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

I 49b [88] Let us call ‘secondary lights’ those lights that emanate from accidental lights. I say, then, that these secondary lights do not emanate from accidental lights by way of reflection, i.e., in the manner of reflection of light from polished bodies. Rather, they emanate from them in the way that the primary or essential lights emanate from self-luminous bodies. Further, if light radiates on any polished bodies, or on bodies some parts of which are polished, it will be reflected from them. And yet a secondary light will emanate from them in the way that light emanates from self-luminous bodies. Let us now explain this state of affairs, again by inspection and experiment—as follows.

[89] Let the experimenter use a chamber into which sunlight may enter so as to reach the floor of the chamber through a hole that is fairly wide but not excessively so. Let him wait until sunlight enters this chamber, and when it enters and appears on the floor, let him close the door, allowing no light to come into the chamber except through the hole. He will then find the chamber illuminated by that light, and will find the light in all parts of the chamber. | I 50a Further, those parts of the chamber’s wall nearer that light will be found more strongly illuminated than the farther parts. Let the experimenter then hold a cup or some other hollow object in that light so that the whole light will enter that object. He will then find that the chamber will turn dark, and that the light that showed on the wall will disappear—except perhaps from an area in the upper parts of the chamber that may be facing the light in the hollow object. Then if he removes the object the chamber will again be illuminated and the light will appear in all parts of it. From this experiment it is evident that the light that appears in all parts of the chamber is but a secondary light that radiates on them from the light appearing on the chamber’s floor.

[90] Let the experimenter then take a sheet of silver and by polishing it make it into a mirror. Experiments made with silver will be clearer than those made with iron mirrors, for the latter dim the lights because of their dark colours, original: "because of their dark colours" refers to the low reflectivity of oxidized iron so that of the lights radiating from them only those that are reflected are I 50b apparent | on account of their strength. (The reason for this will become clear when we speak about reflection.) The experimenter should then place the silver sheet where the sunlight appears, having made sure that the sheet is equal to or larger than the magnitude of the light. If the light exceeds the sheet, he should narrow the aperture so that the entire light may fall on the sheet. When this happens he will find that the light will be reflected from it to one particular place, because reflection can take place only at equal angles (as we shall show when we speak about reflection). He will also find that this light lies on the side opposite to that of the sun; the light due to reflection will appear on the wall opposite the hole or on the ceiling of the chamber if the latter is large. And he will find this light to be strong, similar in strength to the light of the

sun and stronger than any light in the remaining parts of the chamber. And this light will be found to be limited and bounded. When this light appears, let the experimenter observe all parts of the chamber: he will find the chamber to be illuminated, and will find the light that is in it to be stronger and clearer than it was on account of the whiteness of the sheet.

I 51a [91] Now this light has no cause other than the light of the sun which at this moment is on the sheet. For if he lets this light enter into the hollow object in the way mentioned earlier, the light in all parts of the chamber will cease to be visible. But the light cannot be reflected from the sheet except to one particular place which is that where the light appears owing to reflection at this moment, namely the light that is distinct and separate and stronger than all the light in all parts of the chamber. Therefore the light that appears in all parts of the chamber is not due to reflection.

[92] If the experimenter then takes an opaque white object, brings it near to the sheet, and holds it in an oblique direction against the sheet on any side other than that of reflection, he will find that some light clearly appears on the opaque body. Upon this body being moved away from the sheet, the light that is on it will weaken. When he brings it closer still, the light that is on it will become stronger. And when he turns this body round the sheet on all sides I 51b except that of reflection, | confronting the sheet with it, he will find that the light appears on it in all these positions. In addition, he will find that the reflected light remains the same.

[93] Then upon removing the sheet he will also find that of the light in all parts of the chamber, only that which has been reflected will vanish. And if he replaces the sheet with an unpolished body of a pure white colour, he will find that the light in all parts of the chamber gains in strength and increases, without finding in the chamber any reflected light similar to that which was reflected from the polished sheet. If in place of that body he puts a black or dark body, he will find that the light in all parts of the chamber will become dim and weak.

[94] It is therefore evident from this experiment that the light that appears in all parts of the chamber is a secondary light emanating from the accidental light which has reached the floor of the chamber from the light of the sun, and that the radiation of that light on all parts of the chamber is not due to reflection.

I 52a [95] Again, if he similarly examines the light of the moon, he will find that it suffers reflection and also radiates in all directions just as light does | from the essential light of the sun.

[96] Similarly, if an examination is made of the light of fire that radiates on floors and walls and on opaque bodies, light will be found to radiate from it in all opposite directions, as well as being reflected from polished bodies just as happens to all lights.