Lenses and Waves

150 CHAPTER 4
the refracted perpendicular in order to
understand the behavior of strangely
refracted rays, whereas Bartholinus started
from the unrefracted oblique ray.
Huygens was not done yet. He
directed a ray AB through two pieces of
crystal GKVH and LNM, aligned with all
their faces parallel (Figure 55). 144 By
strange refraction one ray BC continues
unrefracted while the other BD is refracted
ordinarily. Upon leaving the crystal the
rays CE and DF become parallel, as
expected. Yet, a curious thing happened
as they entered the second crystal. Ray CE
was not split up into EO and EP, nor was
DF split up into FP and FQ, as ought to be
expected. Instead, CE continued
(unrefracted) along EO and OS, and DF
was refracted (ordinarily) to FQ and QR.
When, on the other hand, AB was not
parallel to the edge of the crystal, or when
both pieces were not parallel to each
other, the rays were split up by the second
piece. A sketch on the preceding page of
his notes shows how the phenomenon
puzzled Huygens. 145 Even if a pair of rays
EP-FP would join, three instead of two
images should have been visible.
Figure 55 Description of polarization.
Huygens’ notes do not reveal whether he just happened to align two
pieces of crystal in this way. The experiment may also have been induced by
his alternative law of strange refraction. The sketches make it clear that he
was considering the unrefracted oblique ray and he may have wondered what
happened if the strange component added to it was equal to the distance EF
between two rays. Huygens may also have been considering the explanation
of strange refraction Pardies had given. If, as Pardies would have it, the
crystal was composed of tiny congruent pieces of crystal, some idea of the
effects might be got by aligning two pieces of the crystal. In that case the
observation would confirm Pardies’ idea as the incident ray would not be
infinitely split up. Whatever induced Huygens to perform the experiment, in
all probability he was the first to observe the phenomenon, polarization as it
is called nowadays.
It puzzled him. In an effort to ‘make sense’ of it, waves returned:
144 Hug2, 177v; OC19, 412-413.
145 Hug2, 177r. Not reproduced in the Oeuvres Complètes.