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

[23] Also, if the object moves farther and farther away, gradually in an orderly manner, it is found that it is perceived as progressively smaller in its entirety until it disappears altogether. And if it continues to move away, it will eventually reach the limit at which it completely disappears so that neither it nor any part of it will be sensed by sight. If it moves farther still, sight will not perceive it.

[24] Again, if the visible object closely approaches the eye, without actually coming into contact with the surface of the eye, it is found to grow in size. Its form becomes indistinct and the minute details of it are so combined that sight fails to discriminate between or identify them. As it approaches the surface of the eye after this condition is reached, it becomes more and more confused, until it comes into contact with the surface of the eye and sight ceases to sense it and perceives its covering effect only.

[25] All that being the case, the distance from which sight properly perceives a visible object is therefore not a single, determinate distance: and the distance at which the form of the object becomes indistinct and its small parts and subtle features become inapparent, indistinct and confused, is not a single, determinate distance. Let us call "moderate distances" all those distances (which are many and variable within a certain range) from which sight perceives the visible object and all those of its parts and properties that can be perceived by sight—this perception of the object and of its properties being such that between it and the real nature of the object and of its properties there exists no appreciable discrepancy, and such that the object’s form produced in the sense-faculty is not so different from its real form as to show an appreciable discrepancy when contemplated and scrutinized by that sight itself 1: The author emphasizes the reliability of vision within a specific range of distance.. And let us call "immoderate distances" those distances at which the visible object disappears, and those at which there disappear those parts of the object that bear an appreciable ratio to the whole object, and the distances at which there disappear those subtle features of the object that may be visible from the moderate distances, and also the distances at which these features become confused and indistinct—regardless of whether these distances are exceedingly far from the eye or exceedingly near to it.

[26] It is thus evident that sight does not perceive any visible object unless the object has some light in it either from itself or from another object; and that the light of many visible bodies appears on the bodies situated opposite them and that their light appears on the eye that perceives them. We must now inquire into the properties of lights and into the manner of their radiation, and

further inquire into the effect of light upon sight; we must subsequently add to this what pertains to the eye, and by careful reasoning work our way to the conclusion.

CHAPTER 3

INQUIRY INTO THE PROPERTIES OF LIGHTS

AND INTO THE MANNER OF RADIATION OF LIGHTS

14a

[1] We find that the light of every self-luminous body radiates on every body opposite to it when there is not between them an opaque or non-transparent body that screens one from the other. For when the sun faces a body on the ground that is not screened from it, its light shines upon that body and is visible, and it simultaneously irradiates every place in all parts of the earth that face it at that time. It is similarly the case with the moon 1: The author identifies the moon as a body that reflects solar light., and also with fire: when the latter lies opposite an opaque body and there is no opaque screen between them and the intervening distance is not excessively large, the light of the fire will radiate on that body and its form will be visible. Again, the light of a fire-brand is found to radiate simultaneously on all bodies surrounding that fire on all sides, and on all opaque bodies above or below it, provided that they are not hidden from it by a screen and their distances are not too large
14b

— whether the fire-brand is small or large, so long as its light is visible on the opaque bodies that face it.

[2] We also find that the radiation of all lights takes place only in straight lines and that no light radiates from a luminous object except in straight lines—provided that the air or transparent body between the luminous object and the body on which the light appears is continuous and of similar transparency.

[3] When this state of affairs is examined at all times it is found to be uniform, suffering no variation or change. This becomes clearly apparent to sense if one examines the lights that enter through holes, slits and doors into dusty chambers. As for the light of the sun, when it enters through a hole into a dark chamber the air of which is cloudy with dust or smoke, the light will
15a

appear to extend rectilinearly from the hole through which the light enters to the place on the chamber’s floor or walls which that light reaches. If the air in the chamber is clear and pure and the extension of the light through it is not visible, and if an experimenter wishes to examine the interval through which the light extends, then let him take an opaque body and, approaching the hole and the place on the chamber’s floor or walls where the light is, let him intercept it by the opaque body: he will find that the light will appear on that opaque body and vanish from the place where it showed on the chamber’s floor or walls. If he approaches any position he