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