Lenses and Waves

156 CHAPTER 4
and his analysis of strangely refracted rays remained separate. As yet,
Huygens had not considered the interaction of waves with Iceland crystal
beyond the observation that it constituted a problem.
This, then, is where the strategy that had been so successful with motion
broke down. Whereas motion only involves observable entities like balls and
pendulums, refraction also entails a consideration of unobservable entities.
As contrasted to the laws of motion, mechanistic causes are involved in the
laws of optics. Huygens believed that a Pardies-like theory of light was a
plausible way to explain refraction, but he had not figured out how it related
to strange refraction. Still, he had committed himself to this theory, and this
was why his earliest study of strange refraction ended inconclusively. In view
of geometrical optics, it may appear strange that Huygens would reject his
law on the basis of mechanistic considerations. Why did he not drop Pardies’
explanation?
Looking at his writings prior to 1672, one would not expect Huygens to
feel so strongly about a mechanistic theory. Prior to the ‘Projet’, Huygens
never considered the nature of refraction more than incidentally. During his
trip to Paris, on 3 January 1661 he discussed refractions – “contre des
Cartes” – with Clerselier and others. 155 In a letter to Moray of 9 June 1662 he
ridiculed Vossius’ ideas on light and said that he had a totally different “…
doctrine concerning refraction …” 156 What it was he did not say. The same
year his brother Lodewijk copied him a letter containing Fermat’s derivation
of the sine law, based on the assumption that light follows the quickest path.
On 8 March 1662 he answered that he admired the ingenuity of Fermat’s
proof, but he found his fundamental assumption “pitoyable” and considered
his doctrine unsatisfactory. According to him, neither Descartes nor Fermat
were capable of proving the law of refraction, “the fundamental theorem of
refractions” – and only experience renders it certain. 157 Apparently Huygens
saw no point in going more deeply into the matter of the cause of refraction.
Despite his deep involvement in dioptrics, Huygens had never elaborated or
adopted a theory of light or refraction. Whether he was simply not really
interested in the matter or had ideas of his own on the back of his head, is
hard to tell. 158
In general, his attitude towards Cartesians and mechanistic philosophizing
had been ambivalent during the previous years. In Paris, as he had already
155 OC22, 544.
156 OC4, 149. “… doctrine touchant la refraction …”.
157 OC4, 71. “Pour faire donc l’accord entre luy [Fermat] et Monsieur des Cartes je dirois que ny l’un ny
l’autre a prouvè la theorem fondamental des refractions, et qu’il n’y a que la seule experience qui nous en
rende certains.”
158 There is one reference to the nature of light in a planned introduction, of 1656, to his treatise on
impact, which appears, however, to be in contradiction with his later views, as Huygens seems to adopt
instantaneous propagation of light: “… if nature as a whole consists of certain corpuscules from the
motion of which every diversity arises, and from the fastest impulse of which light is propagated in a
moment of time and flows throughout the immense expanse of the sky, …” (OC16, 150; translation:
Gabbey, “Huygens and mechanics”, 189)