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.

32 UUNDULATORY TI-IEORY OF OPTICS.
(3) Light polarized in one plane cannot be destroyed
by light polarized in the perpendicular plane.
The first leads at once to the presumption that polarization is not a modification or change of common light, but
a resolution of it into two parts equally related to planes
at right angles to each other; and that the exhibition of a
beam of polarized light requires the action of some peculiar
forces (either those employed in producing ordinary reflection and refraction or those which produce crystalline double
refraction) which will enable the eye to perceive one of these
parts without mixture of the other. This presumption is
strongly supported by the phlonomena of partially-polarized
light. If light falls upon a plate of glass inclined to the
ray, the transmitted light, as we have seen, is partially polarized. If now a second plate of glass be placed in the
path of the transmitted light, inclined at the same angle as
the former plate, but with its plane of reflection at right
angles to that of the former plate, the light which emerges
from it bas lost every trace of polarization; whether it be
examined only with the analyzing plate B, or by the interposition of a plate of crystal in the manner to be explained
hereafter (144). This seems explicable only on the supposition
that the effect of the first plate of glass was to diminish that
part of the light which has respect to one plane (without
totally removing it), and that the effect of the second plate
is to diminish in the same proportion that part of the light
which has respect to the other plane, and therefore that,
after emergence from the second plate, the two portions of
light have the same proportion as before. On considering
this presumption in conjunction with the second and third
conclusions, we easily arrive at this simple hypothesis explaining the whole:
Common light consists of undulations in which the vibrations of each particle are in the plane perpendicular to
the direction of the wave's motion. The polarization of
light is the resolution of the vibrations of each particle
into two, one parallel to a given plane passing through
the direction of the wdve's motion, and the other perpendicular to that plane; which fromn causes that we shall