res which reveal themselves under the scrutiny of polarized
light. When a square of common window-glass is placed between the
Nicols, you see its dim outline, but it exerts no action on the
polarized light. Held for a moment over the flame of a spirit-lamp, on
reintroducing it between the Nicols, light flashes out upon the
screen. Here, as in the case of mechanical action, you have luminous
spaces of strain divided by dark neutral axes from spaces of pressure.

[Illustration: Fig. 40.]

[Illustration: Fig. 41.]

Let us apply the heat more symmetrically. A small square of glass is
perforated at the centre, and into the orifice a bit of copper wire is
introduced. Placing the square between the prisms, and heating the
wire, the heat passes by conduction to the glass, through which it
spreads from the centre outwards. You immediately see four luminous
quadrants and a dim cross, which becomes gradually blacker, by
comparison with the adjacent brightness. And as, in the case of
pressure, we produced colours, so here also, by the proper application
of heat, gorgeous chromatic effects may be evoked. The condition
necessary to the production of these colours may be rendered permanent
by first heating the glass sufficiently, and then cooling it, so that
the chilled mass shall remain in a state of permanent strain and
pressure. Two or three examples will illustrate this point. Figs. 40
and 41 represent the figures obtained with two pieces of glass thus
prepared; two rectangular pieces of unannealed glass, crossed and
placed between the polarizer and analyzer, exhibit the beautiful iris
fringes represented in fig. 42.

[Illustration: Fig. 42.]

Sec. 6. _Circular Polarization._

But we have to follow the ether still further into its hiding-places.
Suspended before you is a pendulum, which, when drawn aside and
liberated, oscillates to and fro. If, when the pendulum is passing the
middle point of its excursion, I impart a shock to it tending to drive
it at right angles to its present course, what occurs? The two
impulses compound themselves to a vibration oblique in direction to
the former one, but the pendulum still oscillates in _a plane_. But,
if the rectangular shock be imparted to the pendulum when it is at the
limit of its swing, then the compounding of the two impulses causes
the suspended ball to describe, not a straight line, but an ellipse;
and, if the shock be competent of itself to produce a vibration of the
sam