Book excerpt: Contents: (1) Historical Overview; & Descartes's Perspectivist Sources; (2) Analysis of Refraction: Cartesian Light-Theory; & A Critical Evaluation; (3) The Foundations of Perspectivist Optics: Perspectivist Light-Theory; Quantization of Light; & Comparison with Descartes's Theory of Light; (4) The Perspectivist Analysis of Refraction: Physical Model; Physical "Explanation" & The Final Cause; (5) Perspectivist Grounds of the Cartesian Proof: Mathematical Implications; From Cosines to Sines; & Descartes Revisited; (6) Cartesian Light-Theory as a Culmination; Toward a Kinetic Theory of Light; & Epistemological Consequences. App.: The Sine-Law Before Descartes; The Fermat-Descartes Controversy; & Kepler, Descartes & the Anaclastic. Illustrations.