is-coloured_ rings produced
by the white light.

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