Optics

The work that first brought Newton to the attention of the scientific community
was not a theoretical or even a mathematical effort; it was a prodigious technical
achievement. In 1668, shortly before his appointment as Lucasian Professor,
Newton designed and constructed a “reflecting” telescope. In previous telescopes,
beginning with the Dutch invention and Galileo’s improvement, light was
refracted and focused by lenses. Newton’s telescope reflected and focused light
with a concave mirror. Refracting telescopes had limited resolution and to
achieve high magnification had to be inconveniently long. (Some refracting telescopes
at the time were a hundred feet long, and a thousand-footer was planned.)
Newton’s design was a considerable improvement on both counts.
Newton’s telescope project was even more impressive than that of Galileo.
With no assistance (Galileo employed a talented instrument maker), Newton cast
and ground the mirror, using a copper alloy he had prepared, polished the mirror,
and built the tube, the mount, and the fittings. The finished product was just six
inches in length and had a magnification of forty, equivalent to a refracting telescope
six feet long.
Newton was not the first to describe a reflecting telescope. James Gregory,
professor of mathematics at St. Andrews University in Scotland, had earlier published
a design similar to Newton’s, but could not find craftsmen skilled enough
to construct it.
No less than Galileo’s, Newton’s telescope was vastly admired. In 1671, Barrowdemonstrated it to the London gathering of prominent natural philosophers
known as the Royal Society. The secretary of the society, Henry Oldenburg, wrote
to Newton that his telescope had been “examined here by some of the most
eminent in optical science and practice, and applauded by them.” Newton was
promptly elected a fellow of the Royal Society.
Before the reflecting telescope, Newton had made other major contributions in
the field of optics. In the mid-1660s he had conceived a theory that held that
ordinary white light was a mixture of pure colors ranging from red, through orange,
yellow, green, and blue, to violet, the rainbow of colors displayed by a prism
when it receives a beam of white light. In Newton’s view, the prism separated
the pure components by refracting each to a different extent. This was a contradiction
of the prevailing theory, advocated by Hooke, among others, that light in
the purest form is white, and colors are modifications of the pristine white light.
Newton demonstrated the premises of his theory in an experiment employing
two prisms. The first prism separated sunlight into the usual red-through-violet
components, and all of these colors but one were blocked in the beam received
by the second prism. The crucial observation was that the second prism caused
no further modification of the light. “The purely red rays refracted by the second
prism made no other colours but red,” Newton observed in 1666, “& the purely
blue no other colours but blue ones.” Red and blue, and other colors produced
by the prism, were the pure colors, not the white.
Soon after his sensational success with the reflecting telescope in 1671, Newton
sent a paper to Oldenburg expounding this theory. The paper was read at a
meeting of the Royal Society, to an enthusiastically favorable response. Newton
was then still unknown as a scientist, so Oldenburg innocently took the additional
step of asking Robert Hooke, whose manifold interests included optics, to
comment on Newton’s theory. Hooke gave the innovative and complicated paper
about three hours of his time, and told Oldenburg that Newton’s arguments were
not convincing.
This response touched off the first of Newton’s polemical battles with his critics.
His first reply was restrained; it prompted Hooke to give the paper in question
more scrutiny, and to focus on Newton’s hypothesis that light is particle-like.
(Hooke had found an inconsistency here; Newton claimed that he did not rely on
hypotheses.) Newton was silent for awhile, and Hooke, never silent, claimed that
he had built a reflecting telescope before Newton. Next, Huygens and a Jesuit
priest, Gaston Pardies, entered the controversy. Apparently in support of Newton,
Huygens wrote, “The theory of Mr. Newton concerning light and colors appears
highly ingenious to me.” In a communication to the Philosophical Transactions of
the Royal Society, Pardies questioned Newton’s prism experiment, and Newton’s
reply, which also appeared in the Transactions, was condescending. Hooke complained
to Oldenburg that Newton was demeaning the debate, and Oldenburg
wrote a cautionary letter to Newton. By this time, Newton was aroused enough to
refute all of Hooke’s objections in a lengthy letter to the Royal Society, later published
in the Transactions. This did not quite close the dispute; in a final episode,
Huygens reentered the debate with criticisms similar to those offered by Hooke.
In too many ways, this stalemate between Newton and his critics was petty,
but it turned finally on an important point. Newton’s argument relied crucially
on experimental evidence; Hooke and Huygens would not grant the weight of
that evidence. This was just the lesson Galileo had hoped to teach earlier in the
century. Now it was Newton’s turn.