Lenses and Waves

1690 - TRAITÉ DE LA LUMIÈRE 225
“Where are explained the causes of what happens to it in reflection and in refraction.
And particularly in the strange refraction of Iceland crystal.” 62
Huygens did not intend to expound the principles of natural philosophy nor
the methodology of mathematical science. The subtitle of Traité was not
something like ‘Treatise of light, where the nature and all properties of light
are wonderfully explained in a clear and most probable way according to the
true philosophy’. He expounded natural philosophical principles insofar as
these want to explain the mathematical laws of optics. He considered the
principle of wave propagation – this ‘principal foundation’ – his main
achievement. The validity of his principle was based upon, and implicitly
confined to, the successful derivation of those laws. Traité de la Lumière
offered an example of the proper use of mechanistic philosophy.
Traité de la Lumière did indeed offer better explanation – more plausible,
more probable – but we value it for its epistemic innovations. The wave
theory had originally been planned as a non-committal, explanatory
introduction to his mathematical theory of dioptrics. The eventual outcome
really exceeded geometrical optics. The few methodological issues Huygens
raised, were passed over as a matter of course. In other words, he does not
seem to have been aware of the epistemically innovative character of the
wave theory. Too modestly, from our point of view, Huygens presented his
wave theory as a better explanation of the laws of optics, instead of a new
way of doing the mathematical science of optics.
6.2 Traité de la Lumière and the advent of physical optics
With Traité de la Lumière, Huygens created a new kind of optics, an instance
of what we would call physical optics. I have argued that his actual interest in
optics was the dioptrics of telescopes but that the phenomenon of strange
refraction rather coincidentally directed him to questions pertaining to the
mechanistic nature of light, which he subsequently subjected to the rigorous
mathematical treatment of his dioptrics. This account of its historical
development sheds new light upon our understanding of Huygens’ science,
as I will argue in the next section. This section deals with lines of
interpretation Huygens’ case suggests for the history of seventeenth-century
optics. I will therefore broaden the outlook of my discussion and see how
the themes in my account of Huygens’ optics may be generalized. I do not
profess to offer a new history of seventeenth-century, rather I want to
suggest some lines of interpretation that I consider important for our
historical understanding of the origin physical optics. A major point of
reference will be, of course, Newton, who created his own particular instance
of a physical optics. Despite fundamental differences in their outlook,
intentions and activities, there are important parallels between the optics of
Huygens and Newton.
62 Traité de la Lumière, title-page. “Où sont expliquées les causes de ce qui luy arrive dans la reflexion, &
dans la refraction. Et particulierement dans l’etrange refraction du cristal d’Islande.”