Descartes' optics theory, artwork. This is the eye diagram from Descartes' book Optics (1637). Three external points are shown depicted on the retina at the back of the eye. The light rays from these three points have been focused on the retina by the structures at the front of the eye, including the lens. The image formed on the retina is being observed by the homunculus at bottom - a human figure that is here being used to represent the unknown process by which the brain interprets a visual image. Artwork from Pioneers of Science (Oliver Lodge, 1893).