Lenses and Waves

214 CHAPTER 6
departure from Paris. In 1681, he fell ill again and he returned to The Hague
in September. In Paris, the climate for Protestants was growing less tolerant
in Paris, and when in 1683 Colbert died, Huygens decided to remain in
Holland. He spent some happy years enjoying the reunion with his brother
until duty in the form of Stadtholder William III (= King William) called
upon Constantijn to go to London in 1688. Their correspondence in these
final years of separation gives once again proof of their intimate
comradeship. In 1687, at the ripe old age of ninety, their father Constantijn
sr. passed away. As the second son, Huygens inherited Hofwijck, the county
house in Voorburg, and the title ‘Lord of Zeelhem’, an estate of the family in
what is now Belgian Limburg. 1 The last years of his life he spent much time
at the seclusion of Hofwijck, where his science experienced somewhat of a
prime with, among other things, contributions to the recent developments in
mathematics.
This chapter begins with the publication history of Traité de la Lumière and
a short outline of his later dioptrics. It continues with a review of
seventeenth-century optics from the perspective of Triaté de la Lumière. The
development of Huygens’ wave theory has revealed some themes that in my
view are important for our understanding of the development of
seventeenth-century optics. I will not offer a worked-out history but rather
sketch the lines of a re-interpretation. In the final part of this chapter, I turn
to Huygens’ science as a whole, in particular his alleged Cartesianism. Read
as a textbook example of Cartesian science, Traité de la Lumière is often seen
as exemplary for Huygens’ science. The eventual Traité de la Lumière should
not, however, be taken at face value. When assessing Huygens’ scholarly
goals and conceptions the winding road of its creation needs to be taken into
account. And it particularly casts doubts on its reputed cartesianist essence.
6.1 Creating Traité de la Lumière
With the solution of the problem of strange refraction, nothing stood in the
way of elaborating the ‘Projet’ of 1672. Yet, it lasted more than ten years
before Huygens put his wave theory to print. In the preface of the eventual
Traité de la Lumière, he mentioned three reasons for the delay:
“One may ask why I have tarried so much with publishing this work. The reason is that
I had written it rather negligently in the language in which one sees it, with the intention
to translate it into Latin, doing so in order to have more attention to things. Upon
which I planned to give it together with another treatise on dioptrics, where I explain
the effects of telescopes, and the other things that also belong to that science. But the
pleasure of the novelty being gone, I have gone on postponing the execution of this
1 Father Constantijn had given the estate Zeelhem - and probably the title too - to his son Constantijn in
1651. Christiaan did not have a title, but he bore ‘Lord of Zuylichem’, for example on the title page of
Horologium Oscillatorium. After their father’s death, Constantijn inherited the house at the ‘Plein’ in The
Hague and the title ‘Lord of Zuylichem’, while Christiaan now became a ‘real’ lord, of Zeelhem. Keesing,
“Wanneer”, 63 and Keesing, Constantijn en Christiaan, 112-113.