 The field of optics has its origins in the Ancient Greek period, when figures like Euclid and Ptolemy wrote works on the subject, but they often focused primarily on the science of vision, analyzing (e.g.) the visual rays that were sometimes thought to extrude from the eye, enabling it to perceive distant physical objects. In the early modern period, Kepler and Descartes each made fundamental contributions to the field, including the discovery of the inversion of the retinal image (in the former case) and an explanation of refraction (in the latter case). Newton’s work helped to shift the focus of optics from an analysis of vision to an investigation of light. In “New theory about light and colors”, published in the Philosophical Transactions in 1672, Newton presented a number of experiments in which sunlight was allowed to pass through one or two prisms in order to probe some of its basic features. The paper recounts a number of experiments that Newton says he had conducted several years earlier. But what precisely counts as a feature of light? Numerous philosophers during the seventeenth century, including Hooke and Huygens, developed doctrines concerning the fundamental physical nature of light in answer to the question: is light a stream of particles (or “corpuscles”), or a wave? Both Hooke and Huygens were wave theorists. This question obviously continued to have relevance into the twentieth century, when wave-particle duality was discovered. In his experiments with the prism, however, Newton apparently sought to investigate something else, viz. what he calls “the celebrated Phenomena of Colours”. Newton’s various prism experiments, which he describes in considerable depth, suggested what he called a “Doctrine” that he expresses in thirteen consecutive numbered propositions. Included in these propositions are the following claims about features of rays of light: first, the rays of light that emerge when sunlight passes through a prism exhibit various colors; second, these colors differ in their “degrees of Refrangibility”, which means that they exhibit and retain an index of refraction, even when they are passed through a second prism; third, these colors—or colorful rays—are not modifications of sunlight itself, but are “Original and connate properties” of it; and, fourth, these facts mean that although ordinary sunlight appears white, or perhaps colorless, to our perception, it actually contains numerous colors within it, which can be experimentally revealed. This final point suggests, in turn, that from Newton’s point of view, colors are not solely perceived, or even perceptible, aspects of physical objects; they can also be conceived of as hidden features of light which cannot be perceived directly under any ordinary circumstance (the physical influence of the prism is required for them to become perceptible).