lved into its two equivalents, which vibrate at right angles to
each other.

[Illustration; Fig. 37.]

In one of these two rectangular directions the ether within the gypsum
is more sluggish than in the other; and, as a consequence, the waves
that follow this direction are more retarded than the others. In both
cases the undulations are shortened when they enter the gypsum, but
in the one case they are more shortened than in the other. You can
readily imagine that in this way the one system of waves may get half
a wave-length, or indeed any number of half wavelengths, in advance of
the other. The possibility of interference here at once flashes upon
the mind. A little consideration, however, will render it evident
that, as long as the vibrations are executed at right angles to each
other, they cannot quench each other, no matter what the retardation
may be. This brings us at once to the part played by the analyzer. Its
sole function is to recompound the two vibrations emergent from the
gypsum. It reduces them to a single plane, where, if one of them be
retarded by the proper amount, extinction will occur.

But here, as in the case of thin films, the different lengths of the
waves of light come into play. Red will require a greater thickness to
produce the retardation necessary for extinction than blue;
consequently when the longer waves have been withdrawn by
interference, the shorter ones remain, the film of gypsum shining with
the colours which the short waves confer. Conversely, when the shorter
waves have been withdrawn, the thickness is such that the longer waves
remain. An elementary consideration suffices to show, that when the
directions of vibration of the prisms and the gypsum enclose an angle
of forty-five degrees, the colours are at their maximum brilliancy.
When the film is turned from this direction, the colours gradually
fade, until, at the point where the directions of vibration in plate
and prisms are parallel, they disappear altogether.

(The best way of obtaining a knowledge of these phenomena is to
construct a model of thin wood or pasteboard, representing the plate
of gypsum, its planes of vibration, and also those of the polarizer
and analyzer. Two parallel pieces of the board are to be separated by
an interval which shall represent the thickness of the film of gypsum.
Between them two other pieces, intersecting each other at a right
angle, are to represent the planes of vibration within the fi