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

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[15] The case of the moon is more manifest. For the light of the crescent moon is visible on the earth’s surface on the second night of the month and on following nights. And, especially when the moon faces a dark place, its light appears in that place though it is still incomplete and faint. Its light then grows every night with the increase of its magnitude until it is full. When this happens its light is found to be stronger than on previous nights. Again, the case of the full moon at rising and setting is similar to that of the sun, and the same is true of the moon at eclipses I 23b when these extend beyond its centre but do not cover the whole moon. Also, if the moonlight that has passed through tiny holes is tested when the moon is full, it is found to expand, and as it recedes from the hole it grows wider. It therefore appears from this expansion that moonlight radiates from every part of the moon and not from a particular part of it, and that the extension of the light of the moon can take place only in straight lines.

[16] This same property also holds for fire. For when a fire is divided into parts by dividing the subject sustaining it, some light will radiate from each of these parts, and the light of each part will be found weaker than that of the whole fire, and the light of a smaller part will be found weaker than that of a larger part. The parts of the fire may also be tested without being divided. To make such a test take a fairly wide copper sheet and make a fairly large circular hole in it; b slide through this hole a well-straightened cylindrical tube of regular circularity and convenient length; let the width of the hole and that of the tube be of the same magnitude and let the tube’s aperture not exceed the thickness of a needle; insert the tube into the hole in the sheet so that its end may be level with the sheet’s surface; attach this sheet to some object at a point above the ground, and let it stand vertically on its edge. Now, in the darkness of night, bring a flame to the vicinity of this sheet and let it be that of a lamp with a broad, bright wick. Hold the flame opposite the hole, then move it closer to the hole until it is so near that no measurable distance exists between them. The area on the side of the tube will then be shaded by the sheet. Let no light be present save the flame being tested, and let this experiment be carried out in a place unswept by winds. Hold an opaque body opposite the end of the tube. The light of the flame will appear on that body. But no light is available except that which has passed through the tube; and no light has passed through the tube a except the light of that part of the flame opposite the tube’s aperture; its area is equal to that of the tube’s aperture. For light proceeds only in straight lines, and no uninterrupted straight lines exist between the light appearing on the body at the end of the tube and any part of the flame other than that opposite the other end of the tube. For the straight lines between this part and the visible light extend inside the tube without the interruption of any opaque body. As for the remaining parts of the body of the

flame, light will proceed from them only to the adjacent end of the tube’s aperture; so that if any of this light enters the end of the tube it will be interrupted by the tube’s wall and abolished and will not pass through the length of the tube. In this case, then, only the light of the part opposite the tube’s end will pass through the length of the tube’s aperture.

[17] The experimenter should then gently move the flame so that another part of it may face the hole, and then inspect the body opposite the end of the tube on which the light is visible. He will find that the light is still visible on that body. If he then moves the body of the flame in all directions, raising and lowering it so that the hole may face one part of the flame after another, he will find that the light appears in all cases on the body opposite the tube. He will also find this light to be weaker than the light of the whole flame when it shows on bodies exposed to the whole bulk of the flame at a distance equal to that between the flame and the place where the light that has passed through that body appears. Let the experimenter narrow the hole by sliding a thin straight body into the tube, thus partly obstructing it, and let him fix this body to the tube’s interior surface. If he tests the light coming through the rest of the tube, he will find it still visible on the body opposite the tube, unless the remaining part of the tube is too narrow. He will also find that the light that appears when the tube is made narrower is smaller and also less visible and weaker than the former light. I 24a Therefore, it appears from this experiment that light radiates from each part of the fire; that the light from a whole fire-brand is stronger than that from a part of it; and that the light from a greater part is stronger than that from a smaller part.

[18] Again, let the experimenter fix the flame close to the hole in the sheet so that it will not move and so that the same part of it will remain opposite the hole; let him then incline the tube so that it will be in an oblique position to the surface of the sheet while its end remains attached to the hole; he should plug any gap (if such appears) at the end of the tube or at the hole in the sheet at its rear; and let him hold the opaque body opposite the tube. He will find that light appears on the opaque body. If he alters the position of the tube by inclining it to another side, and in front of it holds the opaque body on which the light may appear, he will find that the light is still visible on it. By inclining the tube in all directions he will find that the light proceeds from that part of the flame to all sides directly opposed to it. If he then moves the flame I 24b so that another part of it will be opposite the hole, and tests that part too at those inclined positions in which the first part was tested, he will find that the light also proceeds from this part to all opposite sides. If he similarly tests every part of the flame he will find it to be of this description. It appears from this experiment that the light radiates from each part of the flame to every side directly opposed to that part.