Theon of Alexandria, a fourth-century mathematician, also experimented with small apertures, or pinholes. He observed how candlelight passing through a pinhole will create an illuminated spot on a screen that is directly in line with the aperture and the center of the candle. From this observation, which he described in his book The Recension of the Optics of Euclid, Theon deduced that light rays travel in straight lines. Five centuries later, al-Kindi, a ninth century Muslim philosopher, repeated Theon’s candle experiment. Al-Kindi noted that not only does the light from the center of the candle’s flame proceed in a straight line, but the light from each edge of the flame also proceeds in a straight line. Using a diagram, al-Kindi showed that light from the right side of the flame will pass through the aperture and end up on the left side of the screen, while light from the left side of the flame will pass through the aperture and end up on the right side of the screen.

Ibn al-Haytham knew of at least some of these experiments and no doubt was influenced by them. His lamp experiment expanded on the earlier experiments in important ways, however. For one thing, each of the earlier experiments involved only one source of light—the sun or a single candle flame. While Aristotle, Theon, and al-Kindi accurately described the effects of a single light passing through a pinhole, none of them suggests that what is being projected onto the screen is an image of everything on the other side of the aperture. By arranging several different light sources across a large area, Ibn al-Haytham leaves little doubt that an image is being projected onto the screen, even if it is only an image of lights. In the second version of his experiment, where he talks about arranging lamps outside a door, Ibn al-Haytham can be said to be describing the first camera obscura because he is projecting an image from outdoors onto a screen indoors.

As important as Ibn al-Haytham’s experiments with light would prove to be, they make up only small portion of The Book of Optics. Ibn al-Haytham divided his massive work into seven sections, or books. His experiments with lamps come from Book I, which is devoted to “the manner of vision generally.” In addition to descriptions of the properties of light, Book I also contains a chapter on “the structure of the eye.” Drawing on the work of Galen and other medical scholars, Ibn al-Haytham describes the parts of the eye in precise, even graphic detail. He correctly explains how the cornea refracts, or bends, light rays as they enter the eye. He also suggests that the optic nerve carries visual sensations to the brain.

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Copyright © 2008 by Bradley Steffens

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