e amplitude as the first one, the ellipse becomes a circle.

Why do I dwell upon these things? Simply to make known to you the
resemblance of these gross mechanical vibrations to the vibrations of
light. I hold in my hand a plate of quartz cut from the crystal
perpendicular to its axis. The crystal thus cut possesses the
extraordinary power of twisting the plane of vibration of a polarized
ray to an extent dependent on the thickness of the crystal. And the
more refrangible the light the greater is the amount of twisting; so
that, when white light is employed, its constituent colours are thus
drawn asunder. Placing the quartz plate between the polarizer and
analyzer, this vivid red appears; and, turning the analyzer in front
from right to left, the other colours of the spectrum appear in
succession. Specimens of quartz have been found which require the
analyzer to be turned from left to right to obtain the same succession
of colours. Crystals of the first class are therefore called
right-handed, and of the second class, left-handed crystals.

With profound sagacity, Fresnel, to whose genius we mainly owe the
expansion and final triumph of the undulatory theory of light,
reproduced mentally the mechanism of these crystals, and showed their
action to be due to the circumstance that, in them, the waves of
ether so act upon each other as to produce the condition represented
by our rotating pendulum. Instead of being plane polarized, the light
in rock crystal is _circularly polarized_. Two such rays, transmitted
along the axis of the crystal, and rotating in opposite directions,
when brought to interference by the analyzer, are demonstrably
competent to produce all the observed phenomena.

Sec. 7. _Complementary Colours of Bi-refracting Spar in Circularly
Polarized Light. Proof that Yellow and Blue are Complementary._

I now remove the analyzer, and put in its place the piece of Iceland
spar with which we have already illustrated double refraction. The two
images of the carbon-points are now before you, produced, as you know,
by two beams vibrating at right angles to each other. Introducing a
plate of quartz between the polarizer and the spar, the two images
glow with complementary colours. Employing the image of an aperture
instead of that of the carbon-points, we have two coloured circles. As
the analyzer is caused to rotate, the colours pass through various
changes: but they are always complementary. When the one is r