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

one of these objects, put one end of this ruler close to the lower eyelid of one eye and the other end close to the surface of the object, cover the other eye, and, while in this condition, look through the opening in the tube: he will see that part of the object which is opposite the opening of the tube at the end of the ruler. If he covers the opening of the tube with an opaque body, that part of the object will be screened off which he has seen through the opening. Upon removing the cover, he will perceive the same part as he did at first. If, by means of the opaque body, he covers any part of the opening, then there will be screened off only that portion of the visible part situated opposite the covered part of the opening, namely the portion that lies on a straight line with
7a

the eye and the screening body 1: The author uses the term "screening body" to describe the object placed between the observer and the viewed target. — this straightness being secured by the ruler and the straightness of | the tube. For the portion of the visible part which is screened off when a part of the opening is covered always lies together with the eye and the covered part of the opening in a line parallel to the straight line extending along the middle of the ruler's surface parallel to its length. When the cover is removed, the eye will again perceive that same portion of the visible object. That is always found to be so, with no variation or change.

[8] Now when the observer looks at the visible object through the opening in the tube while the ruler lies between the eye and the object, and the opening is obstructed so as to hide that part of the object's surface which the eye formerly perceived, there will exist in this situation between that part of the visible object and the surface of the eye a continuum of air that is free from opaque bodies and an infinity of non-rectilinear distances. For open air exists between one end of the tube and the eye, and likewise between the object and the other end of the tube. But the continuum of air that exists between the eye
7b

and the object is not in this case | rectilinear. And of all the lines that can be imagined between the eye and that part of the visible object, only the straight lines have in this case been interrupted. Thus, if it were possible for sight to perceive an object existing with it in the same atmosphere through non-rectilinear lines, then it would perceive that part of the object opposite the tube's opening after the opening has been obstructed. But we find, when such an object is experimentally examined and observed in the manner we have described, that it ceases to be visible upon closing the opening.

[9] It follows from this experiment, with a necessity that dispels doubt, that sight does not perceive any visible object existing with it in the same atmosphere, this perception being not by reflection, except through the straight lines alone that can be imagined to extend between the surface of the object and the surface of the eye.

[10] Again, we find that sight does not perceive any visible object unless there exists in the object some light which the object either possesses of itself
8a

or which radiates upon it from another object. If | the object is dark and has no

light whatever in it, it will not be perceived or sensed by sight. We also find that when the eye is in a dark place it perceives the objects facing it if they are illuminated by some light and if the intermediate atmosphere is continuous and uninterrupted by any opaque body. If the object is in a dark place that has no light, and the eye is situated in an illuminated place, then that object will not be perceived or sensed by sight. And we find this state of affairs to be uniform and without variation or change. This therefore proves that if the object has some light in it, and it is one of the possible objects of visual perception, and if the light in it is up to the limit that may be perceived by sight, then sight will perceive that object whether or not the air surrounding the eye is illuminated by a different light from that which is in the object.

[11] Further, we find that sight does not perceive any visible object unless the object is of a certain size (by 'size' I mean the measure of the object, be it a
I 8b

body, a surface | or a line), and it does not perceive extremely small objects. It is discovered by reasoning that there exist small bodies which cannot in any way be perceived by sight. For the pupil of a mosquito's eye and similarly small things are not in any way perceptible by sight, even though they are existing bodies. The smallest magnitudes that can be perceived by sight are also related to the strength or weakness of sight. For some small bodies are perceived and sensed by some people but cannot in any way be seen by many others whose sight is not very strong. When all visible objects, including the smallest, are experimentally examined, they are found to be not extremely small. Rather, for any visible object, even a very small one, it is possible to find among existing bodies one which is smaller than that object and which is not
I 9a

sensible to sight. This proves that no visible object is perceptible by sight unless it has a certain size or [it is something] belonging to an object of a certain size, such as colour, shape and the like. | And, therefore, the smallest magnitudes that can be perceived by sight are related to the power of sight.

[12] We also find that sight does not perceive any visible object unless the object is opaque or has some opacity in it. For when the body is extremely transparent (such as rarefied air) sight does not perceive it but perceives what is behind it. Sight does not sense a transparent body unless it is denser than the intermediate air between itself and the eye. But every opaque body has a colour or something like colour, 1: Ibn al-Haytham identifies color as an essential property of opaque bodies required for visual perception. such as the light of the stars and the forms of [self-]luminous bodies. Similarly, no transparent body with any opacity in it can be devoid of colour.

[13] Moreover, we find that when sight perceives some visible object, then moves a considerable distance away from it, the object ceases to be perceived. And we find that when sight moves so far from the object that the object ceases to be visible, it is still able to perceive from the same distance (unless it is too far) another object of a greater size than that of the invisible object. This