The Optics was a product of Kepler's most creative period. It began as
an attempt to give astronomical optics a solid foundation, but soon
transcended this narrow goal to become a complete reconstruction of the
theory of light, the physiology of vision, and the mathematics of
refraction. The result is a work of extraordinary breadth whose
significance transcends most categories into which it might be placed.
It gives us precious insight into Kepler's thought during this crucial
period, an insight all the more valuable in that most of his working
papers from that time have been lost. Second, it is the culmination of a
long and rich tradition in the science of optics, in distinct contrast
with the new optical thought represented by Descartes. And third, it
presents discoveries in the physiology of vision, photometry, and the
geometry of conic sections which have become part of our intellectual
heritage. Especially notable are Kepler's discovery of the inverted
retinal image, his theoretical grounding of the inverse-square
photometric law, and his insights into the relations between the various
conic sections.