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.

DOUBLE REFRACTION OF GLASS UNDER PRESSURE. 383
of blue, in the other there will be an absence of red at
the same point and an excess of blue, &c.
If instead of an analyzing plate we use the doubly-refracting prism described in (117), two images are seen at
once in different positions, every part of one of which is
complementary to the corresponding part of the other. For,
one pencil emerging from the prism consists only of vibrations perpendicular to the plane of refraction of the prism,
and therefore presents to the eye the same image as the
analyzing plate in a given position: the other consists only
of, vibrations in the plane of refraction, and therefore presents the same image as the analyzing plate in the position
differing 900 from the former.
PJioP. 39. Glass under pressure possesses double refraction.
176. This was experimentally shewn by M. Fresnel in
the following manner. A number of prisons were placed as
in fig. 38, and to prevent loss of light a fluid of nearly the
same refractive power was dropped between the adjacent
surfaces. The ends of A, B, C, D, were then violently
pressed by means of screws. On passing a ray of light
through the combination it was divided into two, one
polarized in the plane of the paper and the other in the
perpendicular plane.
177. It is found also that pressure affects the separation of the two rays in crystals which possess the property of double refraction. This leads to the presumption
that double refraction is produced generally by a state of
mechanical constraint in the particles of bodies.
178. According to our preceding theories, since compressed glass possesses double refraction, it ought, when
properly interposed between the polarizing and analyzing
plate, to exhibit colours. This may be seen on squeezing
by means of a screw a piece of glass and holding it in tie
apparatus. But it may be best exhibited by taking a thick
piece of plate glass which is polished at the edges, and