Without breaking ground on the chromatic phenomena presented by crystals, two other sources of colour may be mentioned here. By interference in the earth's atmosphere, the light of a star, as shown by Arago, is self-extinguished, the twinkling of the star and the 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 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 § 1. _Derivation of Theoretic Conceptions from Experience._