CHROMATIC PHENOMENA PRODUCED BY QUARTZ
THE MAGNETIZATION OF LIGHT
RINGS SURROUNDING THE AXES OF CRYSTALS
BIAXAL AND UNIAXAL CRYSTALS
GRASP OF THE UNDULATORY THEORY
THE COLOUR AND POLARIZATION OF SKY-LIGHT
GENERATION OF ARTIFICIAL SKIES.


§ 1. _Action of Crystals on Polarized Light: the Nicol Prism._

We have this evening to examine and illustrate the chromatic phenomena
produced 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 §4, Lecture I., we are able to infer that the one may be
totally reflected, when the other is not. An able optician, named