The publication of "Opticks" represented a major contribution to science, different from but in some ways rivalling the "Principia". "Opticks" is largely a record of experiments and the deductions made from them, covering a wide range of topics in what was later to be known as physical optics. That is, this work is not a geometric discussion of catoptrics or dioptrics, the traditional subjects of reflection of light by mirrors of different shapes and the exploration of how light is "bent" as it passes from one medium, such as air, into another, such as water or glass. Rather, the "Opticks" is a study of the nature of light and colour and the various phenomena of diffraction, which Newton called the "inflexion" of light.