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A Source Book in Medieval Science
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Biographical SummaryWitelo (fl. 1250–1275) A Pole born in Polish Silesia, Witelo (or Vitelo, Vitellio) was educated in Paris (ca. 1253), Padua (ca. 1262–1268), and Viterbo (1269). He was the author of at least two theological treatises, but his major work was the Perspectiva, a treatise on optics in ten books dedicated to William of Moerbeke, whom he had met at Viterbo. Like Moerbeke, Witelo was a Neoplatonist and expressed his neoplatonic views in the preface to the Perspectiva. The theoretical portions of the work were based largely upon Alhazen’s Optics. Witelo also included discussions on the psychological aspects of visior The Perspectiva was influential into the seventeenth century, when Kepler himself read it and used it a a point of departure in his Ad Vitellionem paralipomena quibus astronomiae pars optica traditur (Frankfurt, 1604).
Critical SummaryAlthough Robert Grosseteste greatly stimulated European interest in optics, his own investigations were hampered by the lack of many important sources. Ptolemy’s Optica was just becoming known in the West, and it is doubtful that Grosseteste was familiar with it; Alhazen’s Perspectiva, although translated late in the twelfth or early in the thirteenth century, had not come to Grosseteste’s attention by the time he composed his optical works. Consequently, Grosseteste’s knowledge of optics, though more complete than that possessed by any Western predecessor or contemporary, was primitive by comparison with the optical achievements of Islam; thus his works lack the scope and depth of such a work as Alhazen’s great optical treatise, as even the most casual inspection will reveal. However, by the second half of the century, the most advanced treatises of Greek antiquity and medieval Islam had been rendered into Latin. The most important of these for the development of optics were the Perspectiva of Alhazen, the Optica of Ptolemy, a number of works of Avicenna, and (toward the end of the period) the Catoptrica of Hero of Alexandria. These new riches, especially Alhazen’s Perspectiva, occasioned a dramatic surge in the study of optics, which found its best expression in the works of Roger Bacon, Witelo, and John Pecham. Thus, whether in Alhazen’s Perspectiva (which not only served as the principal source for Western writers but also circulated widely itself) or in the works of Bacon, Witelo, and Pecham which it inspired, the West possessed for the first time treatises in which almost the entire gamut of problems now classified as optical was systematically treated; moreover, in these treatises we find an enormous increase in the sophistication of mathematical techniques employed. Indeed, we find what must be considered a grand synthesis of optical knowledge, comprehensive in scope and incorporating the very best learning of Greece, Islam, and the Latin West. It speaks for the vigor and excellence of this synthesis, that the progress of optics throughout the remainder of the Middle Ages came about as scientists extended the application of principles set forth late in the thirteenth century or as questions raised in the thirteenth century, but incompletely or unsatisfactorily answered, became the objects of inquiry and debate. The selections appearing in this chapter are an attempt to convey the principal doctrines and techniques of this late thirteenth-century synthesis. They are drawn from the works of Roger Bacon, Witelo, and John Pecham, all of whom wrote on optics during the 1260’s and 1270’s. In addition, it has been deemed essential to include selections from the Latin text of Alhazen’s Perspectiva to illustrate further the optical knowledge available in Europe after about 1250 and the origin of many of the ideas expressed by Bacon, Witelo, and Pecham. Since Alhazen and his three thirteenthcentury followers were in such complete agreement on most fundamental issues, it has been unnecessary to present the view of each individual on every question; rather, those passages have been selected that most clearly and concisely present the shared theory. In cases of major disagreement, as on the question of visual rays, alternative views are presented. Finally, it should be noted that the magnitude of the thirteenth-century optical synthesis has made it both possible and necessary to omit all but two selections illustrating optical progress during the later Middle Ages: possible, because the thirteenth-century synthesis provided the framework within which later optics was pursued, so that the views presented here largely represent later medieval views as well; necessary, because late medieval discussions were of a highly particular sort impossible to treat adequately in a source book.
Late Thirteenth-Century Synthesis in Optics
Translations, introduction, and annotation by David C. Lindberg1
17. Witelo: The Debate about Visual Rays
PERSPECTIVA,
It is impossible for sight to be applied to the visible object by rays issuing from the eyes.75
If from the eyes should issue certain rays, by which the visual power is united with external objects, those rays are either corporeal or incorporeal. If corporeal, then when the eye sees stars and the sky, something corporeal issuing from the eye necessarily fills the entire space of the universe between the eye and the visible part of the sky, without diminution of the eye itself. But it is impossible that this should occur and also that it should occur so swiftly, the substance and size of the eye being preserved. If it should be said [instead] that the rays are incorporeal, then those rays do not perceive the visible object, since perception exists only in corporeal things. Therefore, the corporeal eye cannot perceive by the mediation of this insentient incorporeal ray. Nor do such incorporeal things return something to the eye, by which sight could perceive the visible object, since sight occurs only through contact between the eye and the visible form, because there is no action without contact. Therefore, if rays issuing from the eye return nothing to the eye, those rays do not produce sight. But if they return something to the eye, these are lights or colors, which appear by themselves [without the mediation of rays issuing from the eye] and which are multiplied to the eye among the [visual] rays. Therefore, [visual] rays do not cause sight to be applied to the things seen; but something else, which is multiplied to the eye, is by itself the cause of vision. It is therefore impossible for rays of themselves to cause vision, unless perhaps the lines drawn through the points of the forms multiplied from the surfaces of the visible object to the eye are called rays; for, as is evident by Theorem 2 of this book, in order that the object may actually appear, it must be possible to draw straight lines between any point on the surface of the visible object and a given point on the surface of sight. But such rays do not issue from the eyes.76 Therefore, the proposition is evident.
1. Except for section 18, which was translated by Robert B. Burke.
2. Set forth, that is, by Alhazen’s Perspectiva as well as by those thirteenth-century treatises based upon it. One could legitimately argue that this synthesis was actually Islamic and occurred in the eleventh century in the works of Alhazen and Avicenna. Nevertheless, their works did not become available to the West until the thirteenth century, whereupon they inspired further efforts by Bacon, Witelo, and Pecham; thus, so far as the optical knowledge of the West is concerned, the synthesis occurred in the late thirteenth century.
3. Unless otherwise noted, the selections have been translated or reprinted from the following editions: Alhazen, Perspectiva, from Opticae thesaurus Alhazeni Arabis libri septem, edited by Friedrich Risner(Basel, 1572); Witelo, Perspectiva, bound with Alhazen’s Perspectiva in the Opticae thesaurus, but separately paginated; Roger Bacon, The Opus Majus of Roger Bacon, edited by J. H, Bridges (London, 1900); Roger Bacon, De multiplicatione specierum, included in Volume II of Bridges’ edition of the Opus maius. The propositions from John Pecham’s Perspectiva communis (both revised and unrevised versions) are reprinted by permission of the copyright owners, the Regents of the University of Wisconsin, from John Pecham and the Science of Optics: Perspectiva communis edited with an introdution, English translation, and critical notes by David C, Lindberg (Madison, Wis.: University of Wisconsin Press, 1970). The order in which the propositions appear in this source book and their page numbers in my Pecham volume are as follows: Part I, Proposition 27, revised version (p. 109); Propositions 29–34 (pp. 111– 119); Propositions 43, 28, 37, 33, 38 (pp. 127, 109–110, 121, 119, 121–123); Propositions 44–46 (pp. 127–131); Part II, Propositions 6, 20, 30 (pp.161–163, 171–173, 183–185); Part III, Proposition 4 (p.215); Proposition 16 (pp.229–231). Henceforth abbreviated citations will be used.
4. On the history of late medieval optics, see Graziella Federici Vescovini, Studi sulla prospettiva medievale (Turin: Giappichelli, 1965).
74. Risner, p. 87.
75. In the thirteenth century only Witelo followed Alhazen in denying the existence of visual rays.
76. Witelo here distinguishes between rays actually issuing from the observer’s eye and imaginary lines that can be drawn between points in the eye and points on the visible object. This echoes Alhazen’s similar distinction; see note 67.
Contents:
Chicago: Witelo, "Late Thirteenth-Century Synthesis in Optics," A Source Book in Medieval Science in A Source Book in Medieval Science, ed. Edward Grant (Cambridge: Harvard University Press, 1974), 407. Original Sources, accessed March 30, 2023, http://www.originalsources.com/Document.aspx?DocID=LJTZ1A2GVIM7KDB.
MLA: Witelo. "Late Thirteenth-Century Synthesis in Optics." A Source Book in Medieval Science, in A Source Book in Medieval Science, edited by Edward Grant, Cambridge, Harvard University Press, 1974, page 407. Original Sources. 30 Mar. 2023. http://www.originalsources.com/Document.aspx?DocID=LJTZ1A2GVIM7KDB.
Harvard: Witelo, 'Late Thirteenth-Century Synthesis in Optics' in A Source Book in Medieval Science. cited in 1974, A Source Book in Medieval Science, ed. , Harvard University Press, Cambridge, pp.407. Original Sources, retrieved 30 March 2023, from http://www.originalsources.com/Document.aspx?DocID=LJTZ1A2GVIM7KDB.
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