ON BINOCULAR VISION, OR VISION WITH TWO EYES.
We have already seen, in the history of the stereoscope, that in
the binocular vision of objects, each eye sees a different picture
of the same object. In order to prove this, we require only to look
attentively at our own hand held up before us, and observe how some
parts of it disappear upon closing each eye. This experiment proves, at
the same time, in opposition to the opinion of Baptista Porta, Tacquet,
and others, that we always see two pictures of the same object combined
in one. In confirmation of this fact, we have only to push aside one
eye, and observe the image which belongs to it separate from the other,
and again unite with it when the pressure is removed.
It might have been supposed that an object seen by both eyes would be
seen twice as brightly as with one, on the same principle as the light
of two candles combined is twice as bright as the light of one. That
this is not the case has been long known, and Dr. Jurin has proved
by experiments, which we have carefully repeated and found correct,
that the brightness of objects seen with two eyes is only ¹/₁₃th part
greater than when they are seen with one eye.[32] The cause of this
is well known. When both eyes are used, the pupils of each contract
so as to admit the proper quantity of light; but the moment we shut
the right eye, the pupil of the left dilates to nearly twice its size,
to compensate for the loss of light arising from the shutting of the
other.[33]
[32] Smith’s _Opticks_, vol. ii., Remarks, p. 107. Harris makes the
difference ¹/₁₀th or ¹/₁₁th; _Optics_, p. 117.
[33] This variation of the pupil is mentioned by Bacon.
[Illustration: FIG. 6.]
This beautiful provision to supply the proper quantity of light when
we can use only one eye, answers a still more important purpose, which
has escaped the notice of optical writers. In binocular vision, as we
have just seen, certain parts of objects are seen with both eyes, and
certain parts only with one; so that, if the parts seen with both eyes
were twice as bright, or even much brighter than the parts seen with
one, the object would appear spotted, from the different brightness of
its parts. In Fig. 6, for example, (see p. 14,) the areas BFI and CGI,
the former of which is seen only by the left eye, D, and the latter
only by the right eye, E, and the corresponding areas on the other side
of the sphere, would be only half as bright as the portion FIGH, seen
with both eyes, and the sphere would have a singular appearance.
It has long been, and still is, a vexed question among philosophers,
how we see objects single with two eyes. Baptista Porta, Tacquet, and
others, got over the difficulty by denying the fact, and maintaining
that we use only one eye, while other philosophers of distinguished
eminence have adopted explanations still more groundless. The law of
visible direction supplies us with the true explanation.
[Illustration: FIG. 7.]
Let us first suppose that we look with both eyes, R and L, Fig. 7, upon
a luminous point, D, which we see single, though there is a picture
of it on the retina of each eye. In looking at the point D we turn or
converge the optical axes _d_HD, _d′_H′ D, of each eye to the point D,
an image of which is formed at _d_ in the right eye R, and at _d′_ in
the left eye L. In virtue of the law of visible direction the point
D is seen in the direction _d_D with the eye R, and in the direction
_d′_D with the eye L, these lines being perpendicular to the retina
at the points _d_, _d′_. The one image of the point D is therefore
seen lying upon the other, and consequently seen single. Considering
D, then, as a single point of a visible object AB, the two eyes will
see the points A and B single by the same process of turning or
converging upon them their optical axes, and so quickly does the point
of convergence pass backward and forward over the whole object, that
it appears single, though in reality only one point of it can be seen
single at the same instant. The whole picture of the line AB, as seen
with one eye, _seems_ to coincide with the whole picture of it as seen
with the _other_, and to appear single. The same is true of a surface
or area, and also of a solid body or a landscape. Only one point of
each is seen single; but we do not observe that other points are
double or indistinct, because the images of them are upon parts of the
retina which do not give distinct vision, owing to their distance from
the _foramen_ or point which gives distinct vision. Hence we see the
reason why distinct vision is obtained only on one point of the retina.
Were it otherwise we should see every other point double when we look
fixedly upon one part of an object. If in place of _two_ eyes we had
a _hundred_, capable of converging their optical axes to one point,
we should, in virtue of the law of visible direction, see only _one_
object.
The most important advantage which we derive from the use of two
eyes is to enable us to _see distance_, or a third dimension in
space. That we have this power has been denied by Dr. Berkeley, and
many distinguished philosophers, who maintain that our perception of
distance is acquired by experience, by means of the criteria already
mentioned. This is undoubtedly true for great distances, but we
shall presently see, from the effects of the stereoscope, that the
successive convergency of the optic axes upon two points of an object
at different distances, exhibits to us the difference of distance when
we have no other possible means of perceiving it. If, for example, we
suppose G, D, Fig. 7, to be separate points, or parts of an object,
whose distances are GO, DO, then if we converge the optical axes HG,
H′ G upon G, and next turn them upon D, the points will appear to be
situated at G and D at the distance GD from each other, and at the
distances OG, OD from the observer, although there is nothing whatever
in the appearance of the points, or in the lights and shades of the
object, to indicate distance. That this vision of distance is not
the result of experience is obvious from the fact that distance is
seen as perfectly by children as by adults; and it has been proved
by naturalists that animals newly born appreciate distances with the
greatest correctness. We shall afterwards see that so infallible is
our vision of near distances, that a body whose real distance we can
ascertain by placing both our hands upon it, will appear at the greater
or less distance at which it is placed by the convergency of the
optical axes.
We are now prepared to understand generally, how, in binocular vision,
we see the difference between a picture and a statue, and between a
real landscape and its representation. When we look at a picture of
which every part is nearly at the same distance from the eyes, the
point of convergence of the optical axes is nearly at the same distance
from the eyes; but when we look at its original, whether it be a
living man, a statue, or a landscape, the optical axes are converged
in rapid succession upon the nose, the eyes, and the ears, or upon
objects in the foreground, the middle and the remote distances in the
landscape, and the relative distances of all these points from the eye
are instantly perceived. The _binocular relief_ thus seen is greatly
superior to the _monocular relief_ already described.
Since objects are seen in relief by the apparent union of two
dissimilar plane pictures of them formed in each eye, it was a
supposition hardly to be overlooked, that if we could delineate two
plane pictures of a solid object, as seen dissimilarly with each
eye, and unite their images by the convergency of the optical axes,
we should see the solid of which they were the representation. The
experiment was accordingly made by more than one person, and was found
to succeed; but as few have the power, or rather the art, of thus
converging their optical axes, it became necessary to contrive an
instrument for doing this.
The first contrivances for this purpose were, as we have already
stated, made by Mr. Elliot and Mr. Wheatstone. A description of these,
and of others better fitted for the purpose, will be found in the
following chapter.