first shows a single band; that of the second, two; showing that
the second series of tints is not identical with the first, but that
it is produced by the extinction of two colours from the components
of white light. The spectra of the others show series of bands more
and more numerous in proportion to the thickness of the plate, an
array which may be increased indefinitely. The total light, then, of
which the spectrum is deprived by the thicker plates is taken from a
greater number of its parts; or, in other words, the light which
still remains is distributed more and more evenly over the spectrum;
and in the same proportion the sum total of it approaches more and
more nearly to white light.

'These experiments were made more than thirty years ago by the
French philosophers, MM. Foucault and Fizeau.

'If instead of selenite, Iceland spar, or other ordinary crystals,
we use plates of quartz cut perpendicularly to the axis, and turn
the analyzer round as before, the light, instead of exhibiting only
one colour and its complementary with an intermediate stage in which
colour is absent, changes continuously in tint; and the order of the
colour depends partly upon the direction in which the analyzer is
turned, and partly upon the character of the crystal, _i.e._ whether
it is right-handed or left-handed. If we examine the spectrum in
this case we find that the dark band never disappears, but marches
from one end of the spectrum to another, or _vice versa_, precisely
in such a direction as to give rise to the tints seen by direct
projection.

'The kind of polarization effected by the quartz plates is called
circular, while that effected by the other class of crystals is
called plane, on account of the form of the vibrations executed by
the molecules of aether; and this leads us to examine a little more
closely the nature of the polarization of different parts of these
spectra of polarized light.

'Now, two things are clear: first, that if the light be
plane-polarized--that is, if all the vibrations throughout the
entire ray are rectilinear and in one plane--they must in all their
bearings have reference to a particular direction in space, so that
they will be differently affected by different positions of the
analyzer. Secondly, that if the vibrations be circular, they will be