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.)

"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.

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.

"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.

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.

"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.

"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.

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.

"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.

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.

"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.