IN his review of the second volume of the new edition of Kepler's works in NATURE of August 19, p. 306, Prof. H. C. Plummer states that Kepler's formula for the law of refraction is α–β = kα sec β, where α is the angle of incidence and β the angle of refraction. This reduces to the law sin α = n sin β for small angles if (1–k)-l = n. Prof. Plummer also states that as the formula is really empirical, being founded on the flimsiest physical argument, it seems strange that a form so inconvenient for the derivation of β from α should have been adopted.
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HOUSTOUN, R. Kepler's Law of Refraction. Nature 144, 441 (1939). https://doi.org/10.1038/144441b0