963. _Why do we see objects?_
Because the light which is reflected from them enters our eyes and
produces images of their forms upon a membrane of nerves called the
_retina_, just as images are produced upon a mirror.
964. _Why does this enable us to see?_
Because the membrane which receives the images of objects is connected
with the _optic nerve_ which transmits to the brain impressions made by
the reflections of light, just as other nerves convey the effects of
feeling, hearing, tasting, &c.
965. _Why are we enabled to move our eyes?_
Because various muscles are so placed in relation to the eyeball, that
their contraction draws the eye in the direction required. We are thus
enabled to adjust the direction of the eye to the position of the
objects we desire to see, in other words to _set the mirror in such a
position that it will receive the reflection_. (See 517.)
[Verse: "Truly the light is sweet, and a pleasant thing it is for
the eyes to behold the sun."--ECCLESIASTES XI.]
966. _Why are we enabled to see large objects upon so small a surface?_
Because the lenses and humours of the eye _collect the rays of light_
coming from every direction, and, _bringing them into a focus_,
transmit them to the retina, where each ray impresses upon the nervous
surface the qualities it received from the object which reflected it.
[Illustration: Fig. 61.--THE EYEBALL AND ITS MUSCLES.]
A. Portion of _bone_ through which the optic nerve passes in its
communication between the brain and the eye.
B. The _optic nerve_, from before which an _external muscle_ has
been cut away, leaving its two attachments.
C. The _globe_ of the eye.
D. The muscle which turns the eye _outward_, and which is
counteracted by a muscle on the other side.
E. The muscle which passes through a loop, or staple of cartilage
I, and _turns the eye obliquely_. It is counteracted by a muscle
situated underneath.
F. The muscle situated underneath, which turns _the eyeball
upwards_, and is counteracted by
G. The muscle which _turns the eyeball downwards_.
H. The muscle attached to a bone which _turns the eyeball upwards_.
I. The _cartilaginous loop_ through which a muscle passes.
J. The front chamber of the eye filled with a clear fluid.
K. Fragment of the bone by which one of the muscles is fastened.
967. _Why do some persons squint?_
Because it sometimes happens that a muscle of the eye _acts too
powerfully_ for its companion muscle, and draws the eye too much on one
side.
968. _Why does the pupil of the eye look black?_
Because the pupil is an _opening_ through which the rays of light pass
into the chamber of the eye. There is, therefore, nothing in the pupil,
of the eye to reflect light.
[Verse: "Keep me as the apple of thine eye; hide me under the shadow
of thy wings."--PSALM XVII.]
969. _Why is the pupil of the eye larger sometimes than at others?_
Because the _iris_, a ring of extremely fine muscles which surround the
pupil, contracts when too much light falls upon the retina, and dilates
when the light is feeble. It therefore enlarges or diminishes the size
of the pupil to _regulate the admission of light_.
[Illustration: Fig. 62.--SECTION OF THE EYE SEEN FROM BEHIND.]
A. The _pupil_ of the eye through which the light enters.
B. The _iris_, which dilates or contracts, and thereby increases
or lessens the size of the _pupil_.
C. The three coats of the eye, called the _sclerotic_, _choroid_,
and _retina_.
D. The _ciliary processes_, or hair-like muscles, which have a
slight vibratory motion which they impart to the fluids of the eye.
E. The dark coat of the _choroid_, the coat forming the _retina_
removed.
970. _Why have we two eyes?_
Because the field of vision is thereby _much extended_; the _intensity_
of sight is also increased, the impressions upon the brain being
clearer and better defined, just as in a _stereoscope_ the effect of
vision is heightened by a double picture; the sense of sight being more
_constantly_ exercised than any other sense during our waking moments,
_one eye is frequently called upon to give rest to the other_; and
the important faculty of vision, being endangered by the necessary
exposure of some parts of the eye, and the equally necessary delicacy
of an organ formed to receive impressions from so ethereal an element
as light, is rendered the more secure to us, since though one eye may
become enfeebled, diseased, or wholly lost, _the other eye will retain
the blessing of sight_.
[Verse: "The eyes of the Lord are upon the righteous, and his ears
are open unto their cry."--PSALM XXXIV.]
971. _Why, having two eyes, and each eye receiving a reflection upon
its retina, does the brain experience only one impression of an object?_
Because, besides those optical laws which bring upon the two retinas
the exactly corresponding images of the same objects, the optic nerves
_meet_ before they reach the brain, _and blend the impulses which they
convey_.
972. _Why are the eyes provided with eyelids?_
Because the eyes require to be _defended_ from floating particles
in the air, and to be kept _moist and clean_. The eyelids form the
shutters of the eye, defending it when waking, by closing upon its
surface whenever danger is apprehended, moistening its surface when it
becomes dry, and covering it securely during the hours of sleep.
973. _Why are the eyelids fringed with eyelashes?_
Because the eyelashes assist to modify the light, and to protect
the eye, without actually closing the eye-ids. When the eyelids are
partially closed, as in very sunny or dusty weather, the eyelashes
cross each other, forming a kind of shady lattice-work, from the
interspaces of which the eye looks out with advantage, and sees
sufficiently for the guidance of the body.
974. _Why are we able to see at long or short distances?_
Because the _crystalline lens_ of the eye is a moveable body, and is
pushed forward, or drawn back by fine muscular fibres, according to the
distances of the objects upon which we look. By these means its _focus_
becomes adjusted.
975. _Why do we wink?_
Because, by the repeated action of winking, _the eye is kept moist and
clean_, and the watery fluid secreted by little glands in the eyelids,
and at the sides of the eye, is spread equally over the surface,
instead of being allowed to accumulate. But the action of winking, or
brightening the eye, is so instantaneous that it does not impede the
sight.
[Verse: "And the eye cannot say unto the hand, I have no need of
thee; nor again the head to the feet, I have no need of you."--CORINTH.
XII.]
976. _Whence are the humours and secretions of the eye derived?_
From the blood, which flows abundantly to the eyes, and is circulated
in capillary vessels that are spread out upon the membranous coats of
the eye-balls.
[Illustration: Fig. 63.--SECTION OF THE EYE.]
A and B. The _sclerotic_, _choroid_, and _retina_, the three
layers or coats which form the walls of the globe of the eye, and
enclose its humours.
C C. The _iris_.
D. The front chamber of the eye, filled with watery humour.
E. The _pupil_, through which the rays of light pass to
F. The _crystalline lens_.
G G. The _vitreous humour_ enclosed in cells formed by the
_hyaloid membrane_.
H. An _artery_ which supplies blood to the _crystalline lens_, and
which passes through the centre of the _optic nerve_.
G. The _optic nerve_, showing the sheath in which the nerve is
enclosed.
977. _Why do tears form in the eyes?_
Because, under the emotions of the mind, the circulation of blood in
the brain, and in its nearest branches, becomes considerably quickened.
The eyes receive a larger amount of blood, and the secretion of the
lachrymal glands being increased, the fluid overflows, and tears are
formed. The use of tears is probably _to keep the eyes cool during the
excitement of the brain_. They are formed also during _laughing_, but
less frequently.
[Verse: "If the whole body were an eye, where were hearing? if the
whole were hearing, where were smelling?"--CORINTHIANS XII.]
978. _Why do we feel inconvenienced by sudden light?_
Because an excess of light enters the eye before the _iris_ has had
_time to adjust the pupil_ to the amount of light to be received.
979. _Why if we look upon a very bright light, and then turn away, are
we unable to see?_
Because the _iris_ has so reduced the pupil while we were looking at
the bright light, that immediately upon turning to a darker object,
_the pupil is too small_ to admit sufficient rays to enable us to see.
[Illustration: Fig. 64.--CAPILLARY BLOOD-VESSELS OF THE EYE.]
A A. Capillary veins distributed over the _sclerotic coat_.
B. One of the trunks of the _optic nerve_.
C. A _nerve_ communicating with the _ciliary processes_.
D. A _vein_ running parallel with the nerve to the _ciliary
processes_.
E. Side view of the _iris_.
980. _Why do we see better after a short time?_
Because the _iris_ has relaxed and enlarged the pupil, therefore _we
receive more rays of light_ from the comparatively dark object, and are
enabled to see it more clearly.
981. _Why do cats, bats, owls, &c., see in the dark?_
Because their eyes are made highly sensitive to _small quantities of
light_. It is also believed that there are certain properties of light
which affect their eyes, but do not affect ours. In other words, that
there are some rays which are luminous to them which are not luminous
to us. Hence they find _light_ in what we call _darkness_.
[Verse: "He that hath ears to hear, let him hear."--MATTHEW XI.]
982. _Why does the pupil of a cat's eye appear nearly closed by day?_
Because the cat's eye is so sensitive to light that the iris _closes
the pupil almost entirely_ to shut out the too powerful light.