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

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the other hole which has not been stopped. If, therefore, light proceeded on other than straight lines, and if it expanded from the illuminated air into the whole of the air that is continuous with it in other than straight lines, then the whole of the | farther chamber would be uniformly lit when the atmosphere is shining with morning light. For the air in [this chamber] is continuous with the illuminated atmosphere at the time when the light enters the chamber through the holes and also when one of the two holes is stopped. But during the experiment there is no light in the farther chamber other than the lights facing the holes each of which lies in a straight line with the illuminated outside air through the two opposing holes facing it in the two walls of the two chambers, or [the light] in the interval rectilinearly extending between the hole and the illuminated place through the two opposing holes, or whatever radiates from this light into the rest of the chamber.

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[36] [The following] shows that the little light that shines in the rest of the chamber is emanating only from the light opposite the hole inside the chamber, and from nothing else. If one of the two holes is stopped, and the rectilinear interval between the remaining hole and the light passing through it is interrupted with an opaque body, and this body is then brought near the hole so as to make | the light vanish at the place where it showed, the little light that radiated from this light and showed in the rest of the chamber will disappear. This operation, however, does not interrupt the continuity of the air inside the whole of the chamber with the luminous air outside, provided that that opaque body interrupting the rectilinear interval has not touched the hole.

[37] It is evident from this experiment that the light does not proceed from the illuminated air on other than straight lines; and that light rectilinearly radiates from every part of the illuminated air in all opposite directions—because all the separate lights that appear inside the farther chamber face different parts of the illuminated air outside, and also because the similarly illuminated parts of the air are of the same condition.

[38] It is in this manner, therefore, that daylight can be examined and shown to arise from the light of the illuminated atmosphere and to proceed from the atmosphere in straight lines.

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[39] One might object to this assertion, saying: | the whole atmosphere faces the body of the sun at all times and throughout the night; only the earth’s shadow, a narrow cone which constitutes only a small portion of the whole atmosphere is hidden from the body of the sun. Now what appears of the atmosphere facing the earth’s surface at all times is half the whole atmosphere, while the part concealed from the sun at night is a small portion of this half, and most of the air that appears throughout the night facing the earth’s surface

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is illuminated by sunlight. Therefore, if the light that shows in the atmosphere in the morning and evening and that radiates upon the earth’s surface were [the light of] the air that faces the sun and is illuminated by sunlight, then the light would appear in the atmosphere and radiate on the earth’s surface throughout the night, since the larger part of the air facing the earth’s surface is illuminated throughout the night. But no light appears throughout the night in the atmosphere or on the earth’s surface. Therefore, the light that appears in the atmosphere | and on the earth’s surface in the morning and evening is not the light of the air that faces the sun and is illuminated by sunlight.

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[40] In reply to this objection we say: the whole atmosphere is illuminated by sunlight at all times; no part of it is dark or concealed from the sun except the conic umbra conical shadow which is the earth’s shadow. However, the light emanating from the illuminated atmosphere is weak, and the farther it extends the weaker it becomes, this being a characteristic property of light. Thus some light always radiates in all directions from the sunlit atmosphere and penetrates into the atmosphere that is shaded by the earth’s shadow. But this light weakens as it recedes from the sunlit atmosphere from which it proceeds. That being so, the part of the earth’s shadow that is contiguous to the illuminated atmosphere and the part near that, viz. the borders of the shadow, are such that the light radiating on them from the adjacent illuminated atmosphere is fairly strong. But when this light recedes from the borders of the shadow and reaches the middle, or near the middle, of the shadow, | it becomes very weak.

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[41] Now throughout the night the sun is at a distance from the horizon’s circumference, and during most of the night the place on the face of the earth where night falls is situated at or near the middle of the earth’s shadow. Then, when the sun approaches the horizon, the conical shadow will be oblique, and the circumference of the shadow’s base surrounding the earth will approach the place whose horizon-circumference the sun has approached. Thus the place whose horizon-circumference the sun has approached will not be at the limit of the shadow or near the shadow’s border, and the light that issues from the sun and extends alongside the edge of the shadow and close to it will be near the face of the earth. And when the border and limit of the shadow and the light that extends alongside the edge of the shadow and close to the shadow’s border approach [that place on] the face of the earth, the illuminated atmosphere will be near [that place on] the face of the earth, and the light reaching [that place on] the earth’s surface from this atmosphere will be fairly strong. It is for this reason that the eye will perceive the light in the atmosphere at the approach of morning, and this light will reach the earth in the morning. Then, as the sun approaches the horizon, the shadow’s border will approach | the face of the earth, the illuminated atmosphere will approach the eye, and the light reaching the face of the earth will grow stronger. Consequently, as the sun approaches the horizon, the light showing on the face of the earth will gain in strength and clarity.