Kepler’s work in 1603 culiminated in a manuscript presented to the emperor Rudolph II of Austria on January 1, 1604, and later published as Astronomiae Pars Optica [The Optical Part of Astronomy]. In it Kepler described the inverse-square law governing the intensity of light, reflection by flat and curved mirrors, and principles of pinhole cameras, as well as the astronomical implications of optics such as parallax and the apparent sizes of heavenly bodies. He also extended his study of optics to the human eye, and is generally considered to be the first to recognize that images are projected inverted and reversed by the lens on the retina.
Editors. "Johannes Kepler describes how the eye focuses light". The Literary Encyclopedia. First published 01 May 2009
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6089, accessed 28 March 2017.]