Illustration of the British physicist, mathematician and astronomer Isaac Newton (1643-1727) at 23. Newton's most famous works are Principia Mathematica (1687) and Opticks (1704). In Principia, he stated his universal laws of motion and defined his law of universal gravitation. This would form the basis of physics for the next two hundred years. In Opticks, Newton discussed his studies into the spectrum of light, showing that it was a mixture of colours. He also invented the reflecting telescope. From 'Popular Astronomy' by Camille Flammarion (1880).