Lenses and Waves

242 CHAPTER 6
of view. This does not mean that mechanistic philosophy did not influence
Huygens’ studies of motion. Mechanistic views were a source of inspiration,
for example when he equated gravity and circular motion in 1659. 97 Yet, their
role was limited and they were virtually absent in his subsequent analysis of
circular motion. 98 Forces were gradually pushed back and velocity became
Huygens’ ultimate concept of motion. Not just because of the dictates of
mechanistic philosophy, it was the only way he could deal with motion
mathematically.
Huygens lack of interest in the mechanistic nature of light, at least the
absence of any recorded consideration up to the late 1660s, was not singular.
In this sense, his optics is exemplary for his science in general. Before his
move to Paris in 1666, philosophizing about the mechanistic nature of things
is virtually absent in his writings. As I see it, with his brief note on subtle
matter Huygens returned for the first time to the realm of thought that had
made such an impression on his youthful mind. By that time his thinking on
matter and motion had ripened into a thoroughly Galilean conception that
he subsequently injected into mechanistic philosophy. He understood
mechanistic philosophy as an ultimately mathematical idiom. Not the
ontological idiom of Principia Philosophiae, but the mathematical idiom of the
laws of motion of the Discorsi.
The 1669 paper on gravity makes clear that the conception of
mechanistic philosophy that underlies Traité de la Lumière had already taken
shape. In Huygens’ view ‘raisons de mechanique’ invoked matter moving
according to established rules and they were intended to formulate
hypotheses that explained the observable properties of phenomena. As
regards gravity the doctrine had not been realized in full, and in optics it had
not yet taken form. The mechanistic nature of light first enters Huygens’
writings with his approving reference to Pardies’ theory in course of the 1669
debate on gravity.
Huygens versus Newton
In his dispute with Newton on colors Huygens took a more strict position
regarding the need for mechanistic explanation than he did in the ‘Projet’
jotted down at the same time. When in September 1672 Huygens finally
realized the full import of the new theory of light and colors he (grudgingly)
accepted different refrangibility but he did not accept the compound nature
of white light. 99 According to him, Newton first had to solve the difficulty of
explaining it mechanistically. Apparently, Huygens thought that sometimes
mechanistic explanation was more than just an optional subject to satisfy ‘the
mind that loves to know the reason of everything’ as he phrased it in the
‘Projet’.
97
See above, page 96.
98
Yoder, Unrolling time, 17-19.
99
See above, page 88.