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that color perception was caused by pressure exerted
on the eye by swirling vortices. Newton read about
this and didn’t agree.
Descartes had conceived of the notion as a philo-
sophical construct. Newton would only accept exper-
imental proof. He was curious as to how the shape of
the eye affected color perception. To deform his eye-
ball, he placed a bodkin, a kind of blunt needle,
under his eyelid. By carefully moving the bodkin, he
pushed his eye around, producing odd circles of color
and other images.
But not all of his experiments were quite so dan-
gerous. He watched the light of the setting Sun
through the fringes of a feather. This created a dif-
fraction effect, producing bands of light and color as
the incoming sunlight bent around the edges of each
barb. In another effort to understand vision, Newton
stared at an image of the Sun ref lected in a mirror for
as long as possible. When he finally looked away at a
wall, he saw concentric circles of light and color he
called “motion of spirits.”Newton vs. Hooke
Although Newton is given credit for the theory
of gravity, he wasn’t working in a vacuum. Robert
Hooke, Edmond Halley, and others were also
intrigued by the problem.
Hooke was one of the most brilliant scientists
in 17th-century England. He has even been called
England’s Leonardo. Yet we have no contemporary
portrait of the man — a result of Newton’s vendetta
against him.
Hooke came from poor circumstances. He served
as curator of the Royal Society, a scientific fraternity,
in its early years, where he was charged with caring
for the society’s growing collection of equipment and
setting up the experiments for the regular meetings
of its members. His 1665 book, Micrographia, was anThe famous Flower
of Kent apple tree at
Woolsthorpe Manor
sparked Newton’s
interest in gravity.
According to
Newton himself,
when he saw an
apple fall from the
tree one day, he
wondered why the
fruit fell straight
down, rather than
sideways, upward,
or in some other
fashion. FRITZBRUNO/
WIKIMEDIA COMMONSLEFT: Newton’s
own first edition of
Philosophiae Naturalis
Principia Mathematica
includes his
handwritten notes
for changes in a
future publication.
Also note Samuel
Pepys’ name — Pepys
was president of the
Royal Society at the
time of the book’s
publication. ANDREW DUNNRIGHT: The interior
pages of a personal
copy of Principia also
show annotations in
Newton’s hand.
CAMBRIDGE UNIVERSITY LIBRARY