Lenses and Waves

THE 'PROJET' OF 1672 157
experienced during his earlier travels, savants displayed much more concern
for philosophical and metaphysical topics than he did. During his trip to
Paris and London in 1660-1661, he expressed his appreciation for the downto-earth
attitude of the Londoners as compared to the more esoteric bent of
the Parisians. The particular group of Parisians he started to cultivate during
that visit consisted of like-minded ‘mathématiciens’ like Auzout and Petit. 159
In this light, his commitment to a mechanistic theory of light seems
paradoxical.
There is reason to believe that Huygens’ move to Paris brought about a
change in his interests. After 1666, at the Académie, he was confronted with
many discussions about natural philosophical topics. Huygens took part, in
particular, in a discussion on gravity in August 1669. 160 Van Berkel has
suggested that Huygens began to emphasize mechanistic explanations
because he was dissatisfied with the many theories put forward at the
Académie that were not (properly) mechanistic. 161 His opponents in the
discussion on gravity assumed, for example, attractive forces. Huygens’
paper on gravity of 1669 may have had the effect that he saw the value of
discussing natural philosophical questions. In the discussion about gravity,
allusions to the nature of light also come out for the first time. In the notes
he took during this discussion the question is asked how light can be
understood when perfectly hard bodies do not rebound. And he added that
if the corpuscles explaining light were elastic and composed this would
accord with “…l’hypothese du P. Pardies …”. 162 It is reasonable to suggest
that the Parisian scene compelled Huygens to think more and more actively
on questions of mechanistic philosophy than he had done in The Hague.
Pardies himself may have been a decisive factor in this regard. It is not
inconceivable that Pardies’ wave theory showed Huygens that it was possible
to pursue mechanistic philosophizing in a satisfactory manner. Although we
do not know in what manner Huygens intended to treat the mechanistic
causes of refraction, the Pardies-like theory may have offered the kind of
middle course between a non-committal, Barrovian account and a Cartesian
derivation that suited him. Statements in the ‘Projet’ seem to rule out a
Cartesian view, whereas his investigation of strange refraction suggests that
Huygens took questions regarding the nature of light more seriously than
Barrow did. In the next chapter, I discuss what may have attracted Huygens
in Pardies’ theory. I think the example set by Pardies made Huygens realize
that it was possible to treat the nature of light in a ‘comprehensible’ way. 163
He saw no alternative for Pardies’ waves and, strange as it may seem in view
159
Hahn, ”Huygens and France”, 58-59.
160
See below, section 6.3.1.
161
Van Berkel, “Legacy”, 55-59.
162
OC16, 184.
163
I owe this suggestion to Alan Shapiro. It is elaborated in section 5.2.2.