Lenses and Waves

204 CHAPTER 5
from the established laws of optics. It was no ‘truth drawn from experience’,
for it mixed the properties of rays with the properties of waves. The ellipse
construction was inextricably bound up with the principle of wave
propagation.
5.3 A second EUPHKA
With his theory explaining the established laws of refraction as well as the
strange refraction of Iceland crystal, such as we have set it forth in the
preceding section, Huygens addressed the Académie in the summer of 1679,
nearly two years after the EUPHKA of 6 August 1677. We can imagine his
expectations. He would present to his colleagues a truly mechanistic
explanation of the properties of light, firmly founded upon the laws of
motion. Only with his principle could the laws of optics be derived in a
sound and coherent way. In addition, he would present a wonderful
confirmation by explaining the baffling phenomenon of strange refraction
with it. Things could hardly be otherwise. His theory of waves was the only
comprehensible explanation conceivable. It would show what rigorous
thinking could yield. Thinking that was not easily satisfied, but aimed at
rendering matters intelligible without compromise.
At least one member of the Académie was not convinced immediately. It
was Rømer, the same who in 1677 had provided Huygens with observational
proof of the finite speed of light. Rømer’s intervention forced Huygens out
of the safe domain of rational analysis, where the properties of light are
derived from clear and distinct concepts by means of rigorous deduction, to
the empirical domain of tinkering with the unpolished reality of
measurement and experimentation. Huygens managed to counter Rømer’s
objections by measurements acquired by a precise and powerful observing
technique and a ingenious experiment that reveals a remarkable command.
Remarkably, as up to this point Huygens had repeatedly steered clear of
empirical grounds. These measurements of 1679 provided the data of the
eventual Traité de la Lumière which, in other words, date from Huygens’ third
go at strange refraction.
In a letter of 11 November 1677 Huygens had informed Rømer of a
letter he had written to Colbert on October 14. 134 He had praised Rømer’s
“belle invention”, and now added that he had always assumed the same in
order to explain the properties of light. 135 He added further that his
hypothesis to explain strange refraction was so simple and so accurate and
agreed so well with observation that he did not doubt that everyone would
accept it. 136 Replying on December 3, Rømer expressed the opinion that
optical principles that could not account for strange refraction were useless. 137
He was curious after Huygens’ ideas and added that he himself had also done
134
OC8, 41.
135
OC8, 36-37. This letter is quoted on page 161.
136
OC8, 41.
137
OC8, 45.