we are dealing the progress came by way of physics
from the introduction into the questions of mineralogy
of the power to rotate the plane of polarization.

We know that every impression of light is the result
of a vibration. It is as though a rigid rod, clamped in a
vise at one end, should vibrate at the other end, oscill-
ating about a position of equilibrium. If, on the moving
end, there is a polished button making a luminous point,
we can describe with this luminous point an ellipse,
a circle, or a straight line. Let us consider this last
case, the simplest one, and let us call, for sake of argument,
the plane of polarization the plane which contains
the vibrating rod and the luminous line which its
extremity describes. Let us suppose this plane verti-
cal, and the luminous point moving before us in the
line occupied by the hands of a clock indicating six
o'clock, i.e., in a vertical line. As long as only the
air intervenes between the luminous point and our
eye the vibration will not change direction, but there are
many transparent substances which, when traversed
by the vibration, would make it project itself along
the lines of the hands of a clock indicating five minutes
of five for a certain thickness traversed, or ten minutes
of four for a thickness twice as great. In other words,
these substances rotate the plane of polarization to the
left an amount proportional to their thickness. We
call them substances having a left rotary power, or, to
abbreviate, left-handed substances. There exist, further-
more, right-handed substances, of which, mutatis mutandis,
the definition is the same.

Crystallized quartz, the hemihedral form of which
we have just seen, is typically one of these substances
endowed with rotary power; it rotates the plane of
polarization of a ray of light which traverses it in the
direction of the axis, and Biot, in the very careful study

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