Editing François d'Aguilon (section)

=== Perception and the horopter ===
D'Aguilon extensively studied [[stereographic projection]], which he wanted to use a means to aid architects, [[Cosmography|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|Alhazen's]] theory that only light rays orthogonal to the cornea and lens surface are clearly registered.<ref>Gillispie, Charles. C. ed., Dictionary of Scientific biography. 16 vols. New York: Charles Scribner and Sons, 1970</ref> Aguilon was the first to use the term [[horopter]], which is the line drawn through the [[Focus (optics)|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: {{Blockquote|If objects fall upon different rays it can happen that things at different distances can be seen at equal angles. If point C be directly opposite the eyes, A and B, with a circle drawn through the three points, A, B, and C.<ref name="ReferenceA"/> By theorem 21 of [[Euclid|Euclid's]] Third book, any other point D on its circumference which lies closer to the observer than C, will subend an angle ADB which will equal angle ACB. Therefore, objects at C and at D are judged equally far from the eye.<ref name="ReferenceA"/> But this is false, because point C is farther away than D. Therefore a judgment of distance is false when based on the angles between converged axes, quod erat probandum.}}

At first glance, it seems that Aguillon discovered the geometrical horopter more than 200 years before Prevost and Vieth and Muller.<ref name="faculty.fairfield.edu"/> The horopter was then used by architect [[Girard Desargues]], who in 1639 published a remarkable treatise on the conic sections, emphasizing the idea of projection.