Because when an object is viewed at so great a distance that the
optic axes of both eyes are sensibly parallel when directed towards
it, the perspective projections of it, seen by each eye separately,
are similar; and the appearance to the two eyes is precisely the same
as when the object is seen by one eye only. There is, in such case,
no difference between the visual appearance of an object in relief
and its perspective projection on a plane surface; hence pictorial
representations of distant objects, when those circumstances which
would prevent or disturb the illusion are carefully excluded, may be
rendered such perfect resemblances of the objects they are intended to
represent as to be mistaken for them. The Diorama is an instance of
this.--_Professor Wheatstone_; _Philosophical Transactions_, 1838.