Abstract

Available from UMI in association with The British Library. Requires signed TDF. ;Metaphor and analogy are the scaffolding of science. Kepler's theory of the retinal picture could not have been built without the analogy between an eye and a camera obscura, and, two hundred and fifty years later, Charles Darwin devoted most of the first chapter of The origin of Species to discussion of pigeon fanciers. Unlike Darwin, Kepler was bewitched by his own imagination and was led to wonder "how this image or picture is joined to the visual faculty, which is situated in the retina and in the nerve, and whether it is placed within the hollows of the brain, before the soul or tribunal of the visual faculty, or whether the visual faculty, like a magistrate sent by the soul from the administrative chamber of the brain, descends into the optic nerve or retina to meet this image, as though to a lower court" . ;The theory of the retinal image answered the crucial question of medieval optics: the scaffolding had served its purpose and should have been dismantled. Instead, Kepler mistook it for a part of the building: what he construed as a scientific problem was nothing more than the extension of a metaphor. As a result, and in spite of his remarkable achievement, he left the theory of vision in a state of confusion. ;I intend to show that Descartes revolutionized visual theory by manipulating the pair of analogies which had dominated visual theory since its inception and by advancing a theory of depiction which has since become orthodox. Advocates of the first analogy suggest that seeing the world is rather like seeing pictures of it; Kepler's analogy between the eye and a camera obscura, which was anticipated by Leonardo de Vinci, was a variation on this theme. Advocates of the second analogy compare vision to the use of a walking-stick: the air transmits an impression of a visible object to the eye rather as a walking-stick enables us to find our way around in the dark by transmitting pressures to the hand. As I shall show, Descartes' rejection of the pictorial analogy in favour of the walking-stick analogy and his theory of depiction were designed to salvage Kepler's theory of the retinal picture, a pearl of great price, from its murky setting. ;Descartes aimed to show that whilst Kepler's theory of the retinal picture correctly described the optics of the eye, the pictorial analogy was misleading and stood in the way of a mechanistic theory of perception. ;Current visual theory is Cartesian in some respects and un-Cartesian in others. The walking-stick analogy is no longer used, but the explanatory pattern that it served to illustrate is adhered to closely where the perception of brightness and colour are concerned, and the causal theory of depiction commands widespread support. At the same time, the pictorial analogy has not lost its appeal.