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.

358 UNDULATORY THEORY OF OPTICS,
conceive that in one direction of the ray, the plane of polarization of the ordinary ray coincides with the plane of
polarization of light reflected from A. In that case the
light reflected from A will produce in the crystal only the
ordinary ray (92), and consequently the crystalline separation of the rays is of no consequence, because only one of
the rays exists. The ordinary ray emerges therefore from
the crystal just as it entered, unmixed with any other ray,
and therefore falls upon B in the same state as if it had
not passed through the crystalline plate, and therefore, is
not reflected. The same would be true, mutatis mutandis,
if for another direction of the ray the plane of polarization
of the extraordinary ray in the crystal coincided with the
plane of polarization of light reflected from A. Thus if we
determine all the directions of rays in which the plane of
polarization of either the ordinary or the extraordinary ray
coincides with the plane of reflection from A, the rays
passing in those directions will not be capable of reflection
from B, and the appearance presented to the eye by the
rays passing in all these directions will be that of one or
more black lines not necessarily straight, cutting the co,
loured curves before mentioned.
149. If B be turned round its spindle till its plane of
reflection coincides with that of A, the positions determined
by the conditions of (148) will define the directions in which
the light is most highly susceptible of reflection from B, and
therefore one or more bright lines will be seen cutting the
curves. If B be turned to any intermediate position, it will
be found in the same way that the directions of rays, which
make the plane of either the ordinary or the extraordinary
ray to coincide with the plane of reflection either at A or
at B, determine the form of lines which cut all the rings,
and in which the intensity of light is uniformly the same
as if the crystal were not interposed.
These particular cases are pointed out merely as matters
of interest in the general explanation. The determination
of the form of the uncoloured curves will be included in
the general investigation of the intensity of light reflected
in all directions from B.