Some of the chromatic effects of irregular crystallization are beautiful in the extreme.  Could I introduce between our two Nicols a pane of glass covered by those frost-ferns which your cold weather renders now so frequent, rich colours would be the result.  The beautiful effects of the irregular crystallization of tartaric acid and other substances on glass plates now presented to you, illustrate what you might expect from the frosted window-pane.  And not only do crystalline bodies act thus upon light, but almost all bodies that possess a definite structure do the same.  As a general rule, organic bodies act thus upon light; for their architecture implies an arrangement of the molecules, and of the ether associated with the molecules, which involves double refraction.  A film of horn, or the section of a shell, for example, yields very beautiful colours in polarized light.  In a tree, the ether certainly possesses different degrees of elasticity along and across the fibre; and, were wood transparent, this peculiarity of molecular structure would infallibly reveal itself by chromatic phenomena like those that you have seen.

Sec. 4. Colours produced by Strain and Pressure.

Not only do natural bodies behave in this way, but it is possible, as shown by Brewster, to confer, by artificial strain or pressure, a temporary double refracting structure upon non-crystalline bodies such as common glass.  This is a point worthy of illustration.  When I place a bar of wood across my knee and seek to break it, what is the mechanical condition of the bar?  It bends, and its convex surface is strained longitudinally; its concave surface, that next my knee, is longitudinally pressed.  Both in the strained portion and in the pressed portion of the wood the ether is thrown into a condition which would render the wood, were it transparent, double-refracting.  For, in cases like the present, the drawing of the molecules asunder longitudinally is always accompanied by their approach to each other laterally; while the longitudinal squeezing is accompanied by lateral retreat.  Each half of the bar of wood exhibits this antithesis, and is therefore double-refracting.

Let us now repeat this experiment with a bar of glass.  Between the crossed Nicols I introduce such a bar.  By the dim residue of light lingering upon the screen, you see the image of the glass, but it has no effect upon the light.  I simply bend the glass bar with my finger and thumb, keeping its length oblique to the directions of vibration in the Nicols.  Instantly light flashes out upon the screen.  The two sides of the bar are illuminated, the edges most, for here the strain and pressure are greatest.  In passing from longitudinal strain to longitudinal pressure, we cross a portion of the glass where neither is exerted.  This is the so-called neutral axis of the bar of glass, and along it you see a dark band, indicating that the glass along this axis exercises no action upon the light.  By employing the force of a press, instead of the force of my finger and thumb, the brilliancy of the light is greatly augmented.