Opticks

Newton's *Opticks* is a study of light's fundamental nature, based on experiments with prisms, lenses, and glass sheets. It examines how light refracts, diffracts, and mixes into colors, using both spectral light and pigment powders. Published in English in 1704 (with a Latin translation in 1706), it was Newton's second major physical science work and is ranked alongside Kepler's *Astronomiae Pars Optica* and Huygens' *Treatise on Light* as a key text of the Scientific Revolution.

The book is a record of experiments and their deductions, covering what would become physical optics. It does not focus on traditional geometric optics—reflection by mirrors or refraction between media—but on light's nature, color, and diffraction (which Newton called "inflexion"). Newton details his 1672 experiments on dispersion, showing how color arises from selective absorption, reflection, or transmission of light's components. His major finding overturned the ancient belief that pure light is white and becomes colored by mixing with darkness. Instead, he proved light is composed of spectral hues (red, orange, yellow, green, blue, indigo, violet), and all colors, including white, are mixtures of these. He showed each hue refracts at a characteristic angle, but stressed color is a mental sensation, not a property of objects or light itself. For instance, a magenta hue can be mixed from red and violet ends of two spectra, though it does not appear in a single spectrum. By connecting the spectrum's ends, he created a color circle that predicts mixtures and describes perceived similarities among hues. Newton also first outlined multiple-prism arrays, later used as beam expanders in tunable lasers.

Unlike the *Principia*, *Opticks* was published in English, aiding the growth of vernacular science. It is written in a more accessible style, with colloquialisms and metaphors, though Newton's long, qualified sentences can feel dated. It does not use geometric deduction from axioms; instead, axioms define terms, and propositions are demonstrated through specific experiments. The opening declares Newton's intent to prove light's properties by reason and experiments, not hypotheses. A critical experiment showed a color's "degree of refrangibility" (refraction angle) cannot be altered by further reflection, refraction, or filters. The work is a guide to experimental method, using observations to form generalizations and exclude competing explanations through tests. Unlike the *Principia*'s "I make no hypotheses," *Opticks* develops conjectures beyond the evidence—for example, that light behaves as corpuscles, or that perceived colors are harmonically proportioned like musical tones.

Newton originally planned four books but dropped the last on action at a distance. He ended *Opticks* with a set of unanswered questions and assertions called queries. The first queries were brief, but later ones became long essays.