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

[19] It is therefore evident from all that we have said that from every part of every self-luminous body, light radiates in every straight line extending from that part.

5a

[20] This property being manifest in the case of the larger parts of self-luminous bodies, their smaller parts—even when extremely small and as long as they preserve their form—must also be luminous; light will radiate from these parts in the same manner as it does from the larger ones, even though the conditions of smaller parts may be imperceptible. For this property is natural to I 26b self-luminous bodies and inseparable from their essence. Now small and large parts have the same nature as long as they preserve their form. Therefore, the property that belongs to their nature must exist in each part (whether small or large) provided that that part maintains its nature and form. Now the sun and the moon and the other heavenly bodies are not made up of congregated parts; rather, each is a single continuous body whose nature is one and undifferentiated. Nor does one place in them differ in nature from another. Similarly, fire is not an aggregate of parts, but a continuous body; each place in it is similar in nature to the others, and the nature of its smaller parts is similar to that of its large parts, as long as the small parts preserve the form of fire.

5b

[21] The following is therefore clear from all that we have made subject to inspection and explanation, and from the things which we have shown how to test: that the radiation of light from every self-luminous body takes place only in straight lines; I 27a that light radiates from every self-luminous body to all locations between which and the luminous body there exist straight lines which are not interrupted by an opaque body; that light radiates in this manner from every part of the self-luminous body; that the light radiating on one place from the whole of the luminous body is stronger than that radiating from a part of that body upon that place and from that distance; that the light radiating from a larger part is stronger than that radiating from a smaller part; and that this also applies to the small parts of the luminous body even when they cannot be individually examined and their lights are not visible, for this would be due to the inability of sense to perceive what is extremely weak. All this being so, the light shining from a self-luminous body into the transparent air therefore radiates from every part of the luminous body facing that air, I 27b and the light in the illuminated air is continuous and coherent, and it issues from every point on the luminous body in every straight line that can be imagined to extend in the air from that point. It is in this manner that lights radiate from self-luminous bodies into the homogeneously transparent air. Let us call 'primary lights' those lights that radiate from self-luminous bodies.

6a

[22] We find, moreover, that the earth is illuminated at the beginning and the end of day, before sunrise and after sunset, when none of the illuminated

parts at these times is facing the body of the sun or any part of it. But the cause of daylight is none other than the sun, for no light is introduced in day-time that did not exist at night other than sunlight. Again, when the sun has risen above the earth, we find that dwellings and courtyards shaded from the sun by walls or roofs are illuminated although they do not face the sun or any part of it. Likewise, the shadows of mountains and of opaque bodies, indeed all shadows, are found to be lit in day-time although they are screened from the sun by the opaque bodies of which they are shadows. We also find that many dwellings shaded from the sky are lit before sunrise and after sunset although the sun is not yet visible and these places are screened from the sky. Let us, therefore, now inquire into the quality of these lights by subjecting their conditions and properties to inspection and experimentation.

[23] We say, then, that we find that morning light begins when the night is not yet over, extending from the eastern horizon towards the middle of the sky like a straight column. It is found to be weak and barely visible, and the surface of the earth is found to be still in the darkness of night. Then this light becomes stronger, increasing in breadth and length and growing in brightness, while the earth is yet dark. It continues to increase in magnitude and brightness, and the surface of the earth facing that light and exposed to it becomes illuminated with a faint light that is less than the light visible in the atmosphere at that time. The light in the atmosphere then gains in strength and expands until it fills the eastern horizon and reaches the middle of the sky; the atmosphere is then filled with light. Then the light on the ground grows stronger, shining and becoming broad daylight, while the sun is still below the horizon and invisible. After this stage the sun rises and daylight becomes increasingly manifest. And we find that the light at the end of the day behaves in a similar way, after the setting sun disappears below the horizon. The surface of the earth is lit with a manifest light while the atmosphere is illuminated with a strong light, after which the light of the atmosphere continues to weaken and the light on the earth's surface lessens until night falls.

[24] Furthermore, we find that when sunlight irradiates a wall facing a dark place nearby, the latter is turned from darkness to brightness. And if leading to that dark place there is a door facing another wall, then this wall will be illuminated by the sunlight radiating on the outside wall. Those parts of the chamber's floor facing the door and the sunlight will be more strongly illuminated than the rest of the chamber. When the sun goes down, its light no longer radiating on that wall, the place returns to darkness. Similarly, we find that when daylight or moonlight or the light of fire irradiates the wall, a dark place in front of it will be illuminated by that light; when that light ceases the place returns to darkness. Also if, opposite any place illuminated by a strong