changes of colour which it undergoes being due to this cause. Looking
at such a star through an opera-glass, and shaking the glass so as to
cause the image of the star to pass rapidly over the retina, you
produce a row of coloured beads, the spaces between which correspond
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