NEWTON, ISAAC (b. Woolsthorpe, England, 25 December 1642; d. London, England, 20 March 1727), mathematics, dynamics, celestial mechanics, astronomy, optics, natural philosophy.

    on the other hand, understandably thought that he deserved more credit for his own contributions—including hypothesis-based explanations—than Newton was willing to allow him.83 Newton ended the resulting correspondence on a conciliatory note when he wrote in a letter of 5 February 1676, “What Des-Cartes did was a good step. You have added much in several ways, and especially in taking the colours of thin plates into philosophical consideration. If I have seen further it is by standing on the shoulders of Giants.”84

The opening of Newton's original letter on optics suggests that he began his prism experiments in 1666, presumably in his rooms in Trinity, but was interrupted by the plague at Cambridge, returning to this topic only two years later. Thus the famous eighteen months supposedly spent in Lincolnshire would mark a hiatus in his optical researches, rather than being the period in which he made his major discoveries concerning light and color. As noted earlier, the many pages of optical material in Newton's manuscripts85 and notebooks have not yet been sufficiently analyzed to provide a precise record of the development of his experiments, concepts, and theories.

The lectures on optics that Newton gave on the assumption of the Lucasian chair likewise remain only incompletely studied. These exist as two complete, but very different, treatises, each with carefully drawn figures. One was deposited in the University Library, as required by the statutes of his professorship, and was almost certainly written out by his roommate, John Wickins,86 while the other is in Newton's own hand and remained in his possession.87 These two versions differ notably in their textual content, and also in their division into “lectures,” allegedly given on specified dates. A Latin and an English version, both based on the deposited manuscript although differing in textual detail and completeness, were published after Newton's death. The English version, called Optical Lectures, was published in 1728, a year before the Latin. The second part of Newton's Latin text was not translated, since, according to the preface, it was “imperfect” and “has since been published in the Opticks by Sir Isaac himself with great improvements.” The preface further states that the final two sections of this part are composed “in a manner purely Geometrical,” and as such they differ markedly from the Opticks. The opening lecture (or section 1) pays tribute to Barrow and mentions telescopes, before getting down to the hard business of Newton's discovery “that ... Rays [of light] in respect to the Quantity of Refraction differ from one another.” To show the reader that he had not set forth “Fables instead of Truth,” Newton at once gave “the Reasons and Experiments on which these things are founded.” This account, unlike the later letter in the Philosophical Transactions, is not autobiographical; nor does it proceed by definitions, axioms, and propositions (proved “by Experiment”), as does the still later Opticks.88

R. S. Westfall has discussed the two versions of the later of the Lectiones opticae, which were first published in 1729;89 he suggests that Newton eliminated from the Lectiones those “parts not immediately relevant to the central concern, the experimental demonstration of his theory of colors.” Mathematical portions of the Lectiones have been analyzed by D. T. Whiteside, in Newton's Mathematical Papers, while J. A. Lohne and Zev Bechler have made major studies of Newton's manuscripts on optics. The formation of Newton's optical concepts and theories has been ably presented by A. I. Sabra; an edition of the Opticks is presently being prepared by Henry Guerlac.

Lohne finds great difficulty in repeating Newton's “experimentum crucis,”90 but more important, he has traced the influence of Descartes, Hooke, and Boyle on Newton's work in optics.91 He has further found that Newton used a prism in optical experiments much earlier than hitherto suspected—certainly before 1666, and probably before 1665—and has shown that very early in his optical research Newton was explaining his experiments by “the corpuscular hypothesis.” In “Questiones philosophicae,” Newton wrote: “Blue rays are reflected more than red rays, because they are slower. Each colour is caused by uniformly moving globuli. The uniform motion which gives the sensation of one colour is different from the motion which gives the sensation of any other colour.”92

Accordingly, Lohne shows how difficult it is to accept the historical narrative proposed by Newton at the beginning of the letter read to the Royal Society on 8 February 1672 and published in the Philosophical Transactions. He asks why Newton should have been surprised to find the spectrum oblong, since his “note-books represent the sunbeam as a stream of slower and faster globules occasioning different refrangibility of the different colours?” Newton must, according to Lohne, have “found it opportune to let his theory of colours appear as a Baconian induction from experiments, although it primarily was deduced from speculations.” Sabra, in his analysis of Newton's narrative, concludes that not even “the ‘fortunate Newton’ could have been fortunate enough to have achieved this result in such a smooth manner.” Thus one of the most famous examples of the scientific method in operation now seems to have been devised