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

the sun approaches the horizon, the light showing on the face of the earth will gain in strength and clarity until the circumference of the sun's body reaches the circumference of the horizon, so that the border and limit of the shadow and the air illuminated at this time by daylight become close to [that place on] the earth's surface. Now the air that is adjacent and near to [that place on] the earth's surface and that is illuminated by daylight when the sun is close to the horizon before it rises is part of the cone of the earth's shadow; and this air is illuminated only because it is near the illuminated atmosphere facing the sun, since every place whose horizon lies above the sun and which does not face the sun must be inside the cone constituted by the shadow. But this illuminated air is the shadow's border adjoining the illuminated atmosphere that faces the sun.

36b, 37a

[42] Thus the reason why the light does not appear in the atmosphere throughout the night is the remoteness of the illuminated air facing the sun from the face of the earth, and the weakness of the light emanating from the light in this illuminated air, | and the failure of its strength to reach the middle of the earth's shadow. And the reason why the light appears in the atmosphere at dawn and at the beginning of night, and why it irradiates the surface of the earth in the morning and evening, is the nearness to the eye of the illuminated atmosphere facing the sun and the nearness of the shadow's border to the surface of the earth at these times. And for this reason, I mean this nearness, that which first appears of dawn looks narrow and elongated, for the nearest of the shadow's borders to the eye at this time is one straight line, namely the straight line extending on the surface of the shadow's cone passing through the nearest point to the eye at this time on the circumference of the shadow's base. For the eye is not at this moment at the middle of the shadow's cone but displaced from it to that side of the circumference of the shadow's base towards the sun. For the point, towards the sun, which is the extremity of the diameter of the shadow's base passing at this moment through the eye, is nearer to the eye than all the points on the circumference of the shadow's base. | (By the shadow's base I mean here the plane that passes through the position of the eye and intersects the shadow's cone, the line proceeding from this point and extending on the surface of the shadow's cone being in this case the nearest line on this surface to the eye.) The reason is thus evident why light appears in the atmosphere and on the earth's surface in the morning and in the evening but not throughout the night.

[43] Finally, the following might be said: If the light seen in the atmosphere in the morning and in the evening were inherent in the atmosphere but perceived at those times merely on account of its nearness to the eye, then sight would perceive the light in the atmosphere between walls and inside chambers throughout the day, since that atmosphere is illuminated throughout the day

I 37b

and is near the eye. But sight does not perceive the light in such atmosphere; rather, it perceives the light on chamber walls without perceiving any light in the atmosphere between these walls. Therefore, the light seen in the atmosphere in the morning and | in the evening is not a light that is inherent in the air.

[44] In reply to this statement we say: Air is a very transparent body. It is, however, not only extremely transparent but has a little density in it. Therefore, when sunlight irradiates the air, it traverses the air in accordance with the air's transparency, and a small amount of the light is fixed in it in accordance with its slight density. Thus the light that is fixed in a small volume of air is very little because the volume of air is small and because air is very transparent and [only] a little dense and because the quality of the light that is fixed in it is weak. Plenty of light exists, however, in an atmosphere of great depth because of the large volume of air, even though the quality of the light in every small part of it may be weak. But the air existing between walls and inside chambers is small in volume. Therefore, the light in it is scanty on two accounts: the smallness of its volume and the weakness of its quality.

I 38a

[45] Now the extent of the illuminated atmosphere perceived, | and through which the light extends at the time of its perceiving the morning or evening light, is of a great depth in the direction opposite the eye. And at this time the whole atmosphere extending through this depth is illuminated. And every little part of the atmosphere in this depth has a little weak light. And the sizable illuminated portion of this atmosphere opposite the eye at the time of the sight's perception of the light in it, i.e. those parts of which each is equal to the magnitude of the inter-mural air where the light does not show and which extend in depth along the straight line opposite the eye, are, if estimated by our imagination, excessively numerous, on account of the great volume and depth of this air.

I 38b

[46] But if these parts are excessively numerous, and if a little light exists in each of them, and if these many parts rectilinearly extend opposite the eye, then the individually small lights will be multiplied and their strength will be multiplied very many times, for | the eye will perceive them all through one line. But when a small light multiplies many times, it grows in strength and becomes manifest to the sense. That is why light is visible in the illuminated atmosphere, but not in the small [volume] of air inside chambers and between walls or in valleys between mountains or in the air intervening between the eye and the earth's surface, or in any small volume of air.

[47] The difficulty has therefore been clarified, and it has been shown to be true that the light perceived by sight in the atmosphere in the morning and in the evening is the light of the sunlit air facing the sun, and that the light irradiating the surface of the earth before sunrise and after sunset proceeds from the light that exists in the sunlit air facing the sun.