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

b

from this consideration that light radiates from every part of the body of the sun upon every body facing that part, even though the centre of the sun and the remainder of its bulk may be hidden from that body.

[11] Further, when the sun is partially and not completely eclipsed and a part of it remains visible, light radiates from that visible part upon every place on the earth facing it at the time of the eclipse. When the sun is observed at the time of an eclipse that covers most of it and includes its centre, the eclipsed part will be found to grow larger while the remaining part becomes smaller. And yet, from whatever part of the sun that remains, the light will radiate upon the surface of the earth, and that part will be visible in every opposite place and also in every place opposite any portion of that part. And if the light of the sun is examined at the time of eclipse, it will invariably be found to radiate in straight lines, just as it did before the eclipse; further, the light of the sun that appears on the earth at the time of the eclipse will be found to be weaker than a its light before the eclipse. And as the eclipsed part becomes larger and the remaining part smaller, the light visible on the earth becomes weaker. But the remaining part of the sun at the time of an eclipse covering most of the sun is but a part of the sun's circumference. And the condition of the whole circumference of the sun is one and the same. Therefore this consideration makes it manifest that the light of the sun issues from the whole body of the sun and from every place on the sun and not only from a particular place on it.

[12] It is also manifest from this consideration that the straight lines along which the light of the sun extends do not all proceed from the centre of the sun. Rather, the light issues from every part of the body of the sun on every straight line that can be imagined to extend from that part. For when the eclipse covers most of the sun with respect to a particular place on the earth, the centre of the sun is at that time hidden from that place. The straight lines between the centre of the sun and that place are thus interrupted. But the light b still radiates upon that place from the rest of the sun. Thus if the light did not proceed on lines other than the straight lines extending from the sun's centre, it would not be visible at the time of eclipse in those places of the earth from which the centre is hidden. Further, consider those places on the earth with respect to which the sun has descended from the zenith at the time of eclipse in the direction of the exposed, visible part. At this time the light radiates on those places from the exposed part of the sun in a direction inclined towards the side on which the centre of the sun is, and in straight lines that cannot pass through that centre. And the light radiates at this time on every place from which a part of the body of the sun can be seen and with respect to which the eclipse does not cover the whole body of the sun. Therefore, the light of the a sun does not only radiate in straight lines extending from the centre of the sun, but in all the lines that may rectilinearly extend from every part of it.

[13] Further, when the sunlight passes through apertures it is always found to diverge, and as the light recedes from the aperture it becomes wider. This is evident in the case of minute apertures. When sunlight has passed through a minute aperture and appears on a place far removed from the aperture, such light is found to diverge—the area on which the light appears being many times wider than the aperture. As the distance between the aperture and the area where the light appears increases, the light becomes wider. And if the straight interval between the aperture and the visible light is interrupted by an opaque body, the light will be found on that opaque body. But the light on that body will be narrower than that which was visible at the former place. I 20b And as this body approaches the aperture, the patch of light appearing on it will become narrower. And as it is moved farther from the aperture the patch of light appearing on it will grow wider. Thus it is evident from the widening of the light issuing from minute apertures that the light of the sun extends from every part of it, and not just from a particular part.

[14] From this it is also evident that light extends only along straight lines. For if the light extended only from the centre of the sun or from a particular point on it, then the light extending from that point on the lines drawn from it to the narrow aperture would insensibly diverge after passing through the aperture. For the divergence would be determined by the diameter of the aperture, the distance of the sun from the aperture and from the place where the light appears. But as far as sense is concerned there is no appreciable difference between these two distances by comparison with the distance of the I 21a sun. Thus the light issuing from the minute aperture and appearing on the ground (or on some other place) would be equal in magnitude to the aperture, especially if the aperture is cylindrical. It would also come about that if sunlight passed through a narrow cylindrical hole, and the position of the hole were slightly altered so that the straight line extending through its length to the body of the sun would not meet that point on the sun, no light would come out of or go through the hole. Further, if light extended on other than straight lines, then, having come out of a minute aperture, it would extend on non-rectilinear lines. Therefore, the expansion of the light passing through minute apertures is clear proof that the light issues from the whole body of the sun to the aperture, and that it issues in straight lines. That is why when it comes out of the aperture it diverges and widens, this divergence taking place in straight lines. For light diverges as it proceeds from the whole body of the I 21b sun to the narrow aperture, and as it comes out of the aperture and goes forward, another cone opposite the first one is produced, since light proceeds in straight lines. It thus appears from all that we have explained that the light of the sun radiates from every part of the body of the sun to every side directly opposite that part.