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

higher hole and the corresponding point on the outer end of the facing hole. Pass the thread through the first hole and nail down its end outside this higher hole. The experimenter should now enter the western chamber and extend the thread to a point on the ground or on a wall of the chamber. He should subsequently stretch the thread so tightly as to make it exactly straight and in place. This done, he should mark the position of its extremity in the chamber. This place, then, will be in a straight line with the straight interval extending from the first, higher hole to the lower hole facing it. The thread should now be taken out of this hole and put through the other hole, and the same things should be done with it as before. Stretch it to another place in the chamber and mark the position of its extremity. This second position is therefore in a straight line with the straight interval also extending from the first, higher hole to the second hole.

31a

[31] When these two positions in the chamber have been determined, let the experimenter choose a pitch dark but clear night. After nightfall let him enter the chamber and close the door, thus excluding all light from the chambers. Both chambers will then be dark. Let him then enter the western chamber and look into one of the two holes so as to see the sky through the higher hole. He should make sure that none of the large stars whose light may be seen on the earth’s surface is facing the hole. If such a star is there he should wait until it no longer faces the hole. He should also look into the other hole so as to see the sky through the higher hole when there is no large star facing it. He will see the atmosphere from these positions to be dark. Let him then inspect the places he has determined in the chamber opposite the holes. He will find them dark. No light will be visible in them, and the whole chamber will be dark, except for some extremely weak and negligible light from the sky.

31b

[32] He should then wait | until morning. When dawn breaks he should look through the opposing holes until he sees the atmosphere illuminated and, moving from his position so as not to be in front of the hole, look at each one of the places he has determined opposite the holes. He will find them illuminated with a faint light proportionate to the strength of the light in the atmosphere. If no light is visible in the chamber he should wait a while until morning light grows stronger and then look at those places: he will find them illuminated, and the light in each of them will be circular and wider than the hole to the extent required by the expansion of the light. But he will find no light in the rest of the chamber at this time. Whatever light he may find will be weak and perceptible only in proportion to what may be emanating from the light visible in the two spots opposite the holes. Then when he screens one of the two holes, the light will depart from the place facing it, though the light from the other will remain. When he removes the screen the light will return to that place.

I 32a

[33] Let the experimenter then turn to the straight interval | between one of the two holes and the place to which the light proceeds from that hole, and interrupt it with an opaque body. He will find that the light will appear on the opaque body and disappear from the place opposite that hole. If he then moves this body along the straight interval, he will find the light to be always on it. (This interval can be determined with the help of a straight rod or a long ruler by attaching the ruler to the circumference of the hole and thus determine the straightness of the interval with it.) If he then removes the opaque screening body, the light will return to its former place. Similarly, if, in the first chamber, he interrupts the straight interval between the first, higher hole and one of the two holes, he will find that the light will appear on the body interrupting that interval and disappear from the second chamber; and upon removing the screen the light will return to its place. Again, if he examines in both chambers the light passing through the other hole he will find it to be of this description. Now if he makes several holes in the second wall, making sure that each of them directly faces | the first hole (as described in the case of the two former holes) he will find in the second chamber separate lights equal in number to those holes, each of them being directly opposite the first, higher hole.

I 32b

[34] Thus it is clearly proved by this experiment that some light proceeds from the atmosphere illuminated by the morning light to opposite places; that it so proceeds in straight lines; and that the daylight radiating upon the earth before sunrise and after sunset is but a light radiating upon it from the atmosphere illuminated by the sunlight opposite the earth’s surface. If the experimenter also examines in the same manner the luminous atmosphere during the rest of the day, he will find that the light radiates from it in straight lines.

[35] But if some light emanates from the illuminated air to the opposite places, then from every part of the air that is illuminated by any light whatever some light emanates in all directions; and the light emanating from the air will be weaker than that existing in the air;
I 33a

and the strength of the light emanating from the air | will be proportionate to the light existing in it and to the magnitude of that illuminated part of the air. Furthermore, the air inside the second and first chambers is continuous through the first hole with the outer illuminated air from which the light has entered into the second chamber. Now between the illuminated air and the air in the second chamber there exist many curved and sinuous intervals passing through the holes and uninterrupted by any opaque bodies. Thus if one of the two holes is stopped so that the light opposite this stopped hole disappears, then between, on the one hand, the place where the light has vanished and, on the other, the first hole and the outside air, air will continuously extend in many non-rectilinear lines through