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

The same experiment led Newton to two further conclusions, both of real consequence. First, he gave up any hope of “the perfection of Telescopes” based on combinations of lenses and turned to the principle of the reflector; second, he held it to be no longer a subject of dispute “whether Light be a Body.” Observing, however, that it “is not so easie” to determine specifically “what Light is,” he concluded, “I shall not mingle conjectures with certainties.”77

Newton's letter was, as promised, read at the Royal Society on 6 February 1672. A week later Hooke delivered a report in which he criticized Newton for asserting a conclusion that did not seem to Hooke to follow necessarily from the experiments described, which—in any event—Hooke thought too few. Hooke had his own theory which, he claimed, could equally well explain Newton's experimental results.

In the controversy that followed with Hooke, Huygens, and others, Newton quickly discovered that he had not produced a convincing demonstration of the validity and significance of the conclusions he had drawn from his experiments. The objection was made that Newton had not explored the possibility that theories of color other than the one he had proposed might explain the phenomena. He was further criticized for having favored a corporeal hypothesis of light, and it was even said that his experimental results could not be reproduced.

In reply, Newton attacked the arguments about the “hypothesis” that he was said to have advanced about the nature of light, since he did not consider this issue to be fundamental to his interpretation of the “experimentum crucis.” As he explained in reply to Pardies78 he was not proposing “an hypothesis,” but rather “properties of light” which could easily “be proved” and which, had he not held them to be true, he would “rather have ... rejected as vain and empty speculation, than acknowledged even as an hypothesis.” Hooke, however, persisted in the argument. Newton was led to state that he had deliberately declined all hypotheses so as “to speak of Light in general terms, considering it abstractly, as something or other propagated every way in straight lines from luminous bodies, without determining what that Thing is.” But Newton's original communication did assert, “These things being so, it can be no longer disputed, whether there be colours in the dark, nor ... perhaps, whether Light be a Body.” In response to his critics, he emphasized his use of the word “perhaps” as evidence that he was not committed to one or another hypothesis on the nature of light itself.79

One consequence of the debate, which was carried on over a period of four years in the pages of the Philosophical Transactions and at meetings of the Royal Society, was that Newton wrote out a lengthy “Hypothesis Explaining the Properties of Light Discoursed of in my Several Papers,”80 in which he supposed that light “is something or other capable of exciting vibrations in the aether,” assuming that “there is an aetherial medium much of the same constitution with air, but far rarer, subtler, and more strongly elastic.” He suggested the possibility that “muscles are contracted and dilated to cause animal motion,” by the action of an “aethereal animal spirit,” then went on to offer ether vibration as an explanation of refraction and reflection, of transparency and opacity, of the production of colors, and of diffraction phenomena (including Newton's rings). Even “the gravitating attraction of the earth,” he supposed, might “be caused by the continual condensation of some other such like aethereal spirit,” which need not be “the main body of phlegmatic aether, but ... something very thinly and subtilly diffused through it.”81

The “Hypothesis” was one of two enclosures that Newton sent to Oldenburg, in his capacity of secretary of the Royal Society, together with a letter dated 7 December 1675. The other was a “Discourse of Observations,” in which Newton set out “such observations as conduce to further discoveries for completing his theory of light and colours, especially as to the constitution of natural bodies, on which their colours or transparency depend.” It also contained Newton's account of his discovery of the “rings” produced by light passing through a thin wedge or layer of air between two pieces of glass. He had based his experiments on earlier ones of a similar kind that had been recorded by Hooke in his Micrographia (observation 9). In particular Hooke had described the phenomena occurring when the “lamina,” or space between the two glasses, was “double concave, that is, thinner in the middle then at the edge”; he had observed “various coloured rings or lines, with differing consecutions or orders of Colours.”

When Newton's “Discourse” was read at the Royal Society on 20 January 1676, it contained a paragraph (proposition 3) in which Newton referred to Hooke and the Micrographia, “in which book he hath also largely discoursed of this ... and delivered many other excellent things concerning the colours of thin plates, and other natural bodies, which I have not scrupled to make use of so far as they were for my purpose.”82 In recasting the “Discourse” as parts 1, 2, and 3 of book II of the Opticks, however, Newton omitted this statement. It may be assumed that he had carried these experiments so much further than Hooke, introducing careful measurements and quantitative analysis, that he believed them to be his own. Hooke,