XVIII. Contributions to the Physiology of Vision.-Part the First.
On some remarkable, and hitherto unobserved, Phenomena of Binocular Vision.
By CHARLES
WHEATSTONE, F.R.S.,
Professor of Experimental Philosophy at King’s College, London.
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, 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;
and hence pictorial presentations 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. But this similarity no longer exists whent he object
is placed so near the eyes that to view it the optic axes must converge;
under these conditions a different pespective projection of it is seen
by each eye, and these perspectives are more dissimilar as the convergence
of the optic axes becomes greater. This fact may be easily verified by
placing any figure of three dimensions, an outline cube for instance,
at a moderate distance before the eyes, and while the head is kept
perfectly steady, viewing it with each eye successively while the other
is closed. Plate XI. fig. 13. represents the two perspective projections
of a cube; b is that seen by the right eye, and a that
presented to the left eye; the figure being supposed to be placed about
seven inches immediately before the spectator.
The appearances, which are by this simple experiment rendered so obvious,
may be easily inferred from the established law of perspective;
for the same object in relief is, when viewed by a different eye,
seen from two points of sight at a distance from each other equal to
the line joining the two eyes. Yet they seem to have escaped the
attention of every philosopher and artist who has treated of the subjects
of vision and perspective. I can ascribe this inattention to a phenomenon
leading to the important and curious consequences, which will form the
subject of the present communication, only to this circumstance;
that the results being contrary to a principle which was very generally
maintained by optical writers, viz. that objects can
3 B 2