A Source Book in Medieval Science

Author: Witelo

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Late Thirteenth-Century Synthesis in Optics

Translations, introduction, and annotation by David C. Lindberg1

17. Witelo: The Debate about Visual Rays


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.


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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.