Lenses and Waves

THE 'PROJET' OF 1672 143
crystal is easily cleft but only along surfaces parallel to its faces. He noted
that other ways of cutting it had not been successful yet. He evidently
alluded to Bartholinus’ discussion of refraction in planes that are not parallel
to the natural faces of the crystal.
These data were written mainly in French. The investigation continues
with an analysis, written in Latin, of the way rays are refracted by the crystal.
Huygens did not adopt the conclusion Bartholinus had drawn from his
experiments with the crystal, in the form of a ‘law’ of strange refraction.
Huygens proposed an alternative ‘law’.
Bartholinus’ experimenta
To see where he took off in his study of strange refraction and to compare
Huygens’ analysis with Bartholinus’, we first turn to the main argument of
Experimenta crystalli islandici disdiaclastici. As regards its methodological
structure, Experimenta is an interesting work in the history of optics.
Bartholinus explicitly discussed how the main principle for the mathematical
analysis of the phenomenon under consideration - his law of strange
refraction - was derived from and subsequently founded upon empirical
investigation. He did not report literally on his experiments, but stylized and
ordered them into a mathematical argument. 128 This way of integrating
experimental inquiry into mathematical inference is akin to the approach of
men like Pascal and Mariotte, and seems to have been a common strategy in
Figure 48 Double refraction according to Bartholinus.