. 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.

Again, I have here a square of glass which can be inserted into a
press of another kind. Introducing the uncompressed square between the
prisms, its neutrality is declared; but it can hardly be held
sufficiently loosely in the press to prevent its action from
manifesting itself. Already, though the pressure is infinitesimal, you
see spots of light at the points where the press is in contact with
the glass. On turning a screw, the image of the square of glass
flashes out upon the screen. Luminous spaces are seen separated from
each other by dark bands.

Every two adjacent spaces are in opposite mechanical conditions. On
one side of the dark band we have strain, on the other side pressure,
the band marking the neutral axis between both. I now tighten the
vice, and you see colour; tighten still more, and the colours appear
as rich as those presented by crystals. Releasing the vice, the
colours suddenly vanish; tightening suddenly, they reappear. From the
colours of a soap-bubble Newton was able to infer the thickness of the
bubble, thus uniting by the bond of thought apparently incongruous
things. From the colours here presented to you, the magnitude of the
pressure employed might be inferred. Indeed, the late M. Wertheim, of
Paris, invented an instrument for the determination of strains and
pressures, by the colours of polarized light, which exceeded in
accuracy all previous instruments of the kind.

And now we have to push these considerations to a final illustration.
Polarized light may be turned to account in various ways as an
analyzer of molecular condition. It may, for instance, be applied to
reveal the condition of a solid body when it becomes sonorous. A strip
of glass six feet long, two inches wide and a quarter of an inch
t