Treatise on Light (Illustrated): In which are explained the causes of that which occurs in reflexion, & in refraction and particularly in the strange refraction of Iceland crystal

Treatise on Light (French: Traité de la Lumière) is a 1690 book written by the Dutch polymath Christiaan Huygens on his wave theory of light. Huygens' starting point was Descartes' theory, as presented in the Dioptrique, which Huygens aimed to supplant. Huygens' theory is also seen as the historical rival of Newton's theory, which was presented in the Opticks.