Francois d'Aguilon was born on January 4, 1567, in Brussels, in what was then the Spanish Netherlands. His father held a position of some consequence as a secretary to Philip II of Spain, and the household's connection to the Spanish crown placed the young Francois within a world of administrative importance and intellectual ambition. In 1586, he entered the Society of Jesus, becoming a Jesuit in Tournai, and from that moment his life followed the disciplined trajectory of a scholar-priest committed both to faith and to learning.
By 1598, d'Aguilon had moved to Antwerp, which was at the time one of the great centers of European intellectual and commercial life. There he became involved in a project of considerable ambition: helping to plan the construction of the Saint Carolus Borromeus church, an undertaking that brought architecture and Jesuit devotion into close collaboration. His involvement in that project was not merely administrative; it reflected a genuine engagement with the spatial and visual arts that would later inform his scholarly work.
In 1611, d'Aguilon founded a special school of mathematics in Antwerp, fulfilling a vision that the great Jesuit mathematician Christopher Clavius had long held for a dedicated Jesuit mathematical institution. The school drew some of the most talented geometers of the age. Among the notable figures educated there were Jean-Charles della Faille, Andre Tacquet, and Theodorus Moretus. In 1616, the school was joined by Gregoire de Saint-Vincent, himself a mathematician of considerable reputation, adding further luster to an institution that had rapidly established itself as a center of serious mathematical inquiry.
The work for which d'Aguilon is most remembered took the form of an ambitious treatise on optics. His Opticorum Libri Sex, whose Latin title translates as Six Books of Optics, useful for philosophers and mathematicians alike, was published in Antwerp in 1613 by Balthasar I Moretus. The book carried illustrations by Peter Paul Rubens, the great painter who was then at the height of his powers, and the combination of scientific rigor and artistic beauty made the volume one of the most remarkable publications of its era.
The content of the Six Books of Optics ranged widely across the emerging discipline of geometrical optics, which within the Jesuit tradition was treated as a subcategory of geometry. D'Aguilon drew on the work of predecessors including Euclid, Alhazen, Vitello, and Roger Bacon, synthesizing their findings while extending them in significant new directions. One of his most lasting contributions was providing the names that are still used today for stereographic projection and orthographic projection. The projections themselves had been known since antiquity, likely to Hipparchus, but d'Aguilon gave them the terminology through which they have been discussed ever since.
His study of binocular vision represented another area of genuine innovation. D'Aguilon adopted Alhazen's theory that only light rays orthogonal to the cornea and lens surface are clearly perceived, and he built upon this to develop his analysis of the horopter. He was the first to use the term horopter to describe the line drawn through the focal point of both eyes and parallel to the line connecting them, which defines the set of points in space that are seen in their true location. He constructed an instrument to measure the spacing of double images as perceived through the horopter, and in his book he offered a careful geometric argument demonstrating that judgments of distance based solely on the angle between converged visual axes can be false. The argument was sophisticated enough that some historians have credited d'Aguilon with anticipating the formal discovery of the geometrical horopter by more than two centuries before Prevost, Vieth, and Muller addressed the subject in the nineteenth century.
The book's influence extended across multiple generations of mathematicians and scientists. Girard Desargues, the great French mathematician, used d'Aguilon's discussion of projection as a foundation for his own landmark treatise on conic sections published in 1639. Christiaan Huygens also drew inspiration from the work. The poet and diplomat Constantijn Huygens, father of the scientist, encountered d'Aguilon's ideas while still a young man in his early twenties, reflecting the book's broad intellectual reach beyond the purely mathematical community.
While teaching at the Antwerp school, d'Aguilon also carried the responsibilities of a working Jesuit intellectual: he taught logic, syntax, and theology, and was charged with organizing the teaching of geometry and science in ways that would prove useful for geography, navigation, architecture, and the military arts in the Spanish Netherlands. He was working on the completion of the Six Books when death intervened. Francois d'Aguilon died on March 20, 1617, at the age of fifty, before he could see the full reception of his work. The book nonetheless stands as one of the most significant contributions to the science of optics in the early seventeenth century, a monument to the Jesuit intellectual tradition that combined faith, rigorous scholarship, and practical ambition in equal measure.
