The penultimate pre-publication version of J.A.Schuster, ‘Descartes Opticien: The Construction of the Law of Refraction and The Manufacture of its Physical Rationales 1618-1629′, in Stephen Gaukroger, John A. Schuster and John Sutton (eds.) Descartes’ Natural Philosophy: Optics, Mechanics and Cosmology (Routledge, London, 2000), pp. 258-312. Final pagination inserted in brackets throughout.Section 5.2 mentions that the argument concerning the timing and manner of Descartes’ discovery of the (cosecant) law of refraction of light can be supported by the reconstruction of the genealogy of Descartes’ lens theory in the period 1627-37. That work is in Appendix 1 in my Descartes Agonistes—Physico-Mathematics, Method and Corpuscular Mechanism 1618-33 (2013) titled: ‘Descartes, Mydorge and Beeckman: The Evolution of Cartesian Lens Theory 1627-1637’ pp. 603-619.