Isaac Newton Writings

Isaac Newton’s Principia Book 2

In the Principia, Book Two, Lemma II, Newton describes what is essentially the Product Rule for differentiation, applying it to calculate the `moments’ of quantities that are expressed as products of powers of other quantities whose moments are known.


One of the most readable of all the great classics of physical science, Opticks presents a comprehensive survey of 18th-century knowledge of light. Newton describes his experiments with spectroscopy, colors, lenses, reflection, refraction, and more, in language lay readers can easily follow.

Differences between Principia and Opticks

Open Me

Opticks differs in many respects from the Principia. It was first published Read More in English rather than in the Latin used by European philosophers, contributing to the development of a vernacular science literature. The book is a model of popular science exposition: although Newton’s English is somewhat dated, the book can still be easily understood by a modern reader. In contrast, few readers of Newton’s time found the Principia accessible or even comprehensible. Unlike the Principia, Opticks is not developed using the geometric convention of propositions proved by deduction from either previous propositions, lemmas or first principles (or axioms). Instead, axioms define the meaning of technical terms or fundamental properties of matter and light, and the stated propositions are demonstrated by means of specific, carefully described experiments. The first sentence of the book declares My Design in this Book is not to explain the Properties of Light by Hypotheses, but to propose and prove them by Reason and Experiments. In an Experimentum crucis or “critical experiment” (Book I, Part II, Theorem ii), Newton showed that the color of light corresponded to its “degree of refrangibility” (angle of refraction), and that this angle cannot be changed by additional reflection or refraction or by passing the light through a coloured filter. The work is a vade mecum of the experimenter’s art, displaying in many examples how to use observation to propose factual generalisations about the physical world and then exclude competing explanations by specific experimental tests. However, unlike the Principia, which vowed Non fingo hypotheses or “I make no hypotheses” outside the deductive method, the Opticks develops conjectures about light that go beyond the experimental evidence: for example, that the physical behaviour of light was due its “corpuscular” nature as small particles, or that perceived colours were harmonically proportioned like the tones of a diatonic musical scale.

Other Writings


The Newton Project Created in 1998, the Newton Project seeks to make facsimiles and transcriptions of Newton’s manuscripts available in electronic form and to display their original connections, along with full documentation relating to Newton’s reading such as written notes and annotations.

Newton’s Three Laws of Motion  Sir Isaac Newton: The Universal Law of Gravitation  Sir Isaac Newton and the Unification of Physics & Astronomy  These three links are all part of the Astronomy Web Syllabus at the University of Tennessee. A clear, accessible and well-illustrated guide to Newton’s laws.