we have seen that the least obliquity
on the part of the crystals permits light to get through both. Now
suppose, when the two plates are crossed, that we interpose a third
plate of tourmaline between them, with its axis oblique to both. A
portion of the light transmitted by the first plate will get through
this intermediate one. But, after it has got through, _its plane of
vibration is changed_: it is no longer perpendicular to the axis of
the crystal in front. Hence it will, in part, get through that
crystal. Thus, by pure reasoning, we infer that the interposition of a
third plate of tourmaline will in part abolish the darkness produced
by the perpendicular crossing of the other two plates. I have not a
third plate of tourmaline; but the talc or mica which you employ in
your stoves is a more convenient substance, which acts in the same
way. Between the crossed tourmalines, I introduce a film of this
crystal with its axis oblique to theirs. You see the edge of the film
slowly descending, and, as it descends, light takes the place of
darkness. The darkness, in fact, seems scraped away, as if it were
something material. This effect has been called, naturally but
improperly, _depolarization_. Its proper meaning will be disclosed in
our next lecture.

These experiments and reasonings, if only thoroughly studied and
understood, will form a solid groundwork for the analysis of the
splendid optical phenomena next to be considered.

LECTURE IV.

CHROMATIC PHENOMENA PRODUCED BY CRYSTALS IN POLARIZED LIGHT
THE NICOL PRISM
POLARIZER AND ANALYZER
ACTION OF THICK AND THIN PLATES OF SELENITE
COLOURS DEPENDENT ON THICKNESS
RESOLUTION OF POLARIZED BEAM INTO TWO OTHERS BY THE SELENITE
ONE OF THEM MORE RETARDED THAN THE OTHER
RECOMPOUNDING OF THE TWO SYSTEMS OF WAVES BY THE ANALYZER
INTERFERENCE THUS RENDERED POSSIBLE
CONSEQUENT PRODUCTION OF COLOURS
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