ond
to the periods of extinction. Fine scratches drawn upon glass or
polished metal reflect the waves of light from their sides; and some,
being reflected from the opposite sides of the same scratch, interfere
with and quench each other. But the obliquity of reflection which
extinguishes the shorter waves does not extinguish the longer ones,
hence the phenomena of colours. These are called the colours of
_striated surfaces_. They are beautifully illustrated by
mother-of-pearl. This shell is composed of exceedingly thin layers,
which, when cut across by the polishing of the shell, expose their
edges and furnish the necessary small and regular grooves. The most
conclusive proof that the colours are due to the mechanical state of
the surface is to be found in the fact, established by Brewster, that
by stamping the shell carefully upon black sealing-wax, we transfer
the grooves, and produce upon the wax the colours of mother-of-pearl.

LECTURE III.

RELATION OF THEORIES TO EXPERIENCE
ORIGIN OF THE NOTION OF THE ATTRACTION OF GRAVITATION
NOTION OF POLARITY, HOW GENERATED
ATOMIC POLARITY
STRUCTURAL ARRANGEMENTS DUE TO POLARITY
ARCHITECTURE OF CRYSTALS CONSIDERED AS AN INTRODUCTION
TO THEIR ACTION UPON LIGHT
NOTION OF ATOMIC POLARITY APPLIED TO CRYSTALLINE STRUCTURE
EXPERIMENTAL ILLUSTRATIONS
CRYSTALLIZATION OF WATER
EXPANSION BY HEAT AND BY COLD
DEPORTMENT OF WATER CONSIDERED AND EXPLAINED
BEARINGS OF CRYSTALLIZATION ON OPTICAL PHENOMENA
REFRACTION
DOUBLE REFRACTION
POLARIZATION
ACTION OF TOURMALINE
CHARACTER OF THE BEAMS EMERGENT FROM ICELAND SPAR
POLARIZATION BY ORDINARY REFRACTION AND REFLECTION
DEPOLARIZATION

Sec. 1. _Derivation of Theoretic Conceptions from Experience._

One of the objects of our last lecture, and that not the least
important, was to illustrate the manner in which scientific theories
are formed. They, in the first place, take their rise in the desire of
the mind to penetrate to the sources of phenomena. From its
infinitesimal beginnings, in ages long past, this desire has grown and
strengthened into an imperious demand of man's intellectual nature. It
long ago prompted Caesar to say that he would exchange his victories
for a glimpse of the sources of the Nile; it wrought itself into the
atomic theories of Lucretius; it impelled Darwin to those daring
speculations which of late years have so agitated the public mind. But
in no case, while fr