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