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

chooses on the straight line between the hole and the place where the light appears, and intercepts the interval with the opaque body, I 15b the light will appear on that opaque body and vanish from the place in which it formerly appeared. (The straightness of this interval can be tested with a straight rod.) This state of affairs thus shows that the light that entered through the hole extends in a straight line between the hole and the place reached by the light. If the experimenter examines any interval he chooses among the crooked, bent or curved intervals between the hole and the place where the light appears, intercepting it by the opaque body, no light will appear at any point in that interval. It is so with minute holes in opaque bodies. When sunlight irradiates such bodies, it passes through their tiny holes, extending in straight lines. If one tests the straight distance between the tiny hole and the place where the light from the hole appears, the light will be found to extend the whole length I 16a of that straight interval, even if the hole is very small. Let an experimenter take an opaque body and, having made a minute hole in it, let him hold it opposite the body of the sun: he will find that the light goes through the hole, extending on a straight line. If he tests the interval 1: A gap or span of space. on which the light just described has extended by applying a ruler to it, he will find it to be perfectly straight. It is therefore clear from all this that the light of the sun only extends along straight lines.

[4] Similarly, if the light of the moon is tested, it will be found to be of this description. And similarly with the light of the stars: for, in a moonless night, let any of the large stars (such as Venus, or Jupiter at its nearest position to the earth, or also Mars at its nearest position, or Sirius) be opposite a hole giving into a dark chamber: its light will appear in the chamber and will be found opposite the hole. If the observer places his eye in that light and looks towards the hole, he will then see the star facing him. If he observes I 16b the star for some time until it has moved through an appreciable distance, its light in the chamber will be found to have moved from its former place so as to be rectilinearly opposite the star. And as the star moves, that light will move, and the light and the hole and the star will always be found to lie on a straight line.

[5] Then if, with the aid of an opaque body, the experimenter tests the light from the star that appears at the place opposite the hole in the manner we have shown before, by intercepting the straight distance between the place in which the light appears and the hole through which the light enters at any point he chooses on that distance, the light will appear on the opaque body and will vanish from the place in which it previously appeared.

[6] Similarly, if there is a fire facing a hole that leads into a dark chamber, the light of that fire will appear in the chamber opposite the hole. And if one I 17a tests the straight interval between the light and the hole in the way we have mentioned, the light of the fire will be found to pass through every point on it.

The light of the fire may also be tested with a straight rod, provided that the interval between the fire and the hole is short and the interval between the hole and the place where the light appears is also short. For if a straight rod is inserted in the hole through which the light has entered and one end is placed at the point of visible light, its other end will be found at the fire or in a straight line with it, so that the fire, the hole and the light that appears in the chamber after it has entered through the hole will always be found on a straight line.

[7] This property also becomes manifest from the shadows of all kinds of light. For when erect opaque objects are irradiated with light (and) their shadows appear on the ground or on the opaque bodies opposite them, these shadows are always found to extend rectilinearly, and the shadowed regions I 17b are found to be those whose straight distances from the luminous body (the light of which has been cut off from those places) have been intercepted by the objects casting the shadows.

[8] It thus appears from all that we have said that the lights from self-luminous bodies can radiate only in straight lines.

[9] We also find that light radiates from every part of every self-luminous body. And we find that the light that radiates from the whole luminous body is stronger than that which radiates from a part of it. And we find that the light that radiates from a larger part is stronger and more manifest than that which radiates from a smaller part. With regard to the sun, when it begins to rise above the horizon, only a small part of its circumference appears at first, and yet the light of that part radiates upon all facing walls and objects and parts of the earth's surface, while at this moment the centre of the sun is hidden below the horizon and concealed from anything on the earth's surface. Then, as the visible part becomes larger, the light grows and becomes stronger, until the centre of the sun comes up. The light continues to grow until the whole body I 18a of the sun becomes visible. And similarly when the sun sets: for as long as a part of it is visible above the horizon, the light of that part will radiate upon the surface of the earth, even though the centre of the sun and the larger part of its body are hidden from those places which are irradiated by the light of that visible part of the sun.

[10] Now this fact, I mean that the light radiates from the circumference of the sun's body, holds for all horizons. But that part of the sun which is the first to appear at one horizon is not the same as the part which is the first to appear at another horizon—this being due to the motion proper to the sun. Thus the parts of the sun that appear at the beginning of its rising at different horizons are different, especially on different days. And the same holds for the parts of the sun that are the last to set. And, in general, for each place on the earth from which a part of the sun is visible (whether it is a part of the sun's circumference or not), the light will radiate from that part on that place. It is thus manifest