Perhaps because he began work on The Book of Optics while in prison, stripped of all his books and possessions, Ibn al-Haytham does not cite earlier authorities in the book. Instead, he relies on his own observations, demonstrations, and analyses. His approach, he says, will be systematic:

We should distinguish the properties of particulars, and gather by induction what pertains to the eye when vision takes place and what is found in the manner of sensation to be uniform, unchanging, manifest, and not subject to doubt. After which we should ascend in our inquiry and reasonings, gradually and orderly, criticizing premises and exercising caution in regard to conclusions—our aim in all that we make subject to inspection and review being to employ justice, not to follow prejudice, and to take care in all that we judge and criticize that we seek the truth and not be swayed by opinion.

To eliminate opinion and prejudice, Ibn al-Haytham supports his assertions with experimental or mathematical proofs whenever possible. Just five paragraphs after the introduction, for example, Ibn al-Haytham states that straight lines exist between “the surface of the eye” and “each point on the seen surface of the object.” He continues, “An accurate experimental examination of this fact may be easily made with the help of rulers and tubes.”

He then describes how an observer looking through a straight tube will see only the part of an object that lies directly across from the opening of the tube. “If…he covers any part of the opening, then there will be screened off only that portion…that lies on a straight line with the eye and the screening body—this straightness being secured by the ruler and the straightness of the tube,” he writes. “It follows from this experiment, with a necessity that dispels doubt, that sight does not perceive any visible object existing with it in the same atmosphere, this perception being not by reflection, except through straight lines alone that can be imagined to extend between the surface of the object and the surface of the eye.”

The most important discovery in The Book of Optics appears in the very next sentence: “Sight does not perceive any visible object unless there exists in the object some light, which the object possesses of itself or which radiates upon it from another object.” With this simple observation, Ibn al-Haytham solved the mystery of vision that had baffled scholars for centuries. It was light, not the physical “forms” described by the physicists, that traveled from visible objects to the eye. The rays that create vision do not travel out of the eye, as the mathematicians said, but into it. Those rays are light rays. Although Ibn al-Haytham had set out to write a book about vision, he soon realized that vision and light were inextricably linked. Consequently, a significant portion of The Book of Optics is consequently devoted to the study of light. Ibn al-Haytham begins by dividing light into two basic groups: primary light and secondary light. Primary light is the light radiated by an illuminating body, such as a lamp, a fire, the stars, or the sun. Secondary light is primary light that has been reflected off another surface. During the day, for example, the sun provides primary light, while every other visible object—a bird, a tree, a stone, a blade of grass—reflects the light of the sun. Even the atmosphere reflects light, Ibn al-Haytham wrote, which is why the sky brightens even before the sun rises.

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Copyright © 2008 by Bradley Steffens

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