Mathematical tracts on the lunar and planetary theories, the figure of the earth, precession and nutation, the calculus of variations, and the undulatory theory of optics.

356 UNDULATORY THEORY OF OPTICS.
light reflected from A is incapable of being again reflected
from B. If the eye be placed near B so as to observe the
image of A, a very dark spot is seen at its center, and the
whole image, though not quite so black as the central spot,
is very obscure.
145. Now if we interpose between A and B a plate
which possesses double refraction, the image of A is generally seen bright, but sometimes crossed by one or more
dark brushes, and sometimes by rings of circular or more
complicated figure, richly coloured. On inclining the plane
of the interposed plate, the rings generally shift their places
and are succeeded by others; shewing that the peculiar arrangement of colours and brushes depends on the relation
of the direction of the rays to some fixed lines in the interposed plate. There are few substances which when interposed present exactly the same phenomena, but nearly
ail exhibit appearances of the same general character: gorgeous colours, arrayed in symmetrical forms, generally shifting with every change in the position of the interposed
plate, and always altering as B is turned round its spindle.
This class of phenomena is far the most splendid in Optics.
146. The interposition of a piece of common glass
produces no effect. And even a doubly refracting substance
produces no effect if it be placed to receive the light either
before it is polarized at A, or after it is analyzed at B.
It seems therefore that a doubly refracting substance has
generally the power of altering polarized light, in such a
manner that the light, either from losing the character of
polarization, or from a change in the plane, acquires according to certain complicated laws the capability of reflection. It appears however that it exerts no influence on
common light which makes it incapable of polarization as
usual, and that it does not alter polarized light so as to
produce any alteration in the impression made on the eye
unless it is subsequently analyzed.
PRo0I. 30. To explain generally the origin of the coloured rings.