Lenses and Waves

78 CHAPTER 3
Huygens did not explain the ‘secret’ of his new method to his brother. He
urged him not to tell anyone about the plans. Constantijn responded quickly.
On the first of June, Huygens answered two letters – now lost – his brother
had sent on May 18 and 24. 111 Constantijn had sent an ocular with only one
side ground according to his instructions, the other being plane. Apparently,
Constantijn had made some objections to his brother’s design. Huygens did
not agree and urged his brother to make a lens exactly to his directives.
Huygens did not await new lenses,
but immediately tried the one
Constantijn had sent him. A week
later he reported on the disappointing
results. When the objective lens was
covered in ordinary fashion, the
Figure 32 Rendering of Huygens’ sketch.
system performed reasonably well. Yet, the system fell short of his
expectations. According to his design, the quality of the image should be
maintained when the whole objective lens was exposed to light. (Figure 32)
“but uncovering the entire glass I see a bit of coloring which leads me to believe that
there is an inconvenience therein, which results from the angle made by the two
surfaces of the objective at the edges. This necessarily causes colors, in such a way that
by making hyperbolic glasses one encounters the same things when making them very
large.” 112
Huygens here tentatively drew an important conclusion. That is, we
recognize that he was on the right track by suspecting that those colors were
inherent to the refraction of rays and could not be prevented by hyperbolic
lenses. Moreover, his suggestion that the production of colors could be
linked to the angle of the lens’ surfaces was promising in light of Newton’s
later theory of colors. The remark may have been inspired by a measurement
Huygens had performed in November 1665. 113 Having read Hooke’s account,
in Micrographia, of colors produced in thin films of transparent material, he
set out to determine the thickness of the film, which Hooke had not been
able to do. He pressed two lenses together to produce colored rings. The
colors appear where the two lenses nearly meet, a situation comparable to
the thin rim of a glass lens. Whether this measurement and the remark of
1668 are connected is, however, mere speculation. In Micrographia, he also
would have found discussions of prism experiments, and the effect of a
prism may also explain the emphasis on the angle between the faces of the
lens at the edge. Whatever be the case, Huygens did not pursue this line of
thinking. He suspected that the proportions of Constantijn’s lens were not
the gist of the problem, “but before assuring that, I would be pleased to
111
OC6, 218-220.
112
OC6, 220-221. “mais en decouvrant tout le verre je vois un peu de couleurs ce qui me fait croire qu’il y
a un inconvenient de costè la, qui provient de l’angle que font les 2 surfaces de l’objectif vers les bords.
qui cause necessairement des couleurs, de sorte qu’en faisant des verres hyperboliques l’on trouueroit la
mesme chose en les faisant fort grands.”
113
OC17, 341. Huygens’ measurements, as well as the experiments Newton performed at the same time,
are amply discussed in Westfall, “Rings”.