Treatise on LightChristiaan Huygens

Huygens published the Treatise on Light in 1690, though he had largely completed it by 1678. He treats light as a succession of pulses propagating at finite speed through a subtle ethereal medium. Every point on a wave front becomes a source of secondary spherical wavelets; the new front is the envelope of those wavelets. From this principle he derives the laws of reflection and refraction. Later chapters apply the theory to the double refraction of Iceland spar.

The book stands against the corpuscular account Newton develops in the Opticks. The two pictures competed for a century. Young's interference experiments and Fresnel's work on diffraction revived the wave approach in the early nineteenth century. Maxwell later absorbed that picture into electromagnetic theory.