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 thick, 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]