affected in precisely the same way (whatever that may be) in all
positions of the analyzer. This statement merely recapitulates a
fundamental point in polarization. In fact, plane-polarized light is
alternately transmitted and extinguished by the analyzer as it is
turned through 90 deg.; while circularly polarized light [if we could
get a single ray] remains to all appearance unchanged. And if we
examine carefully the spectrum of light which has passed through a
selenite, or other ordinary crystal, we shall find that, commencing
with two consecutive bands in position, the parts occupied by the
bands and those midway between them are plane-polarized, for they
become alternately dark and bright; while the intermediate parts,
_i.e._ the parts at one-fourth of the distance from one band to the
next, remain permanently bright. These are, in fact, circularly
polarized. But it would be incorrect to conclude from this
experiment alone that such is really the case, because the same
appearance would be seen if those parts were unpolarized, _i.e._ in
the condition of ordinary lights. And on such a supposition we
should conclude with equal justice that the parts on either side of
the parts last mentioned (e.g. the parts separated by eighth parts
of the interval between two bands) were partially polarized. But
there is an instrument of very simple construction, called a
"quarter-undulation plate," a plate usually of mica, whose thickness
is an odd multiple of a quarter of a wave-length, which enables us
to discriminate between light unpolarized and circularly polarized.
The exact mechanical effect produced upon the ray could hardly be
explained in detail within our present limits of time; but suffice
it for the present to say that, when placed in a proper position,
the plate transforms plane into circular and circular into plane
polarization. That being so, the parts which were originally banded
ought to remain bright, and those which originally remained bright
ought to become banded during the rotation of the analyzer. The
general effect to the eye will consequently be a general shifting of
the bands through one-fourth of the space which separates each pair.

'Circular polarization, like circular motion generally, may of
course be of two kinds, which differ only in the direction of the
motion