Lenses and Waves

110 CHAPTER 4
“difficulties against Descartes. where would the acceleration come from. he makes light
a tendency to move [conatus movendi], which makes it difficult to understand
refraction as he explains it, at least in my view. … light extends circularly and not in an
instant,…” 9
The concluding words reveal Huygens’ own conception: the nature of light is
to spread out circularly over time. In other words, light consists of waves.
The notes also clarify where Huygens had got the idea to think of light as
waves. “Refraction as explained by Pardies.” 10 Ignace-Gaston Pardies was a
Jesuit father with a keen interest in the mathematical sciences, who actively
participated in the Parisian scientific life, and with whom Huygens
maintained good relations. Pardies had proposed the idea that light consists
of waves and had explained the sine law with it. Huygens listed some
essentials of a wave theory: “transparency without penetration. bodies
capable of this successive movement. Propagation perpendicular to circles.” 11
In the margin he added “vid. micrograph. Hookij”, a reminder to check
Hooke’s alternative wave theory of Micrographia. 12 The original formulation of
Pardies’ theory has been lost, so we cannot know what precisely Huygens
knew of it. He had known of “… the hypothesis of father Pardies …” at
least since August 1669, when he mentioned it in a discussion at the
Académie. 13 On 6 July 1672 Pardies sent him a treatise on refraction that
probably revealed some more details. After Pardies died in 1673, his confrere
Pierre Ango published his explanation of refraction – at least its main lines –
in L’Optique divisée en trois livres (1682). Ango had taken ‘the best parts’ of
Pardies’ theory and blended them with own ideas, but Huygens did not have
a high opinion of Ango’s work. 14 We do not know to what exact extent
Huygens knew Pardies’ theory and derived his own understanding of the
nature of light and refraction from it. We do know that they stood in close
contact over these matters, that Huygens openly acknowledged the
contributions of Pardies, and that the essentials of Pardies’ theory where
central to Huygens’ subsequent attack of strange refraction. He explicitly
recorded the main assumption of Pardies’ derivation of the sine law:
“Propagation perpendicular to circles.” In other words, rays are always
normal to waves. 15
9
OC13, 742. “difficultez contre des Cartes. d’où viendrait l’acceleration. il fait la lumiere un conatus
movendi, selon quoy il est malaisè d’entendre la refraction comme il l’explique, a mon avis au moins. …
lumiere s’estend circulairement et non dans l’instant, …”
10
OC13, 742. “Refraction comment expliquee par Pardies.”
11
OC13, 742. “transparance sans penetration. corps capable de ce mouvement successif. Propagation
perpendiculaire aux cercles.”
12
OC13, 742 note 1.
13
OC16, 184. “… l’hypothese du P. Pardies …” Pardies’ second letter to Newton in their dispute about
colors suggests that Pardies’ wave conception of light was rooted in Grimaldi’s ideas. Shapiro, “Newton’s
definition”, 197.
14
Shapiro, “Kinematic optics”, 209-210. OC10, 203-204.
15
This is discussed below, in section 4.2.2.