hick, is held at the centre between the finger and thumb. On sweeping
a wet woollen rag over one of its halves, you hear an acute sound due
to the vibrations of the glass. What is the condition of the glass
while the sound is heard? This: its two halves lengthen and shorten in
quick succession. Its two ends, therefore, are in a state of quick
vibration; but at the centre the pulses from the two ends alternately
meet and retreat from each other. Between their opposing actions, the
glass at the centre is kept motionless: but, on the other hand, it is
alternately strained and compressed. In fig. 38, A B may be taken to
represent the glass rectangle with its centre condensed; while A' B'
represents the same rectangle with its centre rarefied. The ends of
the strip suffer neither condensation nor rarefaction.

[Illustration: Fig. 38]

If we introduce the strip of glass (_s_ _s'_, fig. 39) between the
crossed Nicols, taking care to keep it oblique to the directions of
vibration of the Nicols, and sweep our wet rubber over the glass, this
is what may be expected to occur: At every moment of compression the
light will flash through; at every moment of strain the light will
also flash through; and these states of strain and pressure will
follow each other so rapidly, that we may expect a permanent luminous
impression to be made upon the eye. By pure reasoning, therefore, we
reach the conclusion that the light will be revived whenever the glass
is sounded. That it is so, experiment testifies: at every sweep of the
rubber (_h_, fig. 39) a fine luminous disk (O) flashes out upon the
screen. The experiment may be varied in this way: Placing in front of
the polarizer a plate of unannealed glass, you have a series of
beautifully coloured rings, intersected by a black cross. Every sweep
of the rubber not only abolishes the rings, but introduces
complementary ones, the black cross being, for the moment, supplanted
by a white one. This is a modification of a beautiful experiment which
we owe to Biot. His apparatus, however, confined the observation of it
to a single person at a time.

[Illustration: Fig. 39.]

Sec. 5. _Colours of Unannealed Glass_.

Bodies are usually expanded by heat and contracted by cold. If the
heat be applied with perfect uniformity, no local strains or pressures
come into play; but, if one portion of a solid be heated and another
portion not, the expansion of the heated portion introduces strains
and pressu