. And, in fact, to convert the circular polarization produced
by this plate from one of these kinds to the other (say from
right-handed to left-handed, or _vice versa_), we have only to turn
the plate round through 90 deg.. Conversely, right-handed circular
polarization will be changed by the plate into plane-polarization in
one direction, while left-handed will be changed into plane at right
angles to the first. Hence if the plate be turned round through 90 deg.
we shall see that the bands are shifted in a direction opposite to
that in which they were moved at first. In this therefore we have
evidence not only that the polarization immediately on either side
of a band is circular; but also that that immediately on the one
side is right-handed, while that immediately on the other is
left-handed[28].

'If time permitted, I might enter still further into detail, and
show that the polarization between the plane and the circular is
elliptical, and even the positions of the longer and shorter axes
and the direction of motion in each case. But sufficient has,
perhaps, been said for our present purpose.

'Before proceeding to the more varied forms of spectral bands,
which I hope presently to bring under your notice, I should like to
ask your attention for a few minutes to the peculiar phenomena
exhibited when two plates of selenite giving complementary colours
are used. The appearance of the spectrum varies with the relative
position of the plates. If they are similarly placed--that is, as if
they were one plate of crystal--they will behave as a single plate,
whose thickness is the sum of the thicknesses of each, and will
produce double the number of bands which one alone would give; and
when the analyzer is turned, the bands will disappear and re-appear
in their complementary positions, as usual in the case of
plane-polarization. If one of them be turned round through 45 deg., a
single band will be seen at a particular position in the spectrum.
This breaks into two, which recede from one another towards the red
and violet ends respectively, or advance towards one another
according to the direction in which the analyzer is turned. If the
plate be turned through 45 deg. in the opposite direction, the effects
will be reversed. The darkness of the bands is, however, not equally
complete