Clerselier to Fermat (in French), 6 May 1662, in P. Tannery and C. Henry (eds.), Œuvres de Fermat, vol. 2 (Paris: Gauthier-Villars et fils, 1894), pp. 464–72.
Fermat to Clerselier (in French), 21 May 1662, in P. Tannery and C. Henry (eds.), Œuvres de Fermat, vol. 2 (Paris: Gauthier-Villars et fils, 1894), pp. 482–4.
On the proof, see Darrigol, 2012, p. 190. On the date of the reading (misprinted as 1808 in early sources), see Frankel, 1974, p. 234n. The full text (with the misprint) is "Mémoire sur les mouvements de la lumière dans les milieux diaphanes", Mémoires de l'Académie des Sciences, 1st Series, vol. X (1810), reprinted in Oeuvres complètes de Laplace, vol. 12 (Paris, Gauthier-Villars et fils, 1898), pp. 267–298. An intermediate version, including the proof but not the appended "Note", appeared as "Sur le mouvement de la lumière dans les milieux diaphanes", Mémoires de Physique et de Chimie de la Société d'Arcueil, vol. 2 (1809), pp. 111–142 & Plate 1 (after p. 494).
H.A. Lorentz, 1907, Abhandlungen über Theoretische Physik, vol. 1, Berlin: Teubner, ch. 14, ss. 12, 13, and ch. 16, s. 18; translated as "H.A. Lorentz on the equivalence of Huygens' construction and Fermat's principle", doi:10.5281/zenodo.3835134, 2020.
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In the translation, some lines and symbols are missing from the diagram; the corrected diagram may be found in Fresnel's Oeuvres Complètes, vol. 2, p. 547.
On the proof, see Darrigol, 2012, p. 190. On the date of the reading (misprinted as 1808 in early sources), see Frankel, 1974, p. 234n. The full text (with the misprint) is "Mémoire sur les mouvements de la lumière dans les milieux diaphanes", Mémoires de l'Académie des Sciences, 1st Series, vol. X (1810), reprinted in Oeuvres complètes de Laplace, vol. 12 (Paris, Gauthier-Villars et fils, 1898), pp. 267–298. An intermediate version, including the proof but not the appended "Note", appeared as "Sur le mouvement de la lumière dans les milieux diaphanes", Mémoires de Physique et de Chimie de la Société d'Arcueil, vol. 2 (1809), pp. 111–142 & Plate 1 (after p. 494).
H.A. Lorentz, 1907, Abhandlungen über Theoretische Physik, vol. 1, Berlin: Teubner, ch. 14, ss. 12, 13, and ch. 16, s. 18; translated as "H.A. Lorentz on the equivalence of Huygens' construction and Fermat's principle", doi:10.5281/zenodo.3835134, 2020.