Before Newton completed his masterpiece, The Principia Mathematica, he had established his reputation with this treatise on the properties of light. Though on a narrower topic, this work is as impressive in its own right as The Principia, for it provided a scientific analysis of light that became the basis of our modern understanding. Based on experiments in which a beam of light was passed through a prism, Newton showed that white light was complex and could be analyzed as a blend of the various colors of the spectrum. Divided into three books, the first describes his experiments with the spectrum. The second deals with the ring phenomenon, in which concentric rings of colors appear in the thin layer of air separating a lens and an underlying plate of glass. The third book describes his work on diffraction. Also discussed is Newton's theory that light consists basically of material corpuscles in motion. Though clearly intended for fellow scientists this classic monument of modern physics is surprisingly readable and understandable for nonspecialists.