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

    raised to any given power, and taken together, may make everywhere the same sum.”72

FIGURE 2. Newton's drawing of the crucial experiment (University Library, Cambridge, MS Add. 4002, fol. 128a). Newton himself was a careful draftsman, but the diagrams have become so corrupt in later editions as to violate the fundamental laws of optics.

Newton's analytic solution of the curve of least descent is of particular interest as an early example of what became the calculus of variations. Newton had long been concerned with such problems, and in the Principia had included (without proof) his findings concerning the solid of least resistance. When David Gregory asked him how he had found such a solid, Newton sent him an analytic demonstration (using dotted fluxions), of which a version was published as an appendix to the second volume of Motte's English translation of the Principia.73

Optics.

The study of Newton's work in optics has to date generally been limited to his published letters relating to light and color (in Philosophical Transactions, beginning in February 1672), his invention of a reflecting telescope and “sextant,” and his published Opticks of 1704 and later editions (in Latin and English). There has never been an adequate edition or a full translation of the Lectiones opticae. Nor, indeed, have Newton's optical manuscripts as yet been thoroughly studied.74

Newton's optical work first came to the attention of the Royal Society when a telescope made by him was exhibited there. Newton was elected a fellow shortly thereafter, on 11 January 1672, and responded by offering the Society an account of the discovery that had led him to his invention. It was, he proudly alleged, “the oddest if not the most considerable detection yet made in the operations of nature”: the analysis of dispersion and the composition of white light.

In the published account Newton related that in 1666 (“at which time I applyed myself to the grinding of Optick glasses of other figures than Spherical”) he procured a triangular glass prism, “to try therewith the celebrated Phaenomena of Colours.” Light from a tiny hole in a shutter passed through the prism; the multicolored image--to Newton's purported surprise--was of “an oblong form,” whereas “according to the received laws of Refraction, I expected [it] should have been circular.” To account for this unexpected appearance, Newton looked into a number of possibilities, among them that “the Rays, after their trajection through the Prisme did not move in curve lines,” and was thereby led to the famous “experimentum crucis.”75 In this experiment Newton used two prisms: the first was employed to produce a spectrum on an opaque board (BC) into which a small hole had been drilled; a beam of light could thus pass through the hole to a second board (DE) with a similar aperture; in this way a narrow beam of light of a single color would be directed to a second prism, and the beam emerging from the second prism would project an image on another board (Fig. 2). Thus, all light reaching the final board had been twice subjected to prismatic dispersion. By rotating the first prism “to and fro slowly about its Axis,” Newton allowed different portions of the dispersed light to reach the second prism.

Newton found that the second prism did not produce any further dispersion of the “homogeneal” light (that is, of light of about the same color); he therefore concluded that “Light it self is a Heterogeneous mixture of differently refrangible Rays”; and asserted an exact correspondence between color and “degree of Refrangibility” (the least refrangible rays being “disposed to exhibit a Red colour,” while those of greatest refrangibility are a deep violet). Hence, colors “are not Qualifications of Light, derived from Refractions, or Reflections of natural Bodies,” as commonly believed, but “Original and connate properties,” differing in the different sorts of rays.76