Lenses and Waves

246 CHAPTER 6
Huygens should have been more cautious regarding claims about such
motions.
Why did Huygens publish Discours? Huygens may have felt that Newton
challenged him on his own territory, having seen things – the inverse square
law – he had admittedly missed. With Principia Newton threatened to eclipse
the stature of Horologium Oscillatorium and of its author as one of Europe’s
most eminent scholars. Fortunately, Huygens still had this old paper lying
around that treated of just the thing Newton refused to do: the mechanistic
nature of gravity. So, by publishing Discours, Huygens could show that he
could not yet be written off and, in one stroke, how such occult qualities
ought to be properly dealt with. Besides, the 1669 paper on gravity was
already going to be published anyway. It had been among the papers he had,
in 1686, allowed De la Hire to publish. 113 Publishing an extended version
offered Huygens the opportunity to make some corrections and additions
with a view to Principia. 114
The confrontations between Huygens and Newton ran a most
unfortunate course. Their philosophical differences prevented a fruitful
exchange of ideas on the mathematical ground they shared. Discours offered
the very kind of explanation Newton refused to give. Not only did it employ
hypothetical objects like the particles and the streams of subtle matter, but it
also assumed laws applicable to them that lacked foundation. It was the kind
of reasoning Newton would not even do in private and one we would not
expect from the author of Traité de la Lumière. For Newton this made no
difference, Huygens’ waves were as speculative as his vortices and neither
had a place in good science.
Still, Huygens and Newton ultimately wanted the same: to do better
where Descartes had been led astray. This meant that the ‘raisons de
mechanique’ ought to be based on the established laws of motion and were
exempt from a priori truth. Descartes, Huygens observed in his comment on
Baillet’s biography,
“… should have proposed to us his system of physics as an essay of what one can say
with probability in this science whilst admitting nothing but principles of
mechanics…” 115
Newton would agree, but he gave a different interpretation. To the
‘mechanical principles’ he added a new, mathematized concept of force. This
was something Huygens could not do. In the end, he remained faithful to
mechanistic philosophy by restricting its principles to their mathematical
core: Galileo’s science of motion. At the same time, however, in his wave
theory he was the first to realize Descartes’ ideal of a mathematical physics.
He freely applied the Galilean laws of motion to the imperceptible particles
113 See above, page 222.
114 Mahoney argues that Discours offered Huygens an opportunity to publish corrected values for the
Earth’s rotation, resulting from the V.O.C. trials of his clock. Mahoney, “Determination”, 259.
115 OC10, 405. “Il devoit nous proposer son systeme de physique comme un essay de ce qu’on pouvoit
dire de vraisemblable dans cette science en n’admettant que les principes de mechanique …”