thumb|upright=1.1|Opticorum libri sex, 1613
François d'Aguilon (; also d'Aguillon or in Latin Franciscus Aguilonius) (4 January 1567 – 20 March 1617) was a Jesuit, mathematician, physicist, and architect from the Spanish Netherlands.
D'Aguilon was born in Brussels; his father was a secretary to Philip II of Spain. The notable geometers educated at this school included Jean-Charles della Faille, André Tacquet, and Theodorus Moretus. It included one of the first studies of binocular vision. It also gave the names we now use to stereographic projection and orthographic projection, although the projections themselves were likely known to Hipparchus. This book inspired the works of Desargues and Christiaan Huygens.
He died in Antwerp, aged 50.
Six Books of Optics
Francois d'Aguilon's Six Books of Optics concerns geometrical optics, which at the time in the Jesuit school was a subcategory of geometry. He taught logic, syntax, and theology while being charged with organizing the teaching of geometry and science which would be useful for geography, navigation, architecture and the military arts in Belgium. His superiors wanted him to synthesize the work of Euclid, Alhazen, Vitello, Roger Bacon and others. Although he died before completing the book, it still consists of six in-depth books, called Opticorum Libri Sex.
Perception and the horopter
D'Aguilon extensively studied stereographic projection, which he wanted to use a means to aid architects, cosmographers, navigators and artists. For centuries, artists and architects had sought formal laws of projection to place objects on a screen. Aguilon's Opticorum libri sex successfully treated projections and the errors in perception. D'Aguillon adopted Alhazen's theory that only light rays orthogonal to the cornea and lens surface are clearly registered. Aguilon was the first to use the term horopter, which is the line drawn through the focal point of both eyes and parallel to the line between the eyes. In other words, it describes how only objects on the horopter are seen in their true location. He then built an instrument to measure the spacing of double images in the horopter as he saw fit.
D'Aguilon expanded on the horopter by saying in his book: