Download the book Treatise on Light was published in 1690 and is probably the largest scientific volume on light published before Newton's Opticks. The book explains how light travels (i.e., that it has a certain velocity), and what happens when it hits a surface (refraction and reflection). A large portion of the book is devoted to the double refraction occurring in Iceland chrystal, and all drawn conclusions are proved geometrically.
Christiaan Huygens (1629 - 1695) was a prominent physicist and astronomer. His main discoveries are the centrifugal force, collision laws for bodies and the argument that light consists of waves. He was a contemporary of Galilei and Descartes, and a member of the French Royal Society since 1663.