كتاب المناظر (The Optics of Ibn al-Haytham, Books I-III)

grows, views diverge, opinions vary, conclusions differ and certainty becomes difficult to obtain.

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[2] Our subject is obscure and the way leading to knowledge of its nature difficult; moreover, our inquiry requires a combination of the natural and the mathematical sciences. It is dependent on the natural sciences because vision is one of the senses and these belong to natural things. It is dependent on the mathematical sciences because sight perceives shape, position, magnitude, movement and rest, in addition | to its being characterized by straight lines; and since it is the mathematical sciences that investigate these things, the inquiry into our subject truly combines the natural and the mathematical sciences.

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[3] Natural scientists have inquired into the nature of this subject according to their art¹ and exerted themselves in it as much as they could. The learned among them settled upon the opinion that vision is effected by a form which comes from the visible object to the eye and through which sight perceives the form of the object. Mathematicians, for their part, have paid more attention to this science¹ than others. They have pursued its investigation, paying attention to its details and divisions.² They have distinguished objects of vision, assigning causes to their particular properties and stating reasons for each of them. All the same, they have continued throughout the ages to disagree about the principles of this subject, with the result that the opinions of the various groups among the practitioners of this art¹ have gone different ways. But for all the disparity in their ranks, their different epochs and the divergence of their views, in general they agree that vision is effected by a ray which issues from the eye to the visible object and by means of which sight perceives the object; that this ray | extends in straight lines whose extremities meet at the centre of the eye; and that each ray through which a visible object is perceived has as a whole the shape of a cone the vertex of which is the centre of the eye and the base is the surface of the visible object. These two notions, I mean the opinion of the physicists and that of the mathematicians, appear to diverge and contradict one another if taken at their face value.

[4] Mathematicians, moreover, differ about the structure of this ray and about the manner of its production. Some take the view that the radial cone is a solid body, continuous and compact. Others think that the ray consists of straight lines which are fine bodies the extremities of which meet at the centre of the eye and divergently extend until they reach the visible object; and that sight perceives those parts of the surface of the object which the extremities of these lines encounter, whereas the parts of the object’s surface that fall between those extremities are not perceived. Thus it comes about that the extremely small parts and minute pores in the surfaces of visible objects are invisible. Again, a group among those who believe the radial cone to be solid

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and compact thinks | that the ray issues from the eye in one straight line until it reaches the object, after which it moves extremely quickly over the length and breadth of the surface of the object — so quickly in fact that the movement is imperceptible — and through this movement the solid cone is produced. Another group believes the matter to be different and that when the eyelids open in front of an object, the cone is immediately produced, all at once, in no sensible time. A group from among all of these thinks the vision-producing ray to be a luminous power which issues forth from the eye to the visible object, and that sensation is brought about by that power. Another group is of the opinion that when the air comes into contact with the eye it receives from the eye only a certain quality which immediately turns the air into a ray through which sight perceives the visible objects.

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[5] Each of those groups was led to its belief by reasonings, arguments, methods and evidence of its own. But the settled view of all those who have inquired into the manner of visual sensation | divides on the whole into the two contrary doctrines which we mentioned earlier. Now, for any two different doctrines, it is either the case that one of them is true and the other false; or they are both false, the truth being other than either of them; or they both lead to one thing which is the truth. In the latter case each of the groups holding those two doctrines would have failed to complete its inquiry and, unable to reach the end, has stopped short of it. Alternatively, one of them may have reached the end but the other has stopped short of it, thus giving rise to the apparent difference between the two doctrines, although the end would have been the same had the investigation been pushed further. Disagreement may also arise in regard to the subject of an inquiry as a result of a difference in methods of research, but when the inquiry is rightly conducted and the investigation intensified, agreement will emerge and the difference will be settled.

[6] That being the case, and the nature of our subject being confused, in addition to the continued disagreement through the ages among investigators who have undertaken to examine it, and because the manner of vision has not been ascertained, we have thought it appropriate that we direct our attention to this subject as much as we can, and seriously apply ourselves to it, and examine it, and diligently inquire into its nature. We should, that is, recommence the inquiry into its principles and premisses, beginning our investigation with an inspection of the things that exist and a survey of the conditions of visible objects. We should distinguish the properties of particulars, and gather by induction what pertains to the eye when vision takes place and what is found in the manner of sensation to be uniform, unchanging, manifest and not subject to doubt. After which we should ascend in our inquiry and reasonings, gradually and orderly, criticizing premisses and exercising caution in regard