താൾ:CiXIV132a.pdf/401

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LXXXIX

our two eyes are not just the same, as each of them corresponds
to a different point of view. By uniting these two images in one
we are able to appreciate not only the surface, but also the shape,
distance and corporality (sensation of relief) of an object and
so the impression will be correct. We make two drawings of
the same object, one as seen with the right and the other as seen
with the left eye. Then the two drawings are made to coincide
by means of a lens, in giving the rays of light the same direction
as if converging from one object. If one eye looks through
such a lens an impression will be caused, as if we had seen the
object itself. This instrument is called Stereoscope, and was
invented by Sir C. Wheatstone (1838) and improved by Sir D.
Brewster. By the stereoscope genuine bank-notes may be very
easily distinguished from forged ones.

F. Polarization.

§ 219. If a ray of light falls upon a mirror and we turn
this mirror in such a manner that
the ray and the mirror form the
same angle with each other, the ray
will be reflected equally well in all
positions of the mirror. But if
the reflected ray is caused to be
reflected again by a second mir
ror and this second mirror is turned
in the same manner as the first one,
the second reflected ray will appear
first clearer, then darker, then not at
all. If the two mirrors are parallel,
it will be reflected best, if they stand at right angles, worst. This
change is called polarization; and the light reflected a second
time we call polarized light. Polarization was discovered by

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