1 The Birth of Science - MSRI

332 11. The Age-Long Recovery
ton’s fierce animosity toward him. (The second had to do with optics, another
of Newton’s main scientific interests. Hooke, being aware of several
phenomena caused by diffraction and interference, rejected Newton’s corpuscular
optics, and instead founded the wave theory of light. He discovered,
for instance, the interference phenomenon now known as “Newton’s
rings”, which is quite incompatible with Newtonian optics. He achieved
a lot more, including the discovery of cells and of “Boyle’s law” on the
compressibility of gases. It is a legacy of Newton’s long-armed hatred that
even today our textbooks only associate Hooke with the study of elastic
forces — a field where his investigations allowed the substitution, at least
in certain cases, of a dynamometer-based definition of force for the one
given in the Principia.)
But the law itself goes back further. It was stated in 1645 by Boulliau,
based on the argument that the sun’s force, like the light it emits, must
decay with distance in inverse proportion to the area reached. 143 Such
considerations were not new even then, having been made by Kepler,
though the latter rejected the analogy between the sun’s motor force and
light, imagining that force, spreading only over the plane of the ecliptic,
was inversely proportional to distance. 144
Continuing our brief backward history: the analogy between the sun’s
“virtue” and the light it sends out was drawn in the thirteenth century
by Roger Bacon, who outlined a quantitative theory of any propagation
along straight lines, and so arrived at the inverse square law, at least implicitly,
since he attributed the weakening of the action with distance to
the decrease in the cone (solid angle) under which the acted-on body is
seen by the agent. 145
We must conclude that knowledge of the dependence of gravitational
force on the distance predated, in medieval and modern times, not only
any connexion with Kepler’s laws but even the statement of the second
principle of dynamics. In other words, this property of gravitational force
was known not only before it was used to explain any phenomenon, but
even before anyone had properly established what should be understood
by “force”. This odd order of ideas becomes intelligible if we assume that
143 Ismaeli Bullialdi Astronomia Philolaica, Paris, Piget, 1645, p. 23. The reference to Philolaus in the
work’s title shows that Boulliau, too, meant to reconstruct Pythagorean astronomy.
144 J. Kepler, Astronomia nova, xxxvi.
145 R. Bacon, Specula mathematica, III, ii. Johannes Combachius first edited and printed this work
(Frankfurt, 1614), but it is now more readily available as the fourth part of the Opus majus. Bacon
calls multiplicatio secundum figuras the law of dependence on distance of an action that radiates in
all directions along straight lines, and he adds that the lines along which it radiates terminate in
the concave surface of a sphere (op. cit., II, iii).
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