The law of refraction and Kepler's heuristics (Q2285859)

Johannes Kepler's most important contribution to optics was published in 1604 as \textit{Ad vitellionem paralipomena, quibus astronomiae pars optica traditur}. The aim was to provide a quantitative law for the refraction of light. He arrived at one by some analogies, but the law he arrived at was incorrect. Thomas Harriot had apparently determined the correct law of refraction, the law of sines, by 1602, which was before Kepler had even set out to find one. Willebrord Snell then arrived at the correct law at about the same time as Descartes in 1620. Descartes published it in 1637 in his \textit{La dioptrique} as an appendix to his \textit{Discours de la méthode}. In a detailed work of historical reconstruction, the authors of this very interesting paper show that the analogies used by Kepler could have led him to the correct law, that there was nothing intrinsically wrong about his approach.