As already stated, the whole of a beam of ordinary light reflected
from glass at the polarizing angle is polarized; a word must now be
added regarding the far larger portion of the light which is
_transmitted_ by the glass. The transmitted beam contains a quantity
of polarized light equal to the reflected beam; but this is only a
fraction of the whole transmitted light. By taking two plates of glass
instead of one, we augment the quantity of the transmitted polarized
light; and by taking _a bundle_ of plates, we so increase the quantity
as to render the transmitted beam, for all practical purposes,
_perfectly_ polarized. Indeed, bundles of glass plates are often
employed as a means of furnishing polarized light. It is important to
note that the plane of vibration of this transmitted light is at right
angles to that of the reflected light.

One word more. When the tourmalines are crossed, the space where they
cross each other is black. But we have seen that the least obliquity
on the part of the crystals permits light to get through both. Now
suppose, when the two plates are crossed, that we interpose a third
plate of tourmaline between them, with its axis oblique to both. A
portion of the light transmitted by the first plate will get through
this intermediate one. But, after it has got through, _its plane of
vibration is changed_: it is no longer perpendicular to the axis of
the crystal in front. Hence it will, in part, get through that
crystal. Thus, by pure reasoning, we infer that the interposition of a
third plate of tourmaline will in part abolish the darkness produced
by the perpendicular crossing of the other two plates. I have not a
third plate of tourmaline; but the talc or mica which you employ in
your stoves is a more convenient substance, which acts in the same
way. Between the crossed tourmalines, I introduce a film of this