Abstract

Early modern physiological optics introduced the concept of correspondence through the study of the con- ditions for the fusion of binocular images. The formulation of this concept has traditionally been ascribed to Christiaan Huygens (1667) and to an experiment often attributed to Christoph Scheiner (1619). How- ever, Scheiner’s experiment had already been conceptualized, first by Ptolemy (90–168 AD), then by Ibn al-Haytham (d. after 1040). The extent of the latter’s knowledge of the mechanisms of binocular vision is analyzed. It is explained why Ibn al-Haytham, who addressed this problem as an experimentalist, suc- ceeded in discovering the theoretical horopter (the locus of points in space that yields single vision) and yet failed to recognize that the horizontal line of the horopter could be described as a circular plane around the viewer’s head, credit for which goes to Vieth (1818) and Müller (1826). Through his experiments, Ibn al-Haytham established the notion of corresponding points, explored the cases of homonymous (direct) and heteronymous (crossed) diplopias, and prepared the ground for the discovery of Panum’s fusional area. Finally, the influence of al-Haytham’s pioneering treatise Kitāb al-manāẓir (Book of Optics) on West- ern science is examined, by studying the translations and commentaries that were available in the Latin world.