shall learn presently.

The quality of two-sidedness conferred upon light by bi-refracting
crystals may also be conferred upon it by ordinary reflection. Malus
made this discovery in 1808, while looking through Iceland spar at the
light of the sun reflected from the windows of the Luxembourg palace
in Paris. I receive upon a plate of window-glass the beam from our
lamp; a great portion of the light reflected from the glass is
polarized. The vibrations of this reflected beam are executed, for the
most part, parallel to the surface of the glass, and when the glass is
held so that the beam shall make an angle of 58° with the
perpendicular to the glass, the _whole_ of the reflected beam is
polarized. It was at this angle that the image of the tourmaline was
completely quenched in our former experiment. It is called _the
polarizing angle_.

Sir David Brewster proved the angle of polarization of a medium to be
that particular angle at which the refracted and reflected rays
inclose a right angle.[17] The polarizing angle augments with the
index of refraction. For water it is 52½°; for glass, as already
stated, 58°; while for diamond it is 68°.

And now let us try to make substantially the experiment of Malus. The
beam from the lamp is received at the proper angle upon a plate of
glass and reflected through the spar. Instead of two images, you see
but one. So that the light, when polarized, as it now is by
reflection, can only get through the spar in one direction, and
consequently can produce but one image. Why is this? In the Iceland
spar as in the tourmaline, all the vibrations of the ordinary light
are reduced to two planes at right angles to each other; but, unlike