Lenses and Waves

158 CHAPTER 4
of his earlier lack of interest for mechanistic explanation, he did not want to
drop they idea of giving an explanation of refraction.
I suspect few contemporaries would have objected if Huygens had passed
over strange refraction in a treatise on dioptrics. Few would realize that it
contradicted his explanation of refraction; even Pardies himself thought that
strange refraction could easily be resolved with his theory. It looks like
Huygens made things difficult for himself by choosing to bring up the
‘difficulté’ of strange refraction. Not only did he decide to include an
account of strange refraction, but he also wanted to reconcile this with his
mechanistic conception of ordinary refraction. According to Huygens, the
causes of the various properties of light could only be plausible when they
were consistent with one another. Therefore, his study of strange refraction
had not yet come to an end.
The question Huygens had posed in the ‘Projet’ – the ‘difficulté’ of
strange refraction – carried the seed of a turn towards a new way to consider
the problem of finding a new law of optics. His attitude towards the
justification of established laws and the way he had searched for a law of
strange refraction had been traditional. Had he been satisfied with the
results, ‘Dioptrique’ would have been a traditional treatise in geometrical
optics, unaffected by the changes initiated by Kepler and Descartes. The
problem of strange refraction turned out to be not traditional, as it was a
problem of waves instead of rays. In the next chapter we shall see how
Huygens took the remaining step, that of analyzing strange refraction on the
level of waves of light.