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Alhazen's Theory of Vision and Its Reception in the West Author(s): David C. Lindberg Source: Isis, Vol. 58, No. 3 (Autumn, 1967), pp. 321-341 Published by: The University of Chicago Press on behalf of The History of Science Society Stable URL: http://www.jstor.org/stable/227990 . Accessed: 21/09/2011 19:13 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press and The History of Science Society are collaborating with JSTOR to digitize, preserve and extend access to Isis. http://www.jstor.org
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Page 1: Alhazen

Alhazen's Theory of Vision and Its Reception in the WestAuthor(s): David C. LindbergSource: Isis, Vol. 58, No. 3 (Autumn, 1967), pp. 321-341Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/227990 .Accessed: 21/09/2011 19:13

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

The University of Chicago Press and The History of Science Society are collaborating with JSTOR to digitize,preserve and extend access to Isis.

http://www.jstor.org

Page 2: Alhazen

Alhazen's Theory of Vision

and Its Reception in the West

By David C. Lindberg*

I

HE MOST SERIOUS problem facing the Muslim heirs of Greek thought was the extraordinary diversity of their inheritance. Among theories of

optics, for instance, Muslim thinkers had the following choice: the emission theory of sight of Euclid and Ptolemy, which postulated visual rays emanat-

ing from the observer's eye; the older Epicurean intromission theory, which reversed the rays and made them corporeal; the combined emission-intromis- sion theories of Plato and Galen; and some enigmatic statements of Aristotle about light as qualitative change in a medium.1 These Greek theories gener- ated a wide assortment of optical theories in Islam, two of which came to dominate. Hunain ibn Ishaq (d. 877), the most prolific translator of scientific works into Arabic, argued for a combined emission-intromission theory in the tradition of Plato and Galen.2 Al-Kindi (d. c. 873) agreed with Hunain that rays are emitted by both the visible object and the eye, although he couched his theory in terms of a general emanation of power having Stoic and Neoplatonic origins and appropriated the geometrical approach to optics appearing in the works of Euclid and Ptolemy.3 Avicenna (Ibn Sina, d. 1037) took exception to the views of Hunain and al-Kindi, denying that visual rays are of any use in explaining the process of sight and insisting on a complete intromission theory.4 These and other Muslim philosophers made important

* University of Wisconsin. A short version of this paper was presented before a joint meet- ing of the American Association for the Ad- vancement of Science and the History of Science Society, December 1966. The paper is a product of research supported by the National Science Foundation.

1 Obviously this attempt at classification ob- scures a host of distinctions. On Greek optics see Arthur Erich Haas, "Antike Lichttheorien," Archiv fiir Geschichte der Philosophie, 1907, 20:345-386; J. Hirschberg, "Die Optik der alten Griechen," Zeitschrift fiir Psychologie und Physiologie der Sinnesorgane, 1898, 16:321-351; Albert Lejeune, Euclide et Ptolemee (Louvain: Bibliotheque de l'Universite, 1948); Lejeune,

Recherches sur la catoptrique grecque (Brus- sels: Palais de Academies, 1957).

2 The Book of the Ten Treatises of the Eye, ascribed to Hunain ibn Ishdq (809-877 A.D.), trans. Max Meyerhof (Cairo: Government Press, 1928), pp. 31-39.

3 Graziella Federici Vescovini, Studi sulla prospettiva medievale (Turin: G. Giappichelli, 1965), Ch. 3; Alkindi, De aspectibus, in A. A. Bjornbo and Sebastian Vogl, "Alkindi, Tideus und Pseudo-Euklid, Drei optische Werke," Abhandlung zur Geschichte der mathemati- schen Wissenschaften, 1912, 26, Pt. 3.

4 Eilhard Wiedemann, "Ibn Sina's An- schauung vom Sehvorgang," Archiv fiir die Ge-

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contributions through their criticisms of Greek theories and their syntheses of disparate elements of Greek thought; moreover, their influence on Western optical thought was far from negligible. Yet, none of them created an in- clusive optical system to rival that of Ptolemy; they dealt with but one or another aspect of sight, usually in the space of a few paragraphs or a few pages.

The first comprehensive and systematic alternative to Greek optical theo- ries was formulated by Alhazen (Ibn al-Haitham, d. c. 1039), a figure of im- mense importance in the history of optics. Alhazen leveled a devastating attack at prevailing optical theories and formulated a grand and viable alter- native. Moreover, he had a profound influence on the West: his principal work on optics (Kitab al-manazir, cited by Western authors as De aspectibus or Perspectiva) 5 was translated into Latin late in the twelfth or early in the thirteenth century and dominated Western optical thought until early in the seventeenth century. Modern optical thought issues, by direct descent, from the work of Alhazen and his immediate followers.

The central feature of Alhazen's system is its theory of direct vision, and with this topic the Perspectiva opens.6 Alhazen notes, first, the effect of bright lights on the eye. "We find," he says, "that when the eye looks into exceed- ingly bright lights, it suffers greatly because of them and is injured. For when an observer looks at the body of the sun, he cannot see it well, since his eye suffers pain because of the light." 7 Clearly this implies an action of bright bodies on the eye, for injury is something inflicted by an agent on a recipient and could not, in the case of the eye, result from emission of the eye's own ray. The phenomenon of the afterimage supports the same position:

schichte der Naturwissenschaften und der Technik, 1913, 4:239-241. This is a German translation of a short work on physics by Avi- cenna. Avicenna expresses similar views in many other works.

5 The only printed edition of this work is Opticae thesaurus Alhazeni Arabis libri septem, nunc primum editi a Federico Risnero (Basel, 1572); the title Opticae thesaurus was given to the book by Risner. Manuscripts of the Arabic text have recently been discovered (see Max Krause, "Stambuler Handschriften islamischer Mathematiker," Quellen und Studien zur Geschichte der Mathematik, Astronomie und Physik, 1936, Abt. B, 3:476, which lists five lo- cated in Istanbul); an edition is being pre- pared by A. I. Sabra. My study has been limited to the Latin text; but it is abundantly clear- from a comparison of English translations of the same section made from both Latin and Arabic texts (Stephen L. Polyak, The Retina [Chicago: Univ. Chicago Press, 1941], pp. 109- 111), from the recent study by Matthias Schramm (Ibn al-Haythams Weg zur Physik [Wiesbaden: Franz Steiner, 1963]), and from a comparison of the Opticae thesaurus with Al- hazen's shorter tracts on optics translated from Arabic by Wiedemann, Baarman, Winter, and others-that the Latin text faithfully repro-

duces the substance of Alhazen's ideas. If it can be shown that there are significant differ- ences between the Arabic and Latin texts, then I must be content with elucidating the Latin tradition, which was influential in the West. I have repeatedly checked the Risner text against earlier Latin manuscripts (British Museum Royal MS 12.G.VII and Bruges MS 512) and find no differences in substance.

The best secondary works on Alhazen's op- tics are Vasco Ronchi, Histoire de la lumiere, trans. J. Taton (Paris: Librairie Armand Colin, 1956), a translation of his Storia della luce (Bo- logna: N. Zanichelli, 1952); H. J. J. Winter, "The Optical Researches of Ibn al-Haitham," Centaurus, 1954, 3:190-210; Vescovini, Studi, Ch. 7; Schramm, Weg zur Physik; and Leopold Schnaase, Die Optik Alhazens (Stargard: A. Muller, 1889). There is also a two-volume study by a contemporary Egyptian physicist, Mustafa Nazif, Ibn al-Haitham: His Optical Researches and Discoveries (Cairo: Nuri Press, 1942-1943).

6 I.e., the Latin text, which seems to lack the first few brief chapters of the Arabic text. Cf. Eilhard Wiedemann, "Zu Ibn al-Haitams Op- tik," Arch. Gesch. Naturw. Tech., 1910, 3:4.

7 Opticae thesaurus, I, Sec. 1. p. 1. All trans- lations are rendered from this edition. Space has not permitted inclusion of the Latin text.

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. . . when an observer looks at a bright fire and allows it to linger in his vision for a long time, if he then transfers his gaze to a weakly illuminated place, he will [continue to] see the same thing [i.e., the brightness]. . . . Fi-

nally this fades away and vision returns to its normal disposition.8

After overwhelming his reader with this and similar data, Alhazen concludes, "All these things indicate that light produces some effect in the eye."9

So far Alhazen has confined his argument to light from what we would call self-luminous objects. Such luminous rays, filtering through mist or dust, had been recognized since antiquity,10 but Alhazen has more in mind than these. He argues that every visible object is seen by the emission of its own

light, though illumination by a self-luminous body is a normal prerequisite: It has been demonstrated above that light issues in all directions opposite any body that is illuminated with any light. Therefore when the eye is oppo- site a visible object and the object is illuminated with light of any sort, light comes to the surface of the eye from the light of the visible object."

However, an observer perceives the color of the visible object as well as its

light. This is a similar process and always accompanies the perception of light:

It has been shown already that the form of color of any colored body, illumi- nated by any light whatsoever, always accompanies the light emanating from that body to any region opposite the body. . . . Therefore the form of the color of a visible body always accompanies the light coming to the eye from the light of the body. And since light and color come to the surface of the eye simultaneously, the eye perceives the color of the visible object on account of the light coming to it from the object. It is proper, therefore, that the eye should not perceive the color of the visible object except through the form of color accompanying the light to the eye; and the form of color is always mixed with the form of light . . .12

Light and color, the first twenty-two visible intentions identified by Al- hazen, are perceived by sense alone without the support of any process of ratiocination. The remaining twenty visible intentions-including such things as remoteness, position, shape, magnitude, motion, rest, and beauty - are perceived visually, but only by processes of recognition, distinction, and argumentation performed by the virtus distinctiva. Light and color re- main the primary visible intentions, and the others are perceived through their mediation.13

Six conditions must be fulfilled if the forms of light and color (issuing

8 Ibid. nately; consequently, I have made no attempt 9 Ibid. to distinguish between them in my English

translation. In the passage quoted, Alhazen lOE.g., Galen, De usu partium, X, 12, in speaks of light, but elsewhere (e.g., p. 14) he

(Euvres anatomiques, physiologiques et medi- speaks of the form of light cales de Galien, trans. Charles Daremberg, Vol. 12 Ibid., p. 7. I (Paris: J.-B. Baillire, 1854), p. 639. 13 Ibid., II, Ch. 2, pp. 34 if. (chapter desig-

11 Opt. thes., I, Sec. 14, p. 7. Here and else- nations go back to the Arabic original; see where, the Latin text of Alhazen's treatise em- n. 23 below). On the meaning of "intention" ploys the terms lux and lumen indiscrimi- see Vescovini, Studi, pp. 64-69, 80-85.

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from an object in all directions) are to enter the eye of the observer and

bring about perception: 14 (1) There must be a certain distance between the

eye and the visible object. (2) The visible object must be directly in front of the eye, that is, within the visual field. (3) The object must be either self- luminous or illuminated by light from another body. (4) The visible object must possess magnitude; that is, lines drawn from the extremities of the visible object to the center of the eye must intercept, on the surface of the glacial humor (crystalline lens), "a segment of sensible magnitude by comparison with the whole surface of the glacial humor."15 (5) The medium or media between the object and the observer must be transparent. (6) The

object must be dense and solid. This final requirement is instructive regard- ing the nature of light. In the first place, the object must be dense and solid because only dense and solid bodies have color and part of the act of vision is perception of the color of the body. Secondly, if the body were not dense and solid (i.e., if it were transparent), the light by which it is illuminated would pass through without opposition. In this event, there would be no light in the surface of the body capable of emanating its form to the observer:

When there is a transparent body opposite the eye and it is illuminated by light from the direction of the observer, the light passes through it and is not fixed in its surface; and thus in the surface of the body opposite the eye there will be no light from which a form can come to the eye.16

Evidently the light (or the form of light) issuing from a nonluminous body is not its own but has been deposited there by an illuminating body.

The foregoing discussion makes it clear (1) that the forms of light and color issue in all directions from self-luminous or illuminated bodies through transparent media and (2) that the forms of light and color make an impres- sion on the eye. But it might still be argued that the forms of light and color do not issue from the visible object unless triggered by rays emanating from the observer's eye; that is, one could still claim that visual rays play a neces- sary role in vision. In order to demonstrate the futility of this hypothesis of visual rays, Alhazen undertakes a long and tightly knit argument. Let us suppose, he says, "that rays issue from the eye and pass through the trans- parent body [between the eye and the object] to the object of sight and that perception occurs by means of those rays." 17 Either these rays take something from the object and return it to the eye, or they do not. If they do not, the eye cannot perceive the object by means of them. But this is counter to the original supposition that rays issue from the eye to perceive the object. Con- sequently, it must be concluded that the rays do transmit something from the object to the eye:

14 Opt. thes., I, Ch. 7, pp. 22-23. World, Alhazen embraced the emission theory, 15 Ibid., I, Sec. 40, p. 23. but I am making no attempt in this article to 16 Ibid., Sec. 42, p. 23. examine the development of Alhazen's theories; 17 Ibid., Sec. 23, p. 14. Here Alhazen assumes cf. Eilhard Wiedemann, "Zur Geschichte der

that which he proposes to disprove. In his Lehre vom Sehen," Annalen der Physik und earlier treatise, On the Configuration of the Chemie, 1890, neue Folge, 39:473.

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Therefore [according to the emission view] those [visual] rays that perceive the visible object transmit something to the eye, by means of which the eye perceives the object. And since the rays transmit something to the eye, by means of which the eye perceives the object, the eye perceives the light and color in the visible object by no other means than through something coming to the eye from the light and color in the object. .. .18

Thus, even in the emission theory, sight is ultimately achieved by communi- cation of something from the object to the eye. Since it has already been demonstrated that the forms of light and color emanate in all directions from the visible object without the hypothesis of visual rays, of what advantage are the visual rays? As Alhazen expresses this impressive argument,

. . . sight occurs only as something of the visible object comes from the ob- ject [to the eye], whether or not rays issue from the eye. Now it has already been declared that sight is achieved only if the body intermediate between the eye and the visible object is transparent, and it is not achieved if the medium is opaque. . . . Since . . . the forms of the light and color in the visible object reach the eye (if they were [originally] opposite the eye), that which comes from the visible object to the eye (through which the eye per- ceives the light and color in the visible object no matter what the situation [with respect to visual rays]) is merely that form, whether or not rays issue [from the eye]. Furthermore, it has been shown that the forms of light and color are always generated in air and in all transport bodies and are always extended to the opposite regions, whether or not the eye is present. There- fore the egress of rays [from the eye] is superfluous and useless.19

Not yet satisfied that he has disposed of the theory of visual rays, Alhazen launches a further attack. If it is assumed, once again, that sight is due to something issuing from the eye, either that thing is body or it is not. If it is body, it follows that when one looks at the vault of the heavens, body flows from the eye to fill the entire space between the heavens and the eye, yet without destroying or diminishing the eye in any way. Since that is obviously impossible, that which flows from the eye is not body. But if that which issues from the eye is not body, it cannot perceive the the object, since "there is no perception except in bodies."20 Thus, by means of a reductio ad absurdum, Alhazen has demonstrated that a ray issuing from the eye cannot be respon- sible for sight; but he has not demonstrated that no ray issues from the eye. However, he concludes that if the rays are not responsible for sight, they are not sensible; therefore they are conjectural, "and nothing ought to be believed except through reason or by sight."21

Although it appears that Alhazen has completely discredited the theory of visual rays, the obscurity of the text has led to recent confusion on this

18 Opt. thes., I, Sec. 23, p. 14. require that the ray issuing from the eye be 19 Ibid. Note Alhazen's appeal to the prin- material.

ciple of economy. 21 ". . . et nihil debet putari nisi per ra- 20 Ibid. This also excludes the possibility, tionem vel a visu" (ibid.). The last three words

Alhazen thinks, that something issuing from in this Latin text are not found in the Risner the eye could take something from the visible edition but are included in British Museum object and return it to the eye; that too would Royal MS 12.G.VII, fol. 7v.

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point.22 Friedrich Risner, editor of the only printed edition of Alhazen's

optics (1572), divided the work into sections and gave each a title. Section 24 of Book I he entitled: "Vision seems to occur through avvavyetav, that is, rays simultaneously received and emitted." 23 Alhazen's text immediately beneath this title begins as follows:

It has been asserted on account of this [i.e., the argument of the previous section] that both schools of thought [presumably emission and intromission] speak the truth and that both beliefs are correct and consistent; but one does not suffice without the other, and there can be no sight except through that which is maintained by both schools of thought.24

A number of historians, deceived by Risner's title, have concluded that in the opening lines of Section 24 Alhazen backs down on his denial of the existence of visual rays.25 But, in fact, Alhazen never admits the real existence of the rays. He is willing only to allow mathematicians, who are concerned with a mathematical account of the phenomena rather than with the real nature of things, to use visual rays to represent the geometrical properties of sight. Indeed, these rays or lines are indispensable if one is to understand how sight occurs, for through them one is able to visualize "the nature of the arrangement according to which the eye is affected by the form [of light or color]."26 But, according to Alhazen, all mathematicians who postulate visual rays "use only imaginary lines in their demonstrations, and they call them 'radial lines.' "27 Moreover, the belief "of those who consider radial

22 There was no confusion in medieval Europe. Roger Bacon (Opus majus, V, 1, Dist. 7, Ch. 3) and John Pecham (Perspectiva commu- nis, I, Props. 44-46) were fully aware of Al- hazen's uncompromising opposition to visual rays as agents that go out and seize something from the object and convey it back to the eye.

23 "Visio videtur fieri per avvav,yetav, id est receptos simul et emissos radios" (Opt. thes., I, Sec. 24, p. 15). Aetius Placita, IV, 13, 11 (Her- mann Diels, Doxographi Graeci [3rd ed., Berlin: W. de Gruyter, 1958], p. 404) uses the term

-vva6vyeta (simultaneous radiation) to describe Plato's double-emission theory of sight (Plato does not use the term himself; see Haas, "Licht- theorien," p. 393, n. 100), and it is clear that Risner is comparing Alhazen's theory to just such a double-emission theory.

It should be noted that Risner added the sections, but the division of chapters goes back to the Arabic original; cf. Risner's edition with the summary of chapters in the Arabic text contained in Lutfi M. Sa'di, "Ibn-al-Haitham (Alhazen), Medieval Scientist," University of Michigan Medical Bulletin, 1956, 22, No. 6:258- 259.

24 "Et declaratum est ex hoc, quod duae sec- tae dicant verum: et quod duae opiniones sint rectae et convenientes: sed non completur al- tera earum, nisi per alteram, neque potest esse

visio, nisi per illud, quod aggregatur ex duabus sectis" (Opt. thes., I, Sec. 24, p. 15). The open- ing phrase is somewhat ambiguous, and the context is of little help. The passage could

equally well be translated, "It has been demon- strated on account of this . . ," which alters the meaning subtly but significantly. The Arabic text is not relevant at this point, since I am concerned with the misleading character of Risner's title and the Latin text immedi- ately following it. However, either translation is capable of being interpreted as an admission by Alhazen that visual rays exist.

25 For example, Ronchi states that Alhazen, "apres avoir affirm6 nettement que la vision ne se fait pas au moyen de rayons emis par l'eil, en vient a une sorte de compromis et avance que la vision semble se faire concur- remment par des rayons recus et par des rayons 6mis" (Histoire, pp. 39-40). Following Ronchi and Risner, I made the same mistake in my article "The Perspectiva communis of John Pecham: Its Influence, Sources, and Content," Archives internationales d'histoire des sciences, 1965, 18:47-48.

26 Opt. thes., I, Sec. 24, p. 15. 27 Ibid., Sec. 23, p. 15. Cf. Galen, De usu

partium, X, 12, in (Euvres, trans. Daremberg, Vol. I, p. 639.

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lines to be imaginary is true, and the belief of those who suppose that some- thing [actually] issues from the eye is false."28 Thus visual rays (or radial

lines) are mere geometrical constructions, useful in demonstrating the proper- ties of sight. They can serve as a mathematical hypothesis, but they have no

physical existence. However, if these rays are imaginary, why imagine them to issue from the eye rather than from the visible object? This is probably a concession, first, to traditional geometrical optics (e.g., the work of Euclid and Ptolemy), which had been combined with belief in visual rays; and sec- ond, to the natural intelligibility of a center of perspective from which rays emanate to perceive visible things.

An intromission theory of vision brings new urgency to the determination of which ocular organ is the sensitive one. Indeed, the question changes from "Which organ is the source of the rays?" to "Which organ receives the rays?"; and this change in question widens the scope of permissible answers. In the visual ray theory, there was little alternative to placing the source of rays at the center of the eye so that the rays would be unrefracted as they emerged and, consequently, capable of accurately determining the location of objects in space.29 The intromission theory, however, opens the question to further

investigation. Nevertheless, Alhazen's conclusion was substantially the same as that of antiquity. Islamic prohibitions against dissection left Muslim in-

vestigators with little choice but to rely on Greek descriptions of the eye and

pronouncements regarding the sensitive organ. Alhazen's description of the

eye varies only in minor details from the descriptions of Galen and Rufus of

Ephesus, and Alhazen even admits that it is drawn from earlier anatomical treatises.30 He identifies four tunics (consolidativa, uvea, cornea, and aranea) and three humors (aqueous humor, vitreous humor, and crystalline lens),31 and follows Galen closely in arguing that the glacial humor (crystalline lens) is the sensitive organ: "If injury should befall the glacial humor, the other tunics remaining sound, sight is destroyed; if the other tunics should be

corrupted, their transparency and the health of the glacial humor being re-

28 Opt. thes., I, Sec. 23, p. 15. 29 Cf. L'Optique de Claude Ptolemee dans

la version latine d'apres I'arabe de l'emir Eu- gene de Sicile, ed. Albert Lejeune (Louvain: Publications Universitaires de Louvain, 1956), pp. 148-149.

30 Opt. thes., I, Sec., 13, p. 7. Galen and Rufus were the most important ancient sources on the anatomy and physiology of the eye, though Alhazen's direct dependence on them cannot be demonstrated. The drawings of the eye con- tained in the Risner edition (p. 6 of Alhazen's Opt. thes. and p. 87 of Witelo's Optica, bound with the Opt. thes.) do not originate with Alha- zen or Witelo, but were taken from the De cor- poris humani fabrica (1st ed., 1543) of Andreas Vesalius; cf. J. Hirschberg, Geschichte der Augenheilkunde, in Graefe-Saemisch Hand- buch der gesamten Augenheilkunde, Vol. XIII (Leipzig: Wilhelm Engelmann, 1908), p. 149. For a description and reproductions of diagrams

taken from Arabic manuscripts of Alhazen's Kitab al-manizir and other Islamic authors, including one dated 1083 and apparently copied by Alhazen's son-in-law, see Polyak, Retina, pp. 114-119 and Figs. 7-12.

31 Galen (2nd half of the 2nd century) iden- tifies the same three humors, but he is a bit ambiguous on the tunics. He describes seven tunics, but these appear to be circles or layers rather than tunics in the usual sense. Cf. Galen, De usu partium, X, 2, trans. Daremberg, Vol. I, pp. 609-614. On Galen's anatomy of the eye and theory of vision, see Hirschberg, "Die Optik der alten Griechen," pp. 347-351; Polyak, Retina, pp. 97-101. Rufus of Ephesus (1st half of the 2nd century) describes four tunics, but they do not correspond exactly to the four tunics described by Alhazen. On Ru- fus, see iEuvres de Rufus d'Atphese, ed. and trans. Charles Daremberg and Ch. Lmile Ruelle (Paris: J. B. Bailliere, 1879), pp. 154, 170-172.

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tained, sight is not destroyed." 32 Alhazen specifies, further, that the sensitive

part of the glacial humor is its front surface, which is concentric with the cornea.33

Thus far a very general account of Alhazen's theory of vision has been

presented: the forms of light and color emanate from bodies in all directions; they pass through the transparent cornea of the eye and fall on the front surface of the glacial humor. But enormous obstacles remain to be overcome. Chief of these is to show how various parts of the same object are distinguished from each other.34 The observer perceives not only the presence of light and color, but particular patterns of light and color. Clearly, vision is more than mere reception of forms; reception occurs in such a way that different objects (or different parts of the same object) perceived at the same time are perceived as being distinct and in their true spatial relationship. It appears as though this could be explained by attributing the perception of different parts of the object, scattered about the visual field, to different parts of the surface of the glacial humor. But this raises a serious difficulty: from each part of the

object the forms of light and color emanate in all directions; consequently, every part of the glacial humor should receive forms of light and color from every part of the object, and total confusion should result.

Alhazen overcame the difficulty by considering the visual field point by point.35 Every point on a visible object radiates the forms of its light and color in all directions, but only the form directed toward the center of curva- ture of the front surface of the eye is incident on the eye perpendicularly and enters without refraction.36 Shifting to the language of geometrical lines, Alhazen points out that from every point on an object there are infinitely many lines incident on the front of the eye, but only one line from each

point is incident perpendicularly and is hence unrefracted. The forms that are unrefracted are most efficacious in vision, and refracted forms yield only an indistinct impression. By thus restricting himself to forms or rays propa- gated rectilinearly, Alhazen has eliminated all possibility of confusion in the

eye: there is a single ray from every point on the object, passing in a straight line toward the center of the observer's eye.37 Because these rays are recti-

32 Opt. thes., I, Sec. 16, p. 8. Cf. Galen, De usu partium, X, 3, trans. Daremberg, Vol. I, p. 608. A similar argument is presented by Hunain ibn Ishaq (Ten Treatises of the Eye, p. 4) and by later authors such as Bacon and Pecham.

33". . . et erit forma ordinata, sicut est ordinata in superficie rei visae, et in parte ista superficiei glacialis" (Opt. thes., I, Sec. 24, p. 15). Cf. ibid., Sec. 40, p. 23. "Oportet, ut cen- trum superficiei glacialis et centrum superficiei visus sint unum punctum" (ibid., Sec. 23, p. 14).

34 In the emission theory, the eye is an organ with directional sensitivity: rays issue forth in all directions, and the location of the object in space is determined by the direction of the ray terminating on it. In discarding the emission theory, Alhazen gave up this direc- tional sensitivity: the front surface of the lens

is sensitive to the presence of light but not to the direction from which it came. This gener- ates the problem of distinguishing various parts of the visual field.

s3 Ibid., Sec. 18, pp. 9-10. 36 It is uncertain in this context whether

"point" means "very small area" or "that which has no part." In Bk. IV, Alhazen argues that reflection of a sensible ray must occur from a sensible point, having a latitude equal to that of the ray; although he gives no indication, Al- hazen might have considered the same analy- sis applicable in the present case. Cf. ibid., IV, Sec. 16, p. 112.

37 Note that all tunics (or, more accurately, all those before the back surface of the lens- see n. 39) are concentric with the cornea, so that rays perpendicular to the cornea are per- pendicular to all tunics.

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linear, they maintain a fixed order, and "the form will be arranged on the surface of the glacial humor just as it is on the surface of the visible ob-

ject."38 A one-to-one correspondence, which insures clear and unconfused

perception, has thus been established between points on the object and points on the surface of the glacial humor.39

When the propagation of forms in straight lines is expressed in geometrical terms, one has a pyramid with base on the visible object and vertex at the center of the observer's eye.40 From each point on the base, a line can be drawn to the vertex in the eye, representing the path of the form by which vision of that point is achieved. This pyramid is a geometrical representation of the process of sight and aids the investigator in understanding the process.41 Alhazen has thus succeeded in restoring the visual pyramid of Euclidean and Ptolemaic optics and, thereby, the inherent intelligibility of Greek emission theories of vision. But, significantly, he has done so within an intromis- sion framework. For the first time an intromission theory of vision has be- come a viable alternative, adequate to compete on geometrical as well as

physical and physiological terms with the theory of visual rays. Alhazen's theory of vision, as presented above, is confined mostly to the first

of the seven books of his Perspectiva. Book II contains his psychology of per- ception. Book III continues in a psychological vein, dealing with the errors of vision, including those associated with binocular vision. Books IV and V are concerned with reflection from plane mirrors and curved mirrors (both con- cave and convex) of spherical, conical, and cylindrical figure. In Book VI Al- hazen discusses the errors in perception resulting from vision by reflected rays (i.e., errors in number, location, and size of images). Book VII is devoted to refraction of rays. There is no doubt that Alhazen contributed to geometrical studies of reflection and refraction, but his significant innovations were limited to his theory of vision. Although he extended Ptolemy's geometrical optics to new cases and to a higher level of sophistication, it was still Ptole-

38 Ibid., I, Sec. 24, p. 15. 39 In this paper I have not probed deeply

into Alhazen's psychology of perception. In brief, he argues that vision is not completed in the glacial humor (lens), but by the virtus dis- tinctiva belonging to the ultimum sentiens, which is situated in the anterior part of the brain. The forms of light and color penetrate the glacial humor and, at the interface separat- ing the glacial humor and vitreous humor (which is before the center of the eye), are re- fracted away from the center of the eye and never actually converge to a vertex. The forms are then conducted through the vitreous humor and hollow optic nerve-all the time maintain- ing their proper disposition-to the optic chias- ma, where they join the forms from the other eye. They continue through the visual spirit to the anterior part of the brain. Cf. Opt. thes., I, Ch. 5, pp. 15 ff.; II, Ch. 1, pp. 24 ff. For a full-length study of Alhazen's psychology of perception, see Hans Bauer, Die Psychologie Alhazens (Beitrdge zur Geschichte der Philoso-

phie des Mittelalters, 1911, 10, Pt. 5). The same general scheme is found in the writings of Bacon, Pecham, and Witelo; on differences between the psychology of Alhazen and his Latin followers, see Vescovini, Studi, Chs. 4, 7.

40 In works translated from the Arabic, the term pyramis is used even when the figure has a round base and hence could aptly be desig- nated by the term conus. See Marshall Clagett, "The De curvis superficiebus Archimenidis: A Medieval Commentary of Johannes de Tinemue on Book I of the De sphaera et cylindro of Archimedes," Osiris, 1954, 11:298.

41 Opt. thes., I, Sec. 24, p. 15. An earlier point can now be clarified: Alhazen is willing for mathematicians to talk about imaginary rays issuing from the eye in pyramidal form because if rays emanate from the center of the eye, the pyramid of vision is formed without further ado; there are no rays not perpendicular to the surface of the eye to interfere with the geometrical scheme.

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maic optics that he was extending. He played the game with more finesse than Ptolemy, but it was still the same game. The novelty of Alhazen's theory of vision had no influence on traditional geometrical optics; not only can geo- metrical optics be pursued without commitment to any particular theory of vision, but Alhazen was even willing to allow "mathematicians" to continue to express themselves in terms of the discredited emission theory.

II

The wave of translations from Arabic to Latin in the twelfth and thirteenth centuries included Alhazen's Kitab al-manazir. The name of the translator and provenance of the translation are unknown, but the treatise was evidently translated in the late twelfth or early thirteenth century. The earliest known Western citation appears in a work of Jordanus de Nemore,42 who flourished in the early part of the thirteenth century, but its diffusion in the first half of the thirteenth century was not sufficiently wide to have brought it to the attention of Robert Grosseteste, who wrote on optics in the first third of the

century.43 The full impact of Alhazen's new optical theories is first seen in the writings of Roger Bacon, John Pecham, and Witelo, all of whom wrote on optics in the 1260's and 1270's.44

The eagerness with which Alhazen's optics was received in the West was doubtless due to its promise of contributing to an already flourishing optical tradition. Kept alive (if barely) by the encyclopedic tradition of the early Middle Ages, optical theory was nourished dramatically by early translations of scientific works into Latin and brought into prominence by Robert Grosse- teste, who revived optical and other scientific studies at Oxford early in the thirteenth century.45 Among the optical treatises available to Bacon, Pecham,

42 See Marshall Clagett, Archimedes in the Middle Ages, Vol. I (Madison: Univ. Wisconsin Press, 1964), p. 669.

43 Richard C. Dales, "Robert Grosseteste's Scientific Works," Isis, 1961, 52:394-402, dates Grosseteste's optical works between 1231 and 1235. Occasionally al-Bitriji (d. early 13th cen- tury) is quoted as having asserted that Alhazen's Perspectiva was circulating in the West during his lifetime: "Nam licet perspectiva Alhacen sit in usu aliquorum sapientium Latino- rum. . ... (Cf. Lucien Leclerc, Histoire de la medecine Arabe, Vol. II [Paris: E. Leroux, 1876], p. 516.) However, this is not a quotation from al-Bitrfji, but from a fragment of the Opus tertium of Roger Bacon, which (in Paris, Bibliotheque Nationale, Latin MS 10264, fol. 186) is wrongly entitled Liber tertius Alpetra- gii; cf. Un fragment inedit de l'Opus tertium de Roger Bacon, ed. Pierre Duhem (Quaracchi: Collegium S. Bonaventurae, 1909), p. 75.

44 On medieval Western optics in general, see A. C. Crombie, Robert Grosseteste and the Ori- gins of Experimental Science, 1100-1700 (Ox-

ford: Clarendon Press, 1953), and Vescovini, Studi. On Pecham, see my "The Perspectiva communis of John Pecham." All texts and translations from Pecham's Perspectiva com- munis are drawn from my forthcoming edition (Univ. Wisconsin Press). The best sources on Bacon's optics are in Roger Bacon Essays, ed. A. G. Little (Oxford: Clarendon Press, 1914), but further work is in order. However, Bacon's theories are readily accessible in the English translation of his Opus majus (The Opus Ma- jus of Roger Bacon, trans. R. B. Burke, Phila- delphia: Univ. Pennsylvania Press, 1928). On Witelo, see Clemens Baeumker, Witelo, ein Phi- losoph und Naturforscher der XIII. Jahrhun- derts (Beitrige zur Geschichte der Philosophie des Mittelalters, 1908, 3, Pt. 2); Baeumker bases some of his argument on the conclusion that Witelo was the author of De intelligentiis, a conclusion now clearly recognized as false. On Witelo, see also my introduction to a forthcom- ing facsimile reprint of the Risner edition (New York: Johnson Reprint, Sources of Science).

45 On Grosseteste's sources, see Crombie, Grosseteste, pp. 116-117.

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and Witelo were works by Greek, Muslim, and Latin authors. There were, of course, Alhazen and Grosseteste. In addition to these two, Pecham cites al-Kindi, Aristotle, the pseudo-Euclidean Catoptrica, and a medieval abridge- ment of Euclid's Optica; he appears to have known, also, Alhazen's De specu- lis comburentibus (a short work distinct from the Perspectiva), and he may have used Ptolemy's Optica.46 Bacon cites all these on optics, as well as Avi- cenna, Averroes, Tideus, Constantinus Africanus, and Augustine.

Though but one of many authorities, Alhazen exerted by far the dominant influence. Bacon continually cites him by name, and Pecham and Witelo

consciously patterned their major optical works after his Perspectiva, respec- tively condensing and expanding its treatment. Pecham continually bows to the authority of Alhazen, whom he cites as "the Author" or "the Physicist"; and Risner, publisher of the printed works of Alhazen (Latin text) and Witelo in a single volume, has indicated their close relationship by elaborate cross- references. But aside from citations and format, the theories of vision ex-

pressed by Bacon, Pecham, and Witelo are essentially the same as Alhazen's.47 All describe the anatomy of the eye similarly, with only small differences in detail. Vision, according to Bacon, Pecham, and Witelo, occurs through rays issuing from the visible object and falling perpendicularly on the surface of the eye and the glacial humor. Nonperpendicular rays are refracted and con- tribute to vision only incidentally. Through the visual pyramid, consisting solely of perpendicular rays issuing from the object and converging toward a vertex at the center of the eye,48 the forms of the light and color of the ob-

ject are arranged on the surface of the glacial humor precisely as on the sur- face of the object; consequently a one-to-one correspondence is established, which insures clarity of vision. In order to establish this theory of vision, Bacon, Pecham, and Witelo even rely on the same evidence and the same arguments as Alhazen. All four, for example, cite the pain experienced by the eye in looking at bright lights as evidence that light and color make some kind of impression on the eye, and Pecham (like Alhazen) begins his treatise with a description of such evidence.49

It should be evident, then-and historians have long agreed on this-that the main outlines of Alhazen's theory of sight, as well as his more abstract geometry of image formation by reflection and refraction and many minor details, were incorporated in the optical works of Bacon, Pecham, and Witelo. Alhazen's theory was comprehensive and systematic; it was superior in almost every respect to anything the West had known before. To some extent it also

46 For a fuller discussion of Pecham's sources, away from the center of the eye at the interface see my edition of the Perspectiva communis. between the glacial humor and vitreous humor.

47 Crombie, Grosseteste, gives Bacon, Pecham, This prevents inversion or reversal of the forms and Witelo far too much credit for experi- as they are conducted through the optic nerves; mental or observational prowess and attaches cf. n. 39. too much significance to Grosseteste's influence 49 Perspectiva communis, I, Prop. 1. On Al- on their optical theories. For the most part, hazen, see above. Cf. Bacon, The Opus Majus Bacon, Pecham, and Witelo did not observe, of Roger Bacon, ed. J. H. Bridges (London: experiment, or read Grosseteste; they read Al- Williams and Norgate, 1900), V, 1, Dist. 5, Ch. 1, hazen and other Islamic and Greek authorities. Vol. II, pp. 30-32;Witelo, Opticae libri decem,

48 As in Alhazen's theory, the rays do not ed. Risner (bound with Opt. thes.), III, pp. actually converge to a vertex, but are refracted 87-88, 91.

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filled an intellectual void; the works of Aristotle, Euclid, Ptolemy, al-Kindi, and Grosseteste said little about the anatomical and physiological details of sight. These treatises dealt primarily with geometrical optics, on which they were in substantial agreement with Alhazen. Conflict was therefore at a mini- mum. Where earlier authorities had spoken, Alhazen agreed; where they had been silent, he provided a theory that was overwhelmingly systematic and

comprehensive. Nevertheless, strong reservations have been expressed about the assimila-

tion of Alhazen's optics in the West, particularly with regard to the properties of the emanated entity. Vasco Ronchi, one of the most articulate and prolific writers on the history of optics in recent years, has repeatedly characterized medieval Western optics as a corruption of Muslim views and a return to the less satisfactory theories of Greek antiquity. According to Ronchi, Alhazen had set the theory of sight on the road leading to Kepler and modern optics by decomposing "l'objet visible en elements punctiformes, ce qui faisait perdre a la vision de l'objet lui-meme le caractere d'une operation globale qu'on ne pouvait scinder." 50 By treating the object a point at a time, Alhazen had escaped the absurdities of Epicurean eidola, while still operating within an intromission framework. "Les 'eidola' et les 'ecorces,'" Ronchi writes, "sont mortes." 51 And so they were. The question is whether they stayed dead. Ronchi thinks not. Eidola and rinds reappeared, he insists, in the "species" of Western optics. Scholastic scientists, with a few exceptions, attempted to reconcile eidola (poorly disguised in new terminology) with Alhazen's revo- lutionary contributions. The result, Ronchi insists, was "une construction grotesque."52 Simply put, Ronchi feels that species are eidola. He sums up this viewpoint most forcefully when, after describing the diffusion of Al- hazen's ideas and the inability of Western scientists to understand them, he writes,

What ensued was an indescribable atrophy of thought. Ideas tended more or less to cluster about the doctrine of species, a new edition of the ancient eidola. These species, however, were produced by the lumen, when it im- pinged upon a body, and they moved along the observer's visual rays as though along rails guiding them toward the eyes. During this motion they contracted in order to be able to enter the pupil. The contraction no longer constituted a serious obstacle [as it had in ancient times] because Ibn al- Haitham's mechanism had provided a sort of justification for it.

In other words, an effort was made to combine the classical with the new. The merger was a monstrosity, with which the philosophers and mathema- ticians of the later Middle Ages tried to reason when confronted by optical problems.53

An important question remains: To precisely which Western thinkers is Ronchi's analysis to be applied? According to the most sympathetic interpre- tation, Ronchi recognizes only three Western philosophers who assimilated Alhazen's revolutionary theories regarding the process of sight: Bacon, Pe-

50 Ronchi, Histoire, p. 38. 53 Ronchi, Optics, The Science of Vision, 51 Ibid., p. 37. On eidola, see below. trans. Edward Rosen (New York: New York 52 Ibid., p. 55. Univ. Press, 1957), p. 32.

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cham, and Witelo. On the other hand, he singles out Dante Alighieri (d. 1321), Gregorius Reisch (d. 1525), and Giambattista della Porta (d. 1615) as

examples of men who failed to grasp Alhazen's theory, and from an examina- tion of their works purports to demonstrate the universal (or almost uni-

versal) failure of scientists of the late Middle Ages to understand or adopt Alhazen's important innovations.54 In Ronchi's view, then, Alhazen's ideas were assimilated by Bacon, Pecham, and Witelo in the thirteenth century, but afterwards forgotten, rejected, or misunderstood until the time of Mau-

rolycus and Kepler at the end of the sixteenth century. But Ronchi has often been misleading, and this "sympathetic interpreta-

tion" of Ronchi's views does not jump out at the reader of his works. In his main work on the history of optics Ronchi gives not the slightest indication that Bacon and Pecham had anything to do with the diffusion of Alhazen's ideas.55 Moreover, Ronchi's condemnation of the idea of species is sweeping; in no work does he admit any exceptions to the grotesqueness or acknowledge any varieties of the idea. Thus he leaves the clear impression that the species concept in all its forms was a departure from the teaching of Alhazen; and this impression is reinforced by the fact that in most of Ronchi's works only Witelo, who never used the term "species," is recognized as a faithful follower of Alhazen.56 Finally, in his most explicit statement of the period during which this "atrophy of thought" occurred, Ronchi writes,

... pendant quatre ou cinq siecles les idees d'Alhazen n'ont eu aucune consequence appreciable. Si, pendant ce laps de temps, on recherche dans les oeuvres le plus remarquables du monde occidental quelles etaient les idees predominantes au sujet de la lumiere, on retrouve celles de la period grecque.57

Since the light of understanding dawns again toward the end of the sixteenth century with the printing of Alhazen's Perspectiva,58 four or five centuries take one back to the eleventh or twelfth century-before the translation of Alhazen's work into Latin. Consequently, the passage must be interpreted as asserting either that Alhazen's theory was not adopted by Bacon, Pecham,

54 Histoire, pp. 45-49, 57-73. These three men are hardly the best representatives (with the possible exception of della Porta) of medie- val or even late-medieval Western optics. Against Ronchi one might even argue that since Dante, Reisch, and della Porta failed to grasp Alhazen's theories, they could not have been serious students of optics. At any rate, Ronchi's thesis would be more impressive if it could be demonstrated with respect to such competent natural philosophers and optical theorists as Theodoric of Freiberg, Blasius of Parma, and Friedrich Risner.

55 Ronchi's main work on the history of optics is Storia della luce, translated into French as Histoire de la lumiere. The only work (to my knowledge) in which Ronchi mentions Bacon and Pecham as followers of Alhazen is his Optics, The Science of Vision, p. 31, and there they receive but two sentences.

56 Histoire, pp. 44-45; Ronchi, "Sul con- tributo di Ibn-al-Haitham alle teorie della vi- sione e della luce," in Actes du VIIIe Congres International d'Histoire des Sciences (Jerusalem, 1953), p. 520.

57 Histoire, p. 45. 58 Ronchi (ibid.) asserts that Risner, publisher

of the 1572 edition, was also its translator from Arabic to Latin. Elsewhere he identifies Witelo as the translator ("Complexities, Advances, and Misconceptions in the Development of the Science of Vision: What is being Discovered?" in Scientific Change, ed. A. C. Crombie [New York: Basic Books, 1963], p. 546). Actually, neither is correct. The book was rendered into Latin by an unknown translator late in the 12th or early in the 13th century, more likely the latter. Risner merely edited the medieval trans- lation, as he relates in the preface to his edi- tion; Witelo had nothing whatsoever to do with it.

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and Witelo or that its incorporation in their works was not an "appreciable consequence."59

But, in fact, Bacon, Pecham, and Witelo fully assimilated the optics of Alhazen, and their works had a very appreciable influence on succeeding generations. The unusually large number of manuscripts and printed editions of Pecham's Perspectiva communis (as least 47 extant manuscripts and 10

printings) and Witelo's Perspectiva (numerous manuscripts and 3 printings) testifies to the diffusion of Alhazen's ideas throughout the later Middle Ages. Nor were these a matter of reproduction without understanding; for example, the 1542 edition of Pecham's Perspectiva communis was altered extensively by its editor, the mathematician and instrument-maker George Hartmann

(1489-1564), with no distortion of the essential ideas. Moreover, Pecham's

Perspectiva communis was the subject of a number of commentaries, the authors of which did not fail to understand the contents.60 Eventually Pe- cham's book became the standard optical text in the medieval university, so that even an elementary education in optics provided an understanding of Alhazen's theories. If late medieval writers on optics did not preserve Al- hazen's ideas on vision in their works, it was not because they failed to

comprehend, but because they were asking different questions.6' Nevertheless, Ronchi's work has raised the question of the relationship of

medieval species to Alhazen's forms and Epicurean eidola; and if I disagree with Ronchi's answer, I at least endorse his question, for it is most important, deserving a detailed and unambiguous answer. I will devote the remainder of this essay to it, restricting myself for the present to the concept of species in its thirteenth-century form; 62 it will be my thesis that from an optical standpoint the Western idea of species is indistinguishable (with one impor- tant exception) from Alhazen's concept of form. Since the standard of com- parison, as Ronchi has posed the problem, is the Epicurean eidolon, let us first consider the latter.

Epicurus describes eidola in his Letter to Herodotus: "particles are con- tinually streaming off from the surface of bodies. . . . And those given off, for a long time retain the position and arrangement which their atoms had when they formed part of the solid bodies."63 Sight occurs as these eidola enter the observer's eye:

We must also consider that it is by the entrance of something coming from external objects that we see their shapes and think of them. For external

59 Ronchi has already admitted, on the same larizers or amateurs like Dante and Reisch. For page, that Witelo faithfully followed Alhazen. example, Theodoric of Freiberg, in his De luce, However, more recently Ronchi has written was concerned not with geometrical optics but that "at the end of the thirteenth century with a causal account of light; cf. William A. A.D . . . [the work of Ibn al-Haitham] had as Wallace, O.P., The Scientific Methodology of yet had no impact on the Western world" Theodoric of Freiberg (Fribourg, Switzerland: (Scientific Change, p. 545). Univ. Press, 1959), pp. 152-161.

60 For a discussion of the medieval commen- 6 I plan to deal with the later development taries on the Perspectiva communis, see my

A * v taries on the Perspectiva communis, see my of the concept of species in a future paper. edition, n. 44 above. The influence of Al- hazen's Perspectiva is explored in more detail 63 Diogenes Laertius, Lives of Eminent Phi- in my introduction to the forthcoming reprint losophers, X, 48, trans. R. D. Hicks (London: of the Risner edition. William Heinemann, 1925), Vol. II, pp. 577-

61 I have in mind serious scientists, not popu- 579.

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things would not stamp on us their own nature of colour and form through the medium of the air which is between them and us, or by means of rays of light or currents of any sort going from us to them, so well as by the entrance into our eyes or minds, to whichever their size is suitable, of certain films coming from the things themselves, these films or outlines being of the same colour and shape as the external things themselves.64

Thus eidola (material images or skins) are stripped from the outer surfaces of objects and propagated through space as coherent units. Because they are of the same shape and color as the object, they faithfully communicate the latter to the observer. But the fatal objection to the Epicurean theory, be- sides its generally simplistic character, is the problem of shrinking the eidola of large objects, so as to squeeze them into the observer's eye, without altering them in shape or color-and this for all possible distances between the object and the observer.

Now it is clear that Alhazen's forms have little in common with Epicurean eidola. In the first place, Alhazen is willing to consider the form of each point (or small part) of the object independently: he refers, for example, to "the form of the light and color that comes from any point of the visible object to the surface of the eye." 65 Since he is dealing with an image of each point rather than a single replica of the whole object, squeezing images through the pupil is no problem. Secondly, Alhazen's forms are not images in the Epicurean sense. They do not consist of pieces of the object; they are not replicas. Rather they are powers representative of the object, capable of producing effects in a recipient. However, noting Alhazen's explanation of reflection on the analogy of mechanical rebound, Ronchi writes, "The idea that the rays of lumen are the trajectories of minute material corpuscles is already expressed in his work." 66 But this is to misunderstand Alhazen. The analogy of mechanical rebound is meant to elucidate the equal angles of reflection, not the nature of the reflected entity. Schramm, basing his argu- ment on a study of both the Latin and Arabic texts of the Kitab al-manazir, has concluded that Alhazen's form is not a three-dimensional body, but rather that which is impressed on a body through qualitative change.67 Form for Alhazen is thus very close to Aristotelian form.

The concept of species, as applied to optics, had its origin in the Neopla- tonic doctrine of emanation. Plotinus (d. 270 A.D.) maintained that causation

64 Ibid., X, 49, Vol. II, p. 579. For a good p. 112. Alhazen also elucidates refraction by account of the Epicurean theory of vision, see means of mechanical analogies (ibid., VII, Sec. Cyril Bailey, The Greek Atomists and Epicurus 8, p. 241). (Oxford: Clarendon Press, 1928), pp. 406-413; 67 Schramm, Weg zur Physik, p. 216. A. I. cf. Haas, "Lichttheorien," pp. 362-370. Sabra writes, ". . he [Alhazen] denies that

65 Opt. thes., I, Sec. 18, p. 9. Alhazen also light is a body . . ." ("Explanation of Optical speaks of a single form for the whole object Reflection and Refraction: Ibn-al-Haytham, on occasion. Descartes, Newton," Actes du dixieme congres,

66 Optics, Science of Vision, p. 30. Cf. Ron- Vol. I, p. 551). Cf. J. Baarman, "Abhandlung chi, Histoire, p. 42, and Ronchi, "The Evolu- iiber das Licht von Ibn al-Haitham," Zeit- tion of the Meaning of 'Light' in Natural Phi- schrift der Deutschen Morgenldndischen Ge- losophy," in Actes du dixieme congres interna- sellschaft, 1882, 36:197-199; this is the Arabic tional d'histoire des sciences (Ithaca, 1962), Vol. text and German translation of Alhazen's short II, p. 725. Cf. Alhazen, Opt. thes., IV, Sec. 18, treatise On Light.

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may occur through a process of emanation. Indeed, all things tend to emanate their power outside themselves. In The Enneads Plotinus writes,

All existences, as long as they retain their character, produce-about them- selves, from their essence, in virtue of the power which must be in them-some necessary, outward-facing hypostasis continuously attached to them and rep- resenting in image the engendering archetypes: thus fire gives out its heat; snow is cold not merely to itself; fragrant substances are a notable instance; for, as long as they last, something is diffused from them and perceived where- ever they are present.68

Light emanating from a luminous body is another example of the same effect. In the same vein, Avicebron (d. c. 1058), whose Fons vitae was available to Grosseteste and other thirteenth-century writers, argues that powers and rays emanate from all simple substances on the analogy of the emanation of light from the sun: "The essences of simple substances do not issue forth; it is rather their powers and rays that flow forth and spread abroad. . . . Just as light flows from the sun into the air, . . . so every simple substance extends its ray and its light and diffuses them into that which is inferior. ". . 69

These ideas were developed by Grosseteste into the doctrine of the multi- plication of species. As he expresses this doctrine in his De lineis, angulis et figuris,

A natural agent propagates its power from itself to the recipient, whether it acts on the senses or on matter. This power is sometimes called species, some- times a similitude, and is the same whatever it may be called; and it will send the same power into the senses and into matter, or into its contrary, as heat sends the same thing into the sense of touch and into a cold body. For it does not act by deliberation and choice, and therefore it acts in one way, whatever it may meet, whether something with sense perception or some- thing without it, whether something animate or inanimate. But the effects are diversified according to the diversity of the recipient.70

Every natural agent propagates its power from itself to surrounding bodies, and this power is called "species" because the effects bear a resemblance to the agent. Hot bodies, for example, emanate species that produce heat in the recipient, and bright bodies emanate species that produce brightness in the observer's eye.71

Roger Bacon and John Pecham adopted Grosseteste's doctrine of the multi-

68 Plotinus, The Enneads, V, 1, 6, trans. 71 As Grosseteste points out in the passage Stephen MacKenna (2nd ed., London: Faber quoted, the effects do not always resemble the and Faber, 1956), p. 374. agent; but this is an insight not possessed by

69 I have translated this from the Latin text all writers on the subject, and even in Grosse- in Avencebrolis (Ibn Gebirol), Fons vitae, III, teste the effects are never totally unlike the 52, ed. Clemens Baeumker (Beitrdge zur Ge- agent. Bacon insists that "haec species sit similis schichte der Philosophie des Mittelalters, 1895, agenti" (De multiplicatio specierum, ed. J. H. 1), p. 196. Similar ideas on the multiplication Bridges, bound with the Opus majus, Vol. II, of power had been expressed by al-Kindi, and p. 410). Cf. Avicebron, Fons vitae, III, 53; these may have been known to Grosseteste as Charles K. McKeon, A Study of the Summa well; cf. Vescovini, Studi, Ch. 3. philosophiae of the Pseudo-Grosseteste (New

70 Quoted by Crombie, Grosseteste, p. 110. York: Columbia Univ. Press, 1948), pp. 87, 94.

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plication of species and incorporated it in their theories of vision drawn from Alhazen. Bacon, the more vocal on the subject of species, explains,

For every efficient cause acts through its own power, which it exercises on the adjacent matter, as the light [lux] of the sun exercises its power on the air (which power is light [lumen] diffused through the whole world from the solar light [lux]). And this power is called "likeness," "image," and "species" and is designated by many other names, and it is produced by substance as well as accident. . . . This species produces every action in this world, for it acts on sense, on the intellect, and on all the matter of the world for the generation of things.72

Pecham's view is similar:

Every natural body, visible or invisible, diffuses its power radiantly into other bodies. The proof of this is by a natural cause, for a natural body acts outside itself through the multiplication of its form. Therefore the nobler it is, the more strongly it acts. And since action in a straight line is easier and stronger for nature, every natural body, whether visible or not, must multiply its species in a continuous straight line; and this is to radiate.73

Now the central question of this essay is whether or not this concept of

species was "a new edition of the ancient eidola." The answer is clear-no! As these quotations from Bacon and Pecham indicate, species are not material replicas, films of matter peeled from the outer surface of the object and pro- pelled through space, but powers or forms diffused from one point to another through the matter already there. The actual mode of multiplication or propagation of species is very clearly described by Bacon:

But a species is not body, nor is it moved as a whole from one place to an- other; but that which is produced [by an object] in the first part of the air is not separated from that part, since form cannot be separated from the matter in which it is unless it should be soul; rather, it produces a likeness to itself in the second part of the air, and so on. Therefore there is no change of place, but a generation multiplied through the different parts of the me- dium; nor is it body which is generated there, but a corporeal form; . . . and it is not produced by a flow from the luminous body, but by a drawing forth out of the potentiality of the matter of the air.74

The multiplication of species is more like the propagation of waves than like the motion of projectiles.

72 Opus majus, IV, Dist. 2, Ch. 1, Bridges, Huius probatio est per causam naturalem, Vol. I, p. 111; cf. De mult. spec., Bridges, Vol. quoniam corpus naturale agit per formam suam II, pp. 407-418. For a careful discussion of se extra se multiplicantem. Ergo quanto nobi- Bacon's concept of species, see Vescovini, Studi, lior tanto est fortior in agendo. Et quia actio pp. 57-60. Bacon distinguishes between lux in directum est facilior et fortior nature, necesse and lumen in the passage quoted, but admits est ut omne corpus naturale seu visibile seu in De mult. spec., "Sed tamen usualiter lucem non visibile suam speciem multiplicet in con- accipimus pro lumine et e contrario" (Bridges, tinuum et directum, et hoc est radiare" (I, Vol. II, p. 409). Prop. 27 of Pecham's revised version of Pers.

corn.). Cf. II, Prop. 5. 73 "Omne corpus naturale visibile seu non 74 Opus majus, V, 1, Dist. 9, Ch. 4, Bridges,

visibile radiose virtutem suam in alia porrigere. Vol. II, pp. 71-72.

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But this does not get to the nub of Ronchi's argument. His claim is not so much that species resemble eidola in materiality as that species, like eidola, are coherent wholes. Alhazen's principal contribution to the-theory of vision had been to substitute a point-by-point analysis of the visual field (with forms issuing from every point) for the coherent eidolon of Epicurean phi- losophy, and Ronchi's view is that the concept of species was a return to coherence. But this is surely not true of the concept in its thirteenth-century (and most influential) form. Bacon himself felt that the difference between species and Alhazen's forms was merely terminological,75 and he was sub- stantially correct: in fact, the optical properties of his "visible species" and Alhazen's "forms of light and color" are identical. Now on the question of coherence, Alhazen, Bacon, and Pecham are all imprecise in their termi- nology. Alhazen frequently speaks of the singular form of a whole object, but this form has the crucial property of susceptibility to a point-by-point analysis. Moreover, when explaining the process of sight or locating images formed by reflection or refraction, Alhazen considers the object a point at a time and appeals to individual rays instead of the unitary forms. This is not unlike our practice of conceiving of a continuous body of radiation as a composite of discrete rays, emanating from individual points on the lumi- nous body.

On the question of coherence, Bacon and Pecham followed Alhazen's lead in every detail. Pecham often refers to the single species of a whole object, explaining, for example, that the "species [singular] produced by a visible object has the essential property of manifesting the object of which it is the likeness,"76 but this species (like Alhazen's form) is susceptible to puncti- form analysis. On other occasions, Pecham speaks of the species of a point, remarking, for example, that "any point of an object seen in a mirror fills the whole surface of the mirror with its species."77 The very foundation of the geometrical and physiological optics of Bacon and Pecham is this ability of the visual field to be analyzed into points for individual treatment; their optics, like Alhazen's, is based on rays (representable by geometrical lines) rather than a coherent species.78 It is evident, then, that Bacon and Pecham fully and successfully incorporated in their optics both Alhazen's

75 In De mult. spec., Bacon says explicitly that Alhazen used the term "form" to denote species: "Forma quidem vocatur in usu Alha- zen, auctoris Perspectivae vulgatae" (Bridges, Vol. II, p. 410). In his Questiones supra librum de causis, Bacon attributes the concept of species to Alhazen, saying, "Item secundo Per- spective [i.e., Alhazen's] dicitur quod lux et colores multiplicant suas species usque ad sensum . . ." (ed. Robert Steele and F. M. De- lorme in Opera hactenus inedita Rogeri Baconi, fasc. XII [Oxford: Clarendon Press, 1935], p. 52).

76". . . species genita a re visibili essen- tiali habet rem ostendere cuius est simili- tudo . . ." (Pers. corn., II, Prop. 5).

77 " . . quilibet punctus rei vise in speculo replet specie sua totam superficiem speculi" (Pers. cor., I, Prop. 3 of the revised version). In De mult. spec., Bacon discusses both the species (singular) representing the whole ob- ject and the species (plural) of the individual parts in the same argument: "Hoc enim non est quia magnitudo faciat suam speciem, sed quia a tota rei magnitudine venit species coloris et lucis, et a tota superficie. Et tunc species coloris venientes a singulis partibus rei visae non confuduntur in una parte pupillae . . . (Bridges, Vol. II, p. 429).

78 Bacon asserts unequivocally that "an in- finite number of rays issues from every point of the agent" (Opus majus, IV, Dist. 3, Ch. 1, Bridges, Vol. I, p. 122).

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punctiform approach and Grosseteste's concept of species. They achieved this simply by endowing species with all the optical properties of Alhazen's forms.

At only one point in their theories of vision did Bacon and Pecham find that species were significantly at odds with Alhazen's "forms of light and color." Species, according to Grosseteste, issue from all natural bodies-from eyes as well as from perceived objects; and he argues that the species issuing from the observer's eye, as well as those emanating from the object, play a role in sight:

Nor is it to be thought that the emission of visual rays is only imagined and without reality, as those think who consider the part and not the whole. But it should be understood that the visible species [issuing from the eye] is a substance, shining and radiating like the sun, the radiation of which, when joined with the radiation of the exterior shining body, entirely com- pletes vision.

Wherefore natural philosophers, treating that which is natural to vision (and passive), assert that vision is produced by intromission. However, mathe- maticians and physicists, whose concern is with those things that are above nature, treating that which is above the nature of vision (and active), main- tain that vision is produced by extramission. Aristotle clearly expresses this part of vision that occurs by extramission in the last book of De animalibus, saying, "A deep eye sees from a distance: for its motion is neither divided nor destroyed, but a visual power leaves it and goes straight to the objects seen." . . . Therefore true perspective is concerned with rays emitted [by the eye].79

In his Commentary on the Posterior Analytics, Grosseteste says further, "Vi- sion is not completed solely in the reception of the sensible form without matter, but is completed in the reception just mentioned and in the radiant energy going forth from the eye."80

Philosophers of the thirteenth century were faced with the task of recon- ciling Alhazen's denial and Grosseteste's affirmation of the existence of visual rays. The situation was complicated, of course, by the presence of other treatises pronouncing on the same subject-the works of al-Kindi, Euclid, Ptolemy, Aristotle, Avicenna, and others. Bacon and Pecham felt they could do justice to all schools of thought by acknowledging with Grosseteste that visual rays exist and, moreover, are required for sight, while ignoring the vis- ual rays in the further development of their theories of sight.81 After at- tempting a rather subtle reconciliation of Aristotle, Ptolemy, al-Kindi, Euclid, Tideus, Augustine, Alhazen, Avicenna, and Averroes, Bacon concludes,

. . . the species of the things of the world are not immediately suited of themselves to bring to completion an action on the eye because of the nobility

79 I have translated this from Ludwig Baur's double-emanation theory of sight. Latin text in Die philosophischen Werke des 80 Quoted by Crombie, Grosseteste, p. 114. Robert Grosseteste (Beitrige zur Geschichte der 81 Relative to optics, Bacon says, "I have Philosophie des Mittelalters, 1912, 9, pp. 72-73). determined not to imitate one author; rather, The exact reference to Aristotle is De genera- I have selected the most excellent opinions tione animalium, V, 1, 781a1-10; this passage is from each" (Un fragment de l'Opus tertium, obviously one of the sources of Grosseteste's ed. Duhem, p. 75).

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of the latter. Therefore these species must be assisted and excited by the species of the eye, which proceeds through the space occupied by the visual pyramid, altering and ennobling the medium and rendering it commensu- rate with sight. Thus the species of the eye prepares for the approach of the species of the visible object and, moreover, ennobles the species of the object so that it is wholly conformable to and commensurate with the nobility of the animate body (i.e. the eye).82

Surprisingly, John Pecham begins by apparently attacking the emission theory. Following Alhazen in every detail, he goes so far as to argue,

By assuming that sight occurs through rays issuing from the eye, mathema- ticians exert themselves unnecessarily. For the manner in which vision occurs is adequately described above [in terms of intromission], by which all the phenomena of vision can be saved. Therefore it is superfluous to posit such [visual] rays.83

But then Pecham finds that he must make the appropriate concessions to the emission theory required by the concept of species and adds,- with more hesi- tation than Bacon,

The natural light of the eye contributes to vision by its radiance. For as Aristotle says, the eye is not merely the recipient of action, but acts itself just as shining bodies do. Therefore the eye must have a natural light in order to alter visible species and make them commensurate with the visual power, for the species are emitted by the light of the sun and moderated with respect to the eye by mixing with the natural light of the eye. . . . Since vision is of the same kind in all animals, and certain animals are able to bestow the mul- tiplicative power on colors by the light of their eyes so as to see them at night, it follows that the light of the eye has some effect on [external] light. Whether it goes beyond this, I do not determine, save only by following in the foot- steps of the Author [i.e., Alhazen], as I have said before.84

Two conclusions emerge from this analysis of Epicurean eidola, Alhazen's forms, and Western species. First, neither Alhazen's forms nor Western

species bear any but the most superficial resemblance to Epicurean eidola.

82 Opus majus, V, 1, Dist. 7, Ch. 4, Bridges, Vol. II, p. 52.

83"Mathematicos ponentes visum fieri per radios ab oculo micantes superflue conari. Visus enim sufficienter fit per modum prescrip- tum, per quem salvari possunt omnia circa visum apparentia. Ergo superfluum est ponere sic radios" (Pers. corn., I, Prop. 44).

84 "Lumen oculi naturale radiositate sua visui conferre. Oculus enim, ut dicit Aristoteles, non solum patitur, sed agit quemadmodum splendida. Lumen igitur naturale necessarium est oculo ad alterandum species visibiles et ef- ficiendum proportionatas virtuti visive, quo- niam ex luce solari diffunduntur sed ex lumine oculi connaturali oculo contemperantur.... quoniam visus in omnibus animalibus est unius rationis cum igitur quedam animalia per lu- men oculorum suorum sufficiant coloribus vir-

tutem multiplicativam dare ut ab eis nocte videri possint, sequitur ut lumen oculi aliquid in lumine operetur. Et an aliquid ulterius fa- ciat non diffinio nisi huius Auctoris, ut dictum est, vestigia sequendo" (ibid., I, Prop. 46). Ba- con's and Pecham's admission that rays issue from as well as enter the observer's eye cannot, of course, be viewed solely as a compromise between Grosseteste and Alhazen. Bacon and Pecham were influenced also by Aristotle and by the Galenic and Platonic traditions and should be considered heirs of these teachings as well. As Pecham points out in the passage quoted, Aristotle speaks of combined emission and intromission; and Pecham's argument about the visual power making the visible species commensurate with sight is based on Aristotle's remarks in De generatione anima- lium, V, 1, 780a5-25.

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Forms and species are powers, not corpuscles or coherent collections of cor-

puscles, and they are susceptible to punctiform analysis. Second and more

important, Alhazen's forms and Western species have identical optical prop- erties and perform identical optical functions-with one exception. Against Alhazen's denial of visual rays, Bacon and Pecham admitted that species emanate from the observer's eye and play a role in sight. However, this ad- mission of visual rays did not otherwise alter the theory of vision gained from Alhazen,85 and Bacon and Pecham almost completely ignored the species emanating from the observer's eye in the further development of their theo- ries. In their theory, as in Alhazen's, powers emanating from natural bodies

produce effects in a recipient, and both theories permit point-by-point analy- sis of the visual field and offer identical descriptions of the act of vision. Thus the admission of visual rays by Bacon and Pecham was of small consequence for the subsequent history of optics. It did not interfere with the transmis- sion, through their works, of the broad outlines and most of the details of Alhazen's theory of vision.

Witelo, whom I have ignored because of his superior status in Ronchi's scheme of things and his failure to use the term "species" or discuss rays emanating from the eye, shared similar views on the properties of the rays and the nature of sight. After a careful study of Witelo's Perspectiva, Alek- sander Birkenmajer concludes that "sous la plume de Witelo le mot 'forme' doit etre identifie avec la species baconienne . . . dans toute la vaste eten- due de ce term chez Bacon."86

It is evident that apart from the question of visual rays, Alhazen's theory of vision and views on the nature of light were transmitted intact to the West. I do not, of course, claim that the concept of species is in all respects identical to Alhazen's concept of form; the former, for example, has meta- physical implications missing from the latter, and Vescovini has called atten- tion to subtle differences in the associated psychologies of perception.87 My point is simply that the two concepts function identically as a basis for a theory of vision and that the theories of vision built upon them by Bacon, Pecham, and Witelo (on the one hand) and Alhazen (on the other) are substantially the same. Bacon, Pecham, and Witelo initiated a Western optical tradition that faithfully transmitted the essence of Alhazen's achievement in optics to Kep- ler and his seventeenth-century contemporaries.

85 Furthermore, the species issuing from the Bridges, Vol. II, pp. 50-51. observer's eye perform none of the functions 86 "ltudes sur Witelo, II," Bulletin inter- that Alhazen denied to visual rays in his refu- national de l'Academie polonaise des sciences tation of the emission theory, as Bacon himself et des lettres, 1920, p. 356. notes in the Opus majus, V, 1, Dist. 7, Ch. 3, 87 Studi, Chs. 4, 7.

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