The French philosopher René Descartes suggested in 1637 that in order to understand the optical properties of the eye, one should study the eyeball of a recently deceased man or that of a freshly killed large animal. Beginning in the late 17th century, optical models provided a more convenient alternative. The lens of the model projects an inverted and reversed image onto a matted screen on the back. Two lenses can be placed in front of the eye to demonstrate the function of corrective lenses for near- and farsightedness.