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

to conclusions, our aim in all that we make subject to inspection and review being to employ justice, not to follow prejudice, and to take care in all that we judge and criticize that we seek the truth and not be swayed by opinion. We may in this way eventually come to the truth that gratifies the heart and gradually and carefully reach the end at which certainty appears; while through criticism and caution we may seize the truth that dispels disagreement and resolves doubtful matters. For all that, we are not free from that human turbidity which is in the nature of man; but we must do our best with what we possess of human power. From God we derive support in all things.

I 4b

[7] We divide this work into seven Books. 1: This refers to the seven books of the Kitāb al-Manāẓir. In Book I we show the manner of vision generally. In Book II we detail the visible properties, their causes and the manner of their perception. | In Book III we show the errors of sight in what it perceives directly, and their causes. In Book IV we show the manner of visual perception by reflection from smooth bodies. In Book V we show the positions of images, namely the forms seen inside smooth bodies. In Book VI we show the errors of sight in what it perceives by reflection, and their causes. In Book VII we show the manner of visual perception by refraction through transparent bodies whose transparency differs from that of air. And with the end of this Book we conclude this work.

[8] We formerly composed a treatise 1: A reference to an earlier, less mature work by Ibn al-Haytham. on the science of optics in which we often followed persuasive methods of reasoning; but when true demonstrations relating to all objects of vision occurred to us, we started afresh the composition of this book. Whoever, therefore, comes upon the said treatise must know that it should be discarded, for the notions expressed in it are included in the content of the present work.

CHAPTER 2

INQUIRY INTO THE PROPERTIES OF SIGHT

[1] We find that sight does not perceive any visible object unless there is some distance between them. For when the object is in contact with the I 5a surface of the eye it is not perceived by sight, even though it is a proper | object of visual perception.

[2] And we find that sight does not perceive any of the visible objects that are situated with it in the same atmosphere, and are not perceived by reflection, unless the object is placed opposite the eye; and provided that between each point on the perceived surface of the object and the surface of the eye a straight line (or lines) can be imagined; and provided that there does not intervene between the surface of the eye and the object any opaque body that interrupts all the straight lines imagined to lie between the surface of the eye and the perceived surface of the object.

I 5b

[3] Further, for any seen object that is situated with the eye in the same air and is not perceived by reflection, we find that if all the straight lines imagined between the surface of the eye and the perceived surface of the object are interrupted by an opaque body, then the object will be concealed from the eye and cease to be perceptible, even though a continuum of air free from opaque objects may still exist between the eye and the object, provided that this continuity is not rectilinear. | Then, if the opaque screen is removed, the sight will perceive the object.

[4] Suppose now that the screen intersects all straight lines between any part of the surface of the object and the surface of the eye, so that every straight line between that part of the object and the point on the surface of the eye through which vision occurs is interrupted by the screen. Then only that part of the object will disappear which is such that the straight lines between it and the point of vision on the surface of the eye have all been interrupted by the screen.

[5] If a survey is made of all visible objects at all times, and if they are experimentally and accurately examined, they will be found to be uniformly as we have described them, with no variation or change. This therefore proves I 6a that for every seen object that exists with the eye in the same atmosphere, and is not perceived by reflection, there exists between each point on the seen surface of the object and a certain point or multiplicity of points on the surface of the eye a straight line or lines which are not interrupted by any opaque body.

[6] An accurate experimental examination of this fact may be easily made with the help of rulers | and tubes. Let the experimenter who wishes to make such an examination [proceed as follows]. Take a very sound and straight ruler and draw along the middle of its surface a straight line parallel to its sides. 1: Ibn al-Haytham describes a rigorous experimental procedure to prove the rectilinear propagation of light. Take a hollow cylindrical tube, very straight in length, perfectly round and ending in parallel circles; let its thickness be the same throughout and let it be fairly wide but not wider than the eye socket; draw on its outer surface a straight line extending from a point on the circumference of one base to the opposite point on the other side; and let this tube be a little shorter in length than the ruler. Divide the line along the middle of the ruler into three parts, and let the intermediate part be of the same length as the line on the surface of the tube; the remaining parts on either side may be of any length. Attach the I 6b tube to the surface of the ruler, placing the line on its exterior upon the intermediate segment of the line in the middle of the ruler's surface; | and make sure that the ends of the tube coincide with the points marking off the middle segment. The tube should be so closely and firmly fastened that it cannot be loosened or displaced.

[7] When the instrument has been perfectly prepared and the experimenter wishes to examine the perception of visible objects by sight, he should aim at