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

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 framing theories, does the imagination create its materials.  It expands, diminishes, moulds, and refines, as the case may be, materials derived from the world of fact and observation.