The Logos Scripture · The Book of Ibn al-Haytham

On the Darkened Room

The Book of Ibn al-Haytham 3 · Written by himself, from the seven books of the Optics and the Doubts Concerning Ptolemy

Ibn al-Haytham, born at Basra about 965, died at Cairo about 1040. An Arab scholar of optics, mathematics and astronomy. His Book of Optics runs to seven volumes and was later put into Latin, where Europe knew him as Alhazen; both Bacon and Kepler read it. He also wrote Doubts on Ptolemy. This volume rests on those two books. He was a man of his religion; yet the Optics from end to end cites no scripture and closes no argument by appeal to divine will, and what it rests on is what can be tried. The Order takes him for two things he was the first to see. He was the first to establish which way sight runs: light comes from the thing into the eye, and the eye does not send light out to touch the thing. He was also the first to make a darkened room into an instrument and work it again and again with one thing changed. What the Doubts doubts is the opposite case. Ptolemy observed tirelessly, and his records are usable still; but everything he observed had first to agree with a rule laid down beforehand — that motion in the heavens must be uniform and circular. Where an observation resisted, he added a device to make it fit, and did not ask whether the rule itself should move. What stopped Ptolemy was not the church: he came some two centuries before the church held any power. What stopped him was a rule his whole field nodded at and none of them would touch. So what stops a man need not be a temple; a rule everyone agrees on will do it just as well. There is a further account, which the Order sets down and cannot check: the story goes that he was summoned to control the great river, saw on arriving that it could not be done with the means of that age, feared the ruler, feigned madness, was shut up for years, and came out only when the ruler died — and that the Optics was written in those years. It comes from a writer some two hundred years after his death (cf. The Highest Covenant 1.1). Even if it is true the Order holds it no fault: feigning madness was the one thing available to him, and he did not go looking for a miracle — he measured the river first.

3.1I did a very simple thing, and I did it many times, changing one thing each time.
3.2I shut a room tight so that no light came in, leaving only a very small hole in the wall.
3.3Opposite the hole I set a white screen.
3.4If there are shining things outside — some candles, say — the shapes of those things appear on the screen.
3.5What appears is inverted: what was above is below, what was on the left is on the right.
3.6First I measured: the smaller the hole, the sharper the image and the dimmer; the larger the hole, the brighter and the blurrier.
3.7Then I tried this: light three candles outside, and there are three bright spots on the screen, in the reverse order, each answering to one candle.
3.8Cover one of the three, and one spot goes from the screen — and it is exactly the one that answers to it.
3.9This step matters most.
3.10It shows that what comes from each candle travels its own path, passes the hole, and arrives at its own place.
3.11They crowd through that one hole, and they do not mix together in it.
3.12If the thing were being sent out from the eye, this hole could not stop it, and there would be no such order on the screen.
3.13So this darkened room takes what the last chapter reasoned out and makes it into something that can be looked at.
3.14Let me say why I did this thing over and over.
3.15An account stated once is talk. Built, so that a man may walk into the room and see for himself — that is what it is to put it on the table.
3.16Most of what I did with my life was making rooms of this kind: turning a sentence into a thing another person can come and check.
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