  The theoretical terrain in optics changed with Kepler's doctrine that vision is mediated by the retinal image and that the retina is the sensitive body in the eye. Previous theorists generally believed that the “crystalline humor,” now known as the lens, was the sensitive body. Descartes accepted Kepler's result and framed a new theory of spatial perception. Some of his theorizing simply adapted Ibn al-Haytham's theories to the newly discovered retinal image. Thus, Ibn al-Haytham held that size is perceived by combining the visual angle that a body subtends with perception of its distance, to arrive at a perception of the true size of the object. (Visual angle is formed by the directions from a vantage point to a seen-object for a given fixation, e.g., the angle formed by the direction to the feet and to the nose of a person standing at moderate distance to us, with the eyes fixed for the moment.) In al-Haytham's scheme, visual angle is registered at the surface of the crystalline humor. Descartes held that size is perceived by combining visual angle with perceived distance, but he now treated visual angle as the extent of an object's projection onto the retina.