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.

822 UNDULATORY THEORY OF OPTICS.
in the plane perpendicular to the plane of reflection, as it
would not furnish any Ordinary ray with Iceland spar, will
not furnish a reflected ray from unsilvered glass, but will be
entirely transmitted. This is verified by experiment: and
thus we have an easy practical test of the polarization of
light. If light incident at the polarizing angle on unsilvered
glass is not susceptible of reflection, it is polarized in the
plane perpendicular to the plane of reflection. And if, on
turning the glass round the incident ray without varying
the inclination, the reflected light does not vanish in any
position of the glass, the light is not polarized. In the
same manner the polarization of a ray may be ascertained
by examining the state of the emergent ray, after incidence
on a plate of tourmaline; if in any position of the tourmaline the emergent ray disappears, the plane of polarization
of the incident ray is perpendicular to the plane then passing
through the ray and through the axis of the tourmaline.
97. From this it will easily be inferred that if two
such plates of tourmaline are placed with their axes at right
angles to each other, no light can pass through them. For
the light which is transmitted by the first is polarized in
the plane of its axis, that is, in the plane perpendicular to
the axis of the second: and therefore is not allowed to pass
through the second. If one of the tourmalines be turned,
light is immediately seen: it increases till the axes of the
tourmalines are parallel. In the same manner if in fig. 24
A be a plate or several parallel plates, of unsilvered glass,
and B an unsilvered glass* (whose posterior surface is
blackened, to prevent reflection there), B being fixed on
a block which turns round a spindle at C in the direction
of AB: and if each of the glass surfaces make with AB an
angle of about 330. 13' (the refractive index of plate glass
for mean rays being about 1,527 = tan 56~. 47'): then on receiving the light of the clouds on A in such a direction that
the reflected light falls on B, and placing the eye to receive
the light reflected from B, it is seen that when the planes
* Instead of reflection from an unsilvered glass, transmission through a plate
of tourmaline may be used.