_Christian Huyghens was born at The Hague, April 14, 1629. He studied law in Breda, but becoming attracted to the study of mathematics he neglected his legal practice for it. In 1655 he improved the method of grinding telescopic lenses, and, assisted by his brother, discovered the sixth satellite of Saturn and the fact that it was belted with rings. In 1657 he presented to the States-General the first pendulum clock. In 1678 he evolved his wave theory of light, and published it at Leyden in 1690. He died at The Hague, June 8, 1695._ THE WAVE THEORY OF LIGHT[9] Proofs in optics, as in every science in which mathematics is applied to matter, are founded upon facts from experience--as for example, that light moves in straight lines, that the angles of incidence and reflection are equal, and that light rays are refracted in accordance with the law of sines [i. e., that the ratio between the sines of the incident and refracted ray is constant for the same substance.] For this last law is now as generally and surely known as either of the others. Most writers in optics have been content to assume these facts, but others more curious have attempted to discover the source and reason of these phenomena, looking upon them as being in themselves interesting data. Yet although they have propounded some ingenious theories, intelligent readers still require a fuller explanation before being entirely satisfied. Therefore I herein offer some considerations on the matter with the hope of making clearer this branch of physics which has not improperly gained the reputation of being very obscure. I feel myself particularly indebted to those that first began to study these profound subjects, and to lead us to hope them capable of orderly explanation. Yet I have been surprised to find these very investigators accepting arguments far from clear as if proof conclusive. No one has yet offered even a probable explanation of the first two remarkable phenomena of light,--why it moves in straight lines, and why rays from any and all directions can cross one another without interference. I shall attempt in this treatise to submit clearer and more probable reasons, along the lines of modern philosophy, first for the transmission of light, and, second, for its reflection when it meets certain bodies.