  In Kepler's new theory of how the eye works,  an image is formed on the retina as a result of refraction by the cornea and lens. For objects at different distances, the focal properties of the system must be changed, just as the focal length of a camera is changed. There were several theories of how this might occur, but Descartes accepted the view that the lens changes shape  or “accommodates” for near and far vision. He then theorized that this change in the shape of the lens must be controlled by muscles, which themselves are controlled by nerve processes in the brain.