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

will not be able to perceive through the holes anything more or less than the same circle on the wall.

[68] When the wooden block has been precisely placed and perfectly and securely mounted in its hole, the experimenter should bore a round hole in the white wall through the same circle drawn on it so as to lead into the chamber behind this wall. Let the circumference of the hole be the same as that of the circle drawn on the wall’s surface, and let the hole extend in the shape of a cone into the body of the wall, becoming wider as it goes deeper inside. Having made this hole, the experimenter should cover it with an opaque, pure white body, such as a white cloth or stone or sheet of paper. Let this body be not smooth; let it cover the entire hole and let its surface be level with the wall's surface.

I 4b

[69] The experimenter should then watch for the morning light. When daylight shines and the light on the exposed white wall becomes strong, but before the wall is irradiated by sunlight, let him enter the chamber having the two holes, close the door and draw over it a thick curtain so that no light will enter through the door or through gaps in it. | He should then cover the inclined hole so that no light will remain in the chamber other than that which enters through the perpendicular hole. Then, opposite this hole, let him hold an opaque, pure white object. He will find some light on it according to the strength of the light that is on the white wall and on the white body covering the hole. He will also find that the visible light on the opaque object is circular and that it diverges in the same way as the essential light issuing from self-luminous bodies original: "essential light issuing from self-luminous bodies" refers to light originating from a source like the sun or a fire, as opposed to "accidental light" reflected from an illuminated surface and passing through cylindrical holes.

I 5a

[70] If, from a point in this light that appears inside the chamber on the opaque object, the experimenter looks towards the white wall, he will see only the white body covering the hole in that wall. When this light has become manifest to the experimenter, let him remove the white body covering the hole and close the door of the chamber into which this hole leads. Then the light which appeared inside the chamber on the opaque object, and which entered through the perpendicular hole, will vanish and nothing of it will be visible. If any light should appear on this object, | this will be according to what may be emanating from the light that reaches the interior of the perpendicular hole.

[71] If any light appears in this case on the opaque object inside the chamber with the two holes, the experimenter should paint black the interior surface of the perpendicular hole original: "the perpendicular hole" refers to the specific tube/aperture used for the experiment (by means of which he is examining the light) so that no visible light will emanate from it to the interior of the chamber next to it. This having been done, no light will appear on the opaque object confronting the perpendicular hole if the white body that covered the opposite hole is removed.

[72] When the light that appeared on the opaque object facing the perpendicular hole disappears upon the removal of the illuminated body covering the opposite hole, the experimenter should replace this white body, thus covering again the hole in the wall. Then the light will again appear on the opaque object inside the chamber, as it did in the former case.

I 45b

[73] It is therefore clear from this experiment | that the light that passes through the perpendicular hole and appears on the opaque object reaches the latter only from the accidental light term: accidental light light reflected from an illuminated surface rather than originating from a self-luminous source on the opposite white body that covered the opposite hole.

I 46a

[74] Now at the time when this body is removed, and the opposite hole open, and no light appears on the opaque object inside the chamber, there exist through the continuous air many curved and sinuous intervals uninterrupted by any opaque bodies, [all of which lie] between, on the one hand, the opaque body inside the chamber — from which the light has disappeared — and, on the other, the rest of the white wall which is wholly exposed to the light and many [other] illuminated walls and the whole illuminated atmosphere. Only the place directly opposite the perpendicular hole has changed. Nevertheless, the light will fail to appear inside the chamber while the hole in the wall remains open and there is no illuminated opaque body directly facing the perpendicular hole. If the white body is replaced so as to cover the outside hole, the light will appear on the object | inside the chamber.

[75] Now let the experimenter turn to the straight interval between the perpendicular hole and the hole in the wall, and interrupt it with an opaque, pure white body at any point he chooses outside the hole: if light radiates on this body, then it will appear on the object inside the chamber. Then let the experimenter turn to the straight interval between the extremity of the perpendicular hole inside the chamber and the object on which the light appears, and interrupt this interval with an opaque object at any point he chooses: the light will vanish from the first object and appear on the second.

I 46b

[76] Therefore, from considering the appearance of the light on the opaque object inside the chamber while the body that shines with the accidental light is fixed at the wall-opening, and the disappearance of the light from this opaque object upon removing the illuminated body at the opening, it is manifest that the light that appears inside the chamber on the opaque object facing the perpendicular hole while the illuminated body is fixed at the opposite opening, | reaches the opaque object only from the accidental light in the illuminated body that is fixed at the opening, and that in this case no other light reaches it.

[77] That light emanates from the accidental light only in straight lines is again manifest from considering the following: that the light appears on the opaque object inside the chamber when the illuminated body is directly facing