Kepler had paused his other work through…
January 1603 CE
Kepler had paused his other work through most of 1603 to focus on optical theory; the resulting manuscript, presented to the emperor on January 1, 1604, is published as Astronomiae Pars Optica (The Optical Part of Astronomy).
In it, Kepler describes 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 extends his study of optics to the human eye, and is generally considered by neuroscientists to be the first to recognize that images are projected inverted and reversed by the eye's lens onto the retina.
The solution to this dilemma is not of particular importance to Kepler as he does not see it as pertaining to optics, although he does suggest that the image is later corrected "in the hollows of the brain" due to the "activity of the Soul."
Today, Astronomiae Pars Optica is generally recognized as the foundation of modern optics (though the law of refraction is conspicuously absent)