ON MONOCULAR VISION, OR VISION WITH ONE EYE.
In order to understand the theory and construction of the stereoscope
we must be acquainted with the general structure of the eye, with the
mode in which the images of visible objects are formed within it, and
with the laws of vision by means of which we see those objects in
the position which they occupy, that is, in the direction and at the
distance at which they exist.
Every visible object radiates, or throws out in all directions,
particles or rays of light, by means of which we see them either
directly by the images formed in the eye, or indirectly by looking at
images of them formed by their passing through a small hole, or through
a lens placed in a dark room or _camera_, at the end of which is a
piece of paper or ground-glass to receive the image.
In order to understand this let H be a very small pin-hole in a shutter
or camera, MN, and let RYB be any object of different colours, the
upper part, R, being _red_, the middle, Y, _yellow_, and the lower
part, B, _blue_. If a sheet of white paper, _br_, is placed behind
the hole H, at the same distance as the object RB is before it, an
image, _br_, will be formed of the same ray and the same colours as the
object RB. As the particles or rays of light move in straight lines,
a _red_ ray from the middle part of R will pass through the hole H and
illuminate the point _r_ with _red_ light. In like manner, rays from
the middle points of Y and B will pass through H and illuminate with
_yellow_ and _blue_ light the points _y_ and _b_. Every other point
of the coloured spaces, R, Y, and B, will, in the same manner, paint
itself, as it were, on the paper, and produce a coloured image, _byr_,
exactly the same in form and colour as the object RYB. If the hole H
is sufficiently small no ray from any one point of the object will
interfere with or mix with any other ray that falls upon the paper.
If the paper is held at _half_ the distance, at _b′y′_ for example, a
coloured image, _b′y′r′_, of half the size, will be formed, and if we
hold it at twice the distance, at _b″r″_ for example, a coloured image,
_b″y″r″_, of _twice_ the size, will be painted on the paper.
[Illustration: FIG. 4.]
As the hole H is supposed to be so small as to receive only one ray
from every point of the object, the images of the object, viz., _br_,
_b′r′_, _b″r″_, will be very faint. By widening the hole H, so as
to admit more rays from each luminous point of RB, the images would
become brighter, but they would become at the same time indistinct,
as the rays from one point of the object would mix with those from
adjacent points, and at the boundaries of the colours R, Y, and B, the
one colour would obliterate the other. In order, therefore, to obtain
sufficiently bright images of visible objects we must use _lenses_,
which have the property of forming distinct images behind them, at a
point called their focus. If we widen the hole H, and place in it a
lens whose focus is at _y_, for an object at the same distance, HY, it
will form a bright and distinct image, _br_, of the same size as the
object RB. If we remove the lens, and place another in H, whose focus
is at _y′_, for a distance HY, an image, _b′r′_, half of the size of
RB, will be formed at that point; and if we substitute for this lens
another, whose focus is at _y″_, a distinct image, _b″r″_, _twice_ the
size of the object, will be formed, the size of the image being always
to that of the object as their respective distances from the hole or
lens at H.
With the aid of these results, which any person may confirm by making
the experiments, we shall easily understand how we see external objects
by means of the images formed in the eye. The human eye, a section
and a front view of which is shewn in Fig. 5, A, is almost a sphere.
Its outer membrane, ABCDE, or MNO, Fig. 5, B, consists of a tough
substance, and is called the _sclerotic_ coat, which forms the _white
of the eye_, A, seen in the front view. The front part of the eyeball,
C_x_D, which resembles a small watch-glass, is perfectly transparent,
and is called the _cornea_. Behind it is the _iris_, _cabe_, or C in
the front view, which is a circular disc, with a hole, _ab_, in its
centre, called the _pupil_, or _black of the eye_. It is, as it were,
the _window_ of the eye, through which all the light from visible
objects must pass. The _iris_ has different colours in different
persons, _black_, _blue_, or _grey_; and the pupil, _ab_, or H, has
the power of contracting or enlarging its size according as the light
which enters it is more or less bright. In sunlight it is very small,
and in twilight its size is considerable. Behind the iris, and close
to it, is a doubly convex lens, _df_, or LL in Fig. 5, B, called the
_crystalline lens_. It is more convex or round on the inner side, and
it is suspended by the _ciliary_ processes at LC, LC′, by which it
is supposed to be moved towards and from H, in order to accommodate
the eye to different distances, or obtain distinct vision at these
distances. At the back of the eye is a thin pulpy transparent membrane,
_rr_ O _rr_, or _vvv_, called the _retina_, which, like the ground
glass of a camera obscura, receives the images of visible objects.
This membrane is an expansion of the optic nerve O, or A in Fig. 5, A,
which passes to the brain, and, by a process of which we are ignorant,
gives us vision of the objects whose images are formed on its expanded
surface. The globular form of the eye is maintained by two fluids which
fill it,—the _aqueous humour_, which lies between the crystalline lens
and the cornea, and the _vitreous humour_, ZZ, which fills the back of
the eye.
[Illustration: FIG. 5, A.]
[Illustration: FIG. 5, B.]
But though we are ignorant of the manner in which the mind takes
cognizance through the brain of the images on the retina, and may
probably never know it, we can determine experimentally the laws by
which we obtain, through their images on the retina, a knowledge of the
direction, the position, and the form of external objects.
If the eye MN consisted only of a hollow ball with a small aperture H,
an inverted image, _ab_, of any external object AB would be formed on
the retina _r_O_r_, exactly as in Fig. 4. A ray of light from A passing
through H would strike the retina at _a_, and one from B would strike
the retina at _b_. If the hole H is very small the inverted image
_ab_ would be very distinct, but very obscure. If the hole were the
size of the pupil the image would be sufficiently luminous, but very
indistinct. To remedy this the crystalline lens is placed behind the
pupil, and gives distinctness to the image _ab_ formed in its focus.
The image, however, still remains inverted, a ray from the _upper_ part
A of the object necessarily falling on the _lower_ part _a_ of the
retina, and a ray from the _lower_ part B of the object upon the upper
part _b_ of the retina. Now, it has been proved by accurate experiments
that in whatever direction a ray AH_a_ falls upon the retina, it gives
us the vision of the point A from which it proceeds, or causes us to
see that point, in a direction perpendicular to the retina at _a_, the
point on which it falls. It has also been proved that the human eye is
nearly spherical, and that a line drawn perpendicular to the retina
from any point _a_ of the image _ab_ will very nearly pass through the
corresponding point A of the object AB,[31] so that the point A is, in
virtue of this law, which is called the _Law of visible direction_,
seen in nearly its true direction.
[31] _Edinburgh Transactions_, vol. xv. p. 349, 1843; or _Philosophical
Magazine_, vol. xxv. pp. 356, 439, May and June 1844.
When we look at any object, AB, for example, we see only one point
of it distinctly. In Fig. 5 the point D only is seen distinctly, and
every point from D to A, and from D to B, less distinctly. The point of
distinct vision on the retina is at _d_, corresponding with the point D
of the object which is seen distinctly. This point _d_ is the centre of
the retina at the extremity of the line AH_a_, called the optical axis
of the eye, passing through the centre of the lens L_h_, and the centre
of the pupil. The point of distinct vision _d_ corresponds with a small
hole in the retina called the _Foramen centrale_, or _central hole_,
from its being in the centre of the membrane. When we wish to see the
points A and B, or any other point of the object, we turn the eye upon
them, so that their image may fall upon the central point _d_. This
is done so easily and quickly that every point of an object is seen
distinctly in an instant, and we obtain the most perfect knowledge
of its form, colour, and direction. The law of distinct vision may be
thus expressed. Vision is most distinct when it is performed by the
central point of the retina, and the distinctness decreases with the
distance from the central point. It is a curious fact, however, that
the most _distinct point d is the least sensitive to light_, and that
the sensitiveness increases with the distance from that point. This is
proved by the remarkable fact, that when an astronomer cannot see a
very minute star by looking at it directly along the optical axis _d_D,
he can see it _by looking away from it_, and bringing its image upon a
more sensitive part of the retina.
But though we see with one eye the direction in which any object or
point of an object is situated, we do not see its position, or the
distance from the eye at which it is placed. If a small luminous point
or flame is put into a dark room by another person, we cannot with one
eye form anything like a correct estimate of its distance. Even in good
light we cannot with one eye snuff a candle, or pour wine into a small
glass at arm’s length. In monocular vision, we learn from experience to
estimate all distances, but particularly great ones, by various means,
which are called the _criteria of distance_; but it is only with both
eyes that we can estimate with anything like accuracy the distance of
objects not far from us.
The _criteria of distance_, by which we are enabled with one eye to
form an approximate estimate of the distance of objects are five in
number.
1. The interposition of numerous objects between the eye and the object
whose distance we are appreciating. A distance at sea appears much
shorter than the same distance on land, marked with houses, trees, and
other objects; and for the same reason, the sun and moon appear more
distant when rising or setting on the horizon of a flat country, than
when in the zenith, or at great altitudes.
2. The variation in the apparent magnitude of known objects, such as
man, animals, trees, doors and windows of houses. If one of two men,
placed at different distances from us, appears only half the size of
the other, we cannot be far wrong in believing that the smallest in
appearance is at twice the distance of the other. It is possible that
the one may be a dwarf, and the other of gigantic stature, in which
case our judgment would be erroneous, but even in this case other
criteria might enable us to correct it.
3. The degree of vivacity in the colours and tints of objects.
4. The degree of distinctness in the outline and minute parts of
objects.
5. To these criteria we may add the sensation of muscular action, or
rather effort, by which we close the pupil in accommodating the eye to
near distances, and produce the accommodation.
With all these means of estimating distances, it is only by binocular
vision, in which we converge the optical axes upon the object, that we
have the power of _seeing distance_ within a limited range.
But this is the only point in which Monocular is _inferior_ to
Binocular vision. In the following respects it is _superior_ to it.
1. When we look at oil paintings, the varnish on their surface reflects
to each eye the light which falls upon it from certain parts of the
room. By closing one eye we shut out the quantity of reflected light
which enters it. Pictures should always be viewed by the eye farthest
from windows or lights in the apartment, as light diminishes the
sensibility of the eye to the red rays.
2. When we view a picture with both eyes, we discover, from the
convergency of the optic axes, that the picture is on a plane surface,
every part of which is nearly equidistant from us. But when we shut one
eye, we do not make this discovery; and therefore the effect with which
the artist gives relief to the painting exercises its whole effect in
deceiving us, and hence, in monocular vision, the _relievo_ of the
painting is much more complete.
This influence over our judgment is beautifully shewn in viewing,
with one eye, photographs either of persons, or landscapes, or solid
objects. After a little practice, the illusion is very perfect, and
is aided by the correct geometrical perspective and _chiaroscuro_ of
the Daguerreotype or Talbotype. To this effect we may give the name of
_Monocular Relief_, which, as we shall see, is necessarily inferior to
_Binocular Relief_, when produced by the stereoscope.
3. As it very frequently happens that one eye has not exactly the same
focal length as the other, and that, when it has, the vision by one eye
is less perfect than that by the other, the picture formed by uniting a
perfect with a less perfect picture, or with one of a different size,
must be more imperfect than the single picture formed by one eye.