The development of optics between 1450 and 1789 can be conveniently divided into two phases bridged by the optical work of Johannes Kepler (1576â€“1630) and distinguished by a radical change in analytic focus. During the first phase, that focus was primarily on sight, not light. During the second, it shifted completely from sight to light.
In 1450 there were two models of how optics worked.
The second of these models of vision harks back to Alhacen (965â€“1040) and his Perspectivist disciples, Roger Bacon (fl. c. 1265), Witelo (fl. c. 1275), and John Pecham (fl. c. 1280). Rejecting visual rays as functionally pointless, these theorists raised light to primacy in the visual process, supposing it to be an intrinsic quality of self-luminous or illuminated bodies. Each point of light on the surface of such bodies is a source of radiation in its own right, spreading outward in all directions in a process of self-replication. The resulting sphere of propagation can be analytically resolved into individual rays, along which point forms of the original light are transmitted. Color, too, is an intrinsic property of bodies. Yet although they are ontologically distinct, light and color are functionally inseparable. Both must be present in objects if they are to be seen, so what actually radiates from them is luminous color. Thus, like Ptolemy, the Perspectivists viewed luminous color as primal for sight.
Unlike Ptolemy, the Perspectivists gave a detailed account of how the optic complex contributes to vision. The eye itself, they assumed, is a sphere. Toward its front lies the crystalline lens, whose anterior surface is concentric with the eye as a whole. The space behind it is filled with vitreous humor, which is optically denser than the glacial humor occupying the lens. At the very back, directly in line with the center of the pupil and the center of the eye, lies the hollow optic nerve, which reaches from the eye to the forefront of the brain. A conduit for visual spirit manufactured in the brain, this nerve transmits the spirit to the lens and thereby sensitizes it. The anterior surface of the lens, meanwhile, is bombarded from all directions by point forms of luminous color radiating from external objects. Because of its visual sensitivity, though, the lens feels only those color forms that strike it orthogonally and thus selects out a formal representation of the object in point-to-point correspondence with it. The composite of all the rays linking the object and its formal representation on the lensâ€™s surface creates a cone of radiation with its base in the object and its vertex at the center of the eye. Mathematically equivalent to Ptolemyâ€™s visual cone, this radiative cone serves much the same function as the basis for spatial perception.