ACTION OF BODIES MECHANICALLY STRAINED OR PRESSED
  ACTION OF SONOROUS VIBRATIONS
  ACTION OF GLASS STRAINED OR PRESSED BY HEAT
  CIRCULAR POLARIZATION
  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.

Sec. 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 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 which b c is the section shall be perpendicular, or nearly so, to the two end surfaces of the rhomb b x, c y.