Lenses and Waves

236 CHAPTER 6
6.3 Traité de la Lumière and Huygens’ oeuvre
Traité de la Lumière looks like a complete and purposively elaborated whole,
an exemplar of a seventeenth-century mathematical physics in which the
principles of optics are derived from a mathematized theory of the
corpuscular nature of light. Traité de la Lumière is often regarded as exemplary
for Huygens’ science as well. In particular, historians has regarded it as a
proof of the fundamental role mechanistic philosophy played in his science.
Yet, it is risky to base an interpretation on the eventual text, as it barely hints
at the winding road towards the final result. In my view, the historicization
of Traité de la Lumière of the preceding chapters sheds new light on Huygens’
science in general.
In the historical literature, Traité de la Lumière is often seen as a direct
response to Descartes’ optics. Sabra first characterizes Huygens’
Cartesianism and then shows how it produced the wave theory. 81 Along
similar lines, E.J. Dijksterhuis calls Traité de la Lumière the high-point of
seventeenth-century mechanistic science and its author the first perfect
Cartesian. 82 In Traité de la Lumière the mechanistic conception of nature was
indeed perfected, but mechanistic science had not given the momentum to
its materialization. Seeing it as a response to Descartes fails to account for
the fact that mechanistic thinking is virtually absent in his optics prior to the
1670s. At a relatively late stage, Huygens began considering the causes of the
laws of optics and only while developing the wave theory did Huygens
become a ‘mechanistic thinker’. The new form Traité de la Lumière gave to
mechanistic explanation was the outcome of questions pertaining to
geometrical optics, rather than some preconceived plan or mechanistic
programme.
I think that Huygens’ commitment to mechanistic philosophy was not as
decisive as is often assumed. His ideas on the nature of light were, of course,
based on prevailing mechanistic conceptions, and Huygens was well aware of
the problems in Descartes’ optics. Yet, questions of mechanistic theory were
not the impetus or drive of his consideration of causes in optics. The
problem of strange refraction set it going and its persistence gave it an
unexpected twist, eventually resulting in the wave theory as we know it.
Huygens’ mechanistic conceptions regarding the nature of light only played a
limited role in the development and establishment of the wave theory. He
displayed a particular lack of interest in elaborating the mechanistic finesses
of his theory, for example by avoiding the question why waves propagate
asymmetrically in Iceland crystal. The strict definition of what counts as a
‘raison de mechanique’ is largely my interpretation, as Huygens himself did
not explicate it in Traité de la Lumière.
81 “There is in fact evidence to show that Huygens first arrived at his views regarding the nature of light
and the mode of its propagation through an examination of Descartes’ ideas.” Sabra, Theories of Light, 198.
82 Dijksterhuis, Mechanization, 503-507.