by the action of crystals, and double-refracting bodies
generally, upon polarized light, and to apply the Undulatory Theory to
their elucidation. For a long time investigators were compelled to
employ plates of tourmaline for this purpose, and the progress they
made with so defective a means of inquiry is astonishing. But these
men had their hearts in their work, and were on this account enabled
to extract great results from small instrumental appliances. For our
present purpose we need far larger apparatus; and, happily, in these
later times this need has been to a great extent satisfied. We have
seen and examined the two beams emergent from Iceland spar, and have
proved them to be polarized. If, at the sacrifice of half the light,
we could abolish one of these, the other would place at our disposal a
beam of polarized light, incomparably stronger than any attainable
from tourmaline.

The beams, as you know, are refracted differently, and from this, as
made plain in Sec.4, Lecture I., we are able to infer that the one may be
totally reflected, when the other is not. An able optician, named
Nicol, cut a crystal of Iceland spar in two halves in a certain
direction. He polished the severed surfaces, and reunited them by
Canada balsam, the surface of union being so inclined to the beam
traversing the spar that the ordinary ray, which is the most highly
refracted, was totally reflected by the balsam, while the
extraordinary ray was permitted to pass on.

Let _b x, c y_ (fig. 34) represent the section of an elongated rhomb
of Iceland spar cloven from the crystal. Let this rhomb be cut along
the plane _b c_; and the two severed surfaces, after having been
polished, reunited by Canada balsam. We learned, in our first lecture,
that total reflection only takes place when a ray seeks to escape from
a more refracting to a less refracting medium, and that it always,
under these circumstances, takes place when the obliquity is
sufficient. Now the refractive index of Iceland spar is, for the
extraordinary ray less, and for the ordinary greater, than for Canada
balsam. Hence, in passing from the spar to the balsam, the
extraordinary ray passes from a less refracting to a more refracting
medium, where total reflection cannot occur; while the ordinary ray
passes from a more refracting to a less refracting medium, where
total reflection can occur. The requisite obliquity is secured by
making the rhomb of such a length that the plane of