Is the phenomenon of magnification by a converging lens inconsistent and therefore unreliable? Can a lens magnify one part of an object but not another? Physics teachers and even students familiar with basic optics1 would answer “no,” yet many answer “yes.” Numerous telescope users believe that magnification is not a reliable phenomenon in that it does not work for stars. This belief was central to the arguments of one of science's most prominent modern critics—a great story of how misunderstanding basic optics helped to yield bad ideas about science. So magnification is a great topic! It is accessible to students. It gives students insight into the workings of a familiar device such as a telescope that even frequent telescope users often lack. And it has a fascinating side story about how misunderstanding basic science led to interesting consequences.

Topics
Lenses, Telescopes, Education, Teaching
1.
The optics principles and methods used in this paper are common to many physics texts. See, for instance, Rayond A. Serway's Principles of Physics (Saunders College Publishing, Forth Worth, 1994), Chaps. 26 and 28.
2.
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3.
Cox and DeWeerd discuss this use of a translucent screen in their article.
4.
This can be found in a variety of sources spanning a great deal of time. Examples: Gary Seronik, “Understanding Binocular Exit Pupils” (04/18/2009), http://www.garyseronik.com/?q=node/13 (Seronik is an associate editor of Sky & Telescope magazine); Mike Weasner, “OK, So What Can I Do with My New Small Telescope?” (3/26/2006), www.meade4m.com/articles/archive/4M_Weasner_1143408274.html (Meade is one of the most prominent manufacturers of small telescopes); Michael Covington, Astrophotography for the Amateur, 2nd ed. (Cambridge University Press, 1999), p. 170; and John Davis, Elements of Astronomy (J. B. Lippincott & Co., Philadelphia, 1868), p. 164.
5.
Galileo Galiei and Johann Kepler, The Sidereal Messenger of Galileo Galilei, and a part of the preface to Kepler's Dioptrics, translation with notes by E.S. Carlos (Rivingtons, London, 1880), pp. 38 and 40. Galileo's telescope actually used a diverging lens for the eyepiece, placed ahead of where the real image formed, to create an enlarged erect virtual image.
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7.
These “images” are not the stars themselves but the diffraction patterns.
8.
Aperture roughly an inch.
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10.
seconds of arc (1/3600 of a degree).
11.
Galileo Galilei, “Reply to Ingoli,” in M. Finocchiaro, The Galileo Affair — A Documentary History (University of California Press, Los Angeles, 1989), pp. 180 and 174.
12.
A star of first magnitude star is bright; one of sixth magnitude is the faintest visible to an average unaided eye under dark skies.
13.
Galileo Galilei, Dialogue Concerning the Two Chief World Systems — Ptolemaic and Copernican, translated by S. Drake with foreword by Albert Einstein, 2nd ed. (University of California Press, Los Angeles, 1967), p. 359.
14.
The sizes Galileo gave are consistent with the expected diameters of visible central maxima of diffraction patterns formed by the telescopes he used.
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John Preston, Gonzalo Munévar, and David Lamb, The Worst Enemy of Science? Essays in Memory of Paul Feyerabend (Oxford University Press, New York, 2000).
16.
“Papal visit scuppered by scholars,” BBC News (Jan. 15, 2008); http://news.bbc.co.uk/2/hi/europe/7188860.stm.
17.
Paul Feyerabend, Against Method, 3rd ed. (Verso, New York, 1993, reprinted 2001). See pp. 86–105, 125, and 92.
18.
Or had another philosopher possessed sufficient understanding so as to correct him.
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