, Notes and Records of the Royal Society London, 50(2) (1996), 195–210.

¹⁵G. L'E. Turner, Physis, Rivista internationale di storia della scienza, 8, part I (1966), 99–128. ‘Decorative Tooling on 17^th and 18^th Century Microscopes Telescopes’.

¹⁶G. L'E. Turner, Physis, Rivista internationale di storia della scienza, 8, part I (1966), 99–128. ‘Decorative Tooling on 17^th and 18^th Century Microscopes Telescopes’.

¹⁷G. L'E. Turner, Notes and Records Royal Society London, 24(1) (1969), 91–108. James Short, F.R.S. and His Contribution to the Construction of Reflecting Telescopes.

¹⁸Yuri Petrunin, private communication, Telescope Engineering Company, Golden Colorado.

¹⁹John Dollond op. cit.

²⁰ Encyclopedia Britannica, third edition 1797, The section on the ‘telescope’.

²¹ Rees's Encyclopedia, Volume under ‘Telescope.

²²Encyclopedia Britannica op. cit.

²³Rees's Encyclopedia op. cit.

²⁴Encyclopedia Britannica op. cit.

²⁵H.G. Conrady, A.R.C.S. Transaction of the Optical Society, 25 (1924), 219–226. ‘Study of the Significance of the Foucault Knife-Edge Test When Applied to Refracting Systems’.

²⁶John Dollond, Philosophical Transactions of the Royal Society, Vol. 48, p. 103, ‘A Letter from Mr. John Dollond to Mr. James Short, F.R. S. concerning an Improvement of refracting Telescopes’. Read March 1, 1753.

²⁷Mary Margaret Robischon, ‘Scientific Instrument Makers in London During the Seventeenth and Eighteenth Century’, A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (History) in the University of Michigan, 1983. Pyefinch reference: p. 298.

²⁸King op. cit., p. 158.

²⁹Robischon op. cit.

³⁰Robischon op. cit.

³¹Gloria Clifton, Directory of British Scientific Instrument Makers 1550–1851 (Phillip Wilson Publishers Ltd. London, 1995), p. 87.

³²Clifton op. cit., p. 112.

³³Louis Bell, The Telescope, first edition *McGraw Hill Book Company, Inc, New York, 1922), p. 90.

³⁴Lloyd op. cit., p. 350.

³⁵Potter, Richard Potter, An Elementary Treatise on Optics, part 2 (London, Taylor, Walton and Maberly, 1851), pp. 80–81.

³⁶George Airy, ‘On the Principles and Construction of the Achromatic Eye-Piece of Telescopes, and on the Achromatism of Microscopes’, Transactions of the Cambridge Philosophical Society, 2 (1827).

³⁷Benjamin Martin, Philosophia Britannica, 3(Appendix II) (London, 1787), p. 391.

³⁸Willach op. cit.

³⁹Robischon op. cit.

⁴⁰Willach op. cit.

⁴¹William Ellison, Amateur Telescope Making Book 1, edited by Albert Ingalls (Scientific American, Inc. 1948), p. 114. This material was originally written by Ellison for a series of articles published in the English Mechanic, March–October 1918.

⁴²Clifton op. cit., p. 196.

⁴³John Dollond op. cit..

⁴⁴Nevil, Maskelyne, ‘A Theorem of the Aberration of the Rays of Light refracted through a Lens, on account of the Imperfection of the Spherical Figure’, Philosophical Transactions of the Royal Society, LII (Part I) (1761), 17–21.

⁴⁵Potter op. cit..

⁴⁶George S. Monk, Light, Principles and Experiments, second edition (Dover Publications, 1963), pp. 48–50, 421–423.

⁴⁷Peter Barlow, ‘Rules and Principles for Determining the Dispersive Ratio of Glass; and for Computing the Radii of Curvature for Achromatic Object-Glasses, Submitted to the Test of Experiment’, Philosophical Transactions (1827), 231.

⁴⁸Benjamin Martin, New Elements of Optics (London. 1759).

⁴⁹Rees's Encyclopedia op. cit.

⁵⁰Peter Dollond op. cit.

⁵¹Willach op. cit.

⁵²Gunnar Pipping, The Chamber of Physics, Instruments in the History of Sciences Collections of the Royal Swedish Academy of Sciences, Stockholm (Almqvist and Wiksell, Uppsala, 1977).

⁵³Harry Woolf, The William and Mary Quarterly, 3 ^rd Ser., 13(4) (Oct. 1956), 499–518. British Preparations for Observing the Transit of Venus of 1761.

⁵⁴Willach op. cit.

⁵⁵Edmond Halley, ‘An Instance of the Excellence of the Modern Algebra’, Philosophical Transactions, 17 (1693), 960–969.