MIRRORS AND THE PERSPECTIVE*
The profound change in the theory of vision reached by Ibn al-Haytham, may be characterized as a systematic introduction of new norms -mathematical and experimental to treat traditional problems in which light and vision are united. Until then light had been considered to be the instrumentality of the eye and to see an object was to illuminate it. In order to construct a theory of light, it was necessary to begin with a theory of vision; but to establish a theory of vision required taking position on the propagation of light. Each task immediately involved the other and each theory borrowed the language of the other.
The optics of Aristotle, like that of Euclid, and even that of Ptolemy, comprised both factors. In order to introduce the new norms systematically, a better differentiation forced itself on Ibn al-Haytham. But how did it happen? To understand this capital theoretical transformation, we propose to go back to the theory of burning mirrors, as it was constituted particularly by al-Kindi and other scientists. This point, very neglected in the history of optics and the theory of vision, seems to us quite important to grasp the preparation of Ibn al-Haytham's revolution.