"On looking
in a mirror," says he, "we may observe that, if we shut one eye, the
pupil of the other dilates." To this question: "But why, then, have we
two eyes?" he responds: "In order that one may remain if the other
gets injured." Despite the reasoning of the learned philosopher, we
may be permitted to believe that the reason that we have two eyes is
for seeing better and especially for perceiving the effects of
perspective and the relief of objects. We have no intention of setting
forth here the theory of binocular vision; one simple experiment will
permit any one to see that the real place of an object is poorly
estimated with one eye. Seated before a desk, pen in hand, suddenly
close one eye, and, at the same time, stretch out the arm in order to
dip the pen in the inkstand; you will fail nine times out of ten. It
is not in one day that the effects of binocular vision have been
established, for the ancients made many observations on the subject.
It was in 1593 that the celebrated Italian physicist Porta was the
first to give an accurate figure of two images seen by each eye
separately, but he desired no apparatus that permitted of
reconstituting the relief on looking at them. Those savants who, after
him, occupied themselves with the question, treated it no further
than from a theoretical point of view. It was not till 1838 that the
English physicist Wheatstone constructed the first stereoscopic
apparatus permitting of seeing the relief on examining simultaneously
with each of the eyes two different images of an object, one having
the perspective that the right eye perceives, and the other that the
left eye perceives.
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