170 Discoveries of Huygens respecting Polarisatim. 
Huygens next proceeds to explain his theory of double re- 
fraction, founded on the hypothesis of light being the undulations 
of an ethereal medium. He supposes the ordinary refraction to 
be produced by spherical undulations, propagated through the 
crystal, while the extraordinary refraction arises from spheroidal 
undulations, the form of the generating ellipse being determined 
by the ratio of the two refractions. He then proceeds to shew 
that the deviation of the extraordinary ray, calculated upon this 
hypothesis, agrees precisely with observation. As this part of 
Huygens’ treatise is of a less elementary nature than the rest, 
we shall make it the subject of a separate paper in our next 
Number, 
Huygens' Discovery (f the Polarisation of Light. — After he 
had drawn up his treatise on double refraction, Huygens disco- 
vered what he calls “ a wonderful phenomenon,” or the pola- 
risation of the two pencils of light formed by Iceland spar. Ha- 
ving taken two pieces of Iceland crystal, and held them with a 
space between, he found that if all the sides of the one piece 
were parallel to tl^ose of the other, as in Fig. 4., the two pencils, 
BC, BD, formed by the double refraction of the ray AB, were 
not doubly refracted in passing through the second crystal in the 
lines EF, GH ; the pencil HG, which had been regularly refract- 
jed by the first crystal, being now only regularly refracted by the 
second, while the pencil CE, which bad been extraordinarily 
refracted by the first, was only extraordinarily refracted by the 
second. The very same result took place, not only in the posi- 
tion shewn in the figure, but in all other positions, where the 
principal sections of the two crystals were in the same plane, and 
whether the two surfaces were parallel or not. “ Now, it is 
w^onderful,” says Huygens, ‘‘ why the rays CE and DG, inci- 
dent from the air upon the lower crystal, should not divide 
themselves like the first ray AB. .One would say that the ray 
DG, in passing through the upper crystal, must have lost what 
was required to move the matter which served for the extraordi- 
nary refraction ; and, in like manner, that CE had lost what 
was necessary to move the matter which served for the ordinary 
j’efraction. There is, however, another fact which overturns 
this reasoning, which is, that when we arrange the two crystals, 
so that the planes which form the principal sections cut one ano- 
