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

it, being either at the opening in the wall or at any point on the straight interval between the two holes outside the chamber; that the light on the opaque object inside the chamber disappears upon the opposite illuminated body being removed, though the light remains on the rest of the wall and in the whole air which is illuminated by daylight and which is continuous with the air in the hole, and on many [other] illuminated walls between which and the perpendicular hole there exists a continuum of air.

7a

[78] For, between, on the one hand, the opaque object on which the light shows inside the chamber, and, on the other, the rest of the white wall illuminated by daylight and many [other] illuminated walls | and the air illuminated by daylight, there exist infinitely many distances, curved and sinuous and arc-shaped, which are continuous between these places and the opaque object inside the chamber, and extend through the intervening continuous air. And, upon removing the illuminated white body opposite the hole, there disappears only the light at the extremities of the straight intervals between the object inside the chamber and the light.

[79] Furthermore, if the experimenter observes the light that appears on the opaque object inside the chamber while the object faces the illuminated white body, he will find it weaker than the accidental light on the opposing body outside. Then if he moves this opaque object away from the hole along the line of opposition he will find that the light visible on it weakens, and as the object recedes from the hole this light weakens progressively.

7b

[80] Having tried all these things, the experimenter should now plug the perpendicular hole, open the inclined hole, blacken | its interior surface, original: "blacken its interior surface" to prevent internal reflections and confront it with the opaque object. He should also cover the opening in the white wall with the white body. He will find that the light appears on the opaque object inside the chamber.

[81] Again, if he interrupts the straight interval between the opening in the wall and the inclined hole with the white body at any point he chooses on this distance, while this white body is irradiated by light, he will find the light visible on the opaque object inside the chamber. Then if he removes the illuminated white body facing the inclined hole outside, the light will vanish from the opaque object; no light will be visible on it. Upon the white body being returned to the opposite opening or to the straight interval between it and the inclined hole, the light will again be visible on the opaque object inside the chamber — as was the case with the perpendicular hole. It is therefore evident from this experiment that the light which appears inside the chamber 8a on the opaque object facing the inclined hole | reaches that object only in a straight line and only from the opposing [white] body.

[82] If the experimenter, while still examining the inclined hole, moves the opaque object inside the chamber away from the hole, he will find that the

light appearing on that object weakens. And as the object is moved farther from the hole the light appearing on it becomes weaker.

I 48b

[83] The experimenter should then open both the perpendicular and the inclined holes at the same time and confront each of them with a white, opaque object — having covered the opening in the wall with the [other] white body. He will find that the light is visible at the same time on each of the two objects facing the perpendicular and the inclined hole. But it has been shown that, when the illuminated body is fixed at the opening, the light comes to these two places only from that body, when the intervening air between it and each of the perpendicular and inclined holes is continuous and uninterrupted by any opaque bodies. Therefore it is clear from this experiment that the light which simultaneously appears | in both places inside the chamber reaches them only from that opposing illuminated body which is in the opening.

[84] Similarly, if the experimenter bores a number of holes in the inserted wooden block — each facing the opening in the white wall and in the ratio mentioned earlier refers to the previous page's mathematical geometric ratio — and confronts all these holes, while open, with a large opaque object, he will at the same time see as many lights on this object as there are holes, and each light will be directly opposite the illuminated body in the outside opening. From this experiment it is therefore clear that from that body illuminated by daylight, light rectilinearly radiates in all directions, and that this radiation always takes place as long as that body is illuminated.

I 49a

[85] Having established this property of daylight the experimenter should now observe the [same] place when sunlight shines on that wall and examine it in the foregoing manner. He will find the case of sunlight to be the same as that of daylight, except that | the light coming from the sun’s light will be stronger and clearer.

[86] Similarly, if he examines moonlight he will find it to be of this description; original: "of this description" implies it follows the same laws of rectilinear propagation and accidental reflection and, again, if he examines the light of fire he will also find it to be of the same description. To examine the light of fire let him obtain a strong fire and place it opposite the white wall so as to illuminate this wall. Let him cover the opening in the wall with the white body as before, and close the door to the chamber that has the two holes. Let no light remain in the chamber. If he examines the light of the fire in the foregoing manner, he will find that the radiation from the light of the fire which appears on the body covering the opening behaves in the same way as the radiation of the [other] lights — differing only in respect of strength and weakness.

[87] From all these experiments it is therefore clearly evident that from the accidental lights in opaque bodies light radiates in all facing directions; that the radiation of light from them takes place only in straight lines; that the light emanating from the accidental light is weaker than that accidental light; and that the emanating light becomes weaker as it goes farther.