Treatise on Light: In Which Are Explained the Causes of That Which Occurs in Reflection & Refraction (: Où Sont Expliquées les Causes de ce qui Luy Arrive Dans la Reflexion & Dans la Refraction) is a book written by Dutch polymath Christiaan Huygens that was published in French in 1690. The book describes Huygens's conception of the nature of light propagation which makes it possible to explain the laws of geometrical optics shown in Descartes's Dioptrique, which Huygens aimed to replace.
