The senses by which the mind obtains most of its knowledge are the sight and the hearing. In this chapter we will look at the organ or instrument of sight.
The eye a beautiful instrument.
The eye is a very beautiful instrument. It is very nicely made, and it has a great many different parts. You are not old enough yet to understand all about these parts, but there are some things about them that I can explain to you.
What we call the white of the eye is a strong, firm sort of bag. It is filled mostly with a jelly-like substance. It is this that makes it a firm ball. If it were empty it would be like a bag. Into the open part of this, in front, is fitted a clear window. The light goes in here. It can not get in at the sides of the eyeball, through the thick white of the eye.
Its window and dark chamber.
Through this very clear window you can look into the bag or ball of the eye. You can not look through the jelly-like substance that is there, and see the very back of the inside of the eyeball; but it is like looking into a dark chamber. The reason that it is so dark is, that it is lined with something almost black. If this were not so, the eyes would be dazzled with the light that commonly goes into them, just as they now are when the light is very bright indeed.
Inside of the front window of the eye that I have told you aboutthere is a fluid as clear as water. In this fluid you see a sort of curtain with a round opening in it. This opening is called the pupil of the eye. It is not always of the same size. When there is a very bright light, it is small; but when the light is dim, it is large, for then you want all the light that you can get in that dark chamber where the jelly is. You can see the pupil change in its size if you look into the eye of any one while you bring a light very near, and then move it off quickly.
The iris the curtain in the eye.
The curtain in which this opening is we call the iris. It is circular. Its outer edge is fastened all round to the inside of the eyeball. The watery fluid, that I told you is inside of the window of the eye, is on both sides of this curtain. It would not do to have the jelly here, for the curtain would not move easily in that in changing the size of its opening.
The pupil a round opening in it.
The iris is, you know, of different colors in different persons. When it is blue, we say that the person has a blue eye; and if it is quite dark, we say that he has a black eye; and so of other colors. This curtain makes the eye very beautiful; but its chief use is, as you see, to regulate the quantity of light that goes into the eye. When there is a great deal of light, the curtain is drawn in such a way as to have the round opening very small; but when there is little light, it is drawn so as to make this opening large. This curtain must be made very nicely, or it would be puckered when the opening in it is changed in this way. No man could make a curtain of this shape, and have it work like this: it would be a very awkward thing if he should undertake it. He could not possibly make it so that the round opening in it could be madesmaller and larger without wrinkling. But look at this beautiful curtain in the eye, and see how smooth it is, and how perfectly round its edge keeps, as the size of the pupil is changed. Did you ever see any thing work more prettily and easily than this does?
The pupil in the eye of the cat and the horse.
The opening in the curtain is different in different animals. In the cat it is of this shape; in the horse it is shaped in this way. You can see the difference in the size of the cat’s pupil in different lights: if you look at her eyes in a bright sunlight, and then again in the evening, you will see that it is very much larger in the evening than it is in the day. When the sun is very bright, her pupil is a mere chink, like this; but in the evening it is very wide open, shaped in this way.
The images in the eye’s dark chamber.
But I have not yet told you how you see. It is done in this way. The light that goes in through the pupil makes an image or picture there of every thing that is before the eye. It makes the image on a very thin sheet spread out on the back part of thedark chamber where the jelly is; it is just as light makes images of things in a looking-glass, or in the smooth, still water; the only difference is, that the image or picture in the eye is very small. When you see a tree pictured in the still water, the picture is as large as the tree itself; but the picture that the light makes of the tree in that dark chamber of your eye is very small. The picture in your eye of a whole landscape, with all its trees, houses, hills, etc., does not cover over a space larger than a ten cent piece.
But how does the mind in the brain know any thing about these pictures? It knows about them by means of a nerve, that goes from the brain to the eye, and is spread out where the pictures or images are made. It would do no good to have the pictures made in the eye, if the nerve could not tell the mind about them. The eye might be perfect, and yet there might not be any seeing. It is as necessary to have the nerve in good order as it is the eye itself. It is not your eye that sees; it is your mind, and in seeing it uses both the nerve and the eye.
Why we have two eyes.
You have two eyes. When you look at one thing, say a house, there is a picture of the house in both eyes. The two nerves tell the mind in the brain about the two pictures. How is this? Why does not the mind see two houses? It is because the pictures in the two eyes are exactly alike, and both nerves, therefore, tell exactly the same story; if they did not, then the mind would see two houses; that is, it would see double, as it is called. You can see double by pressing one eye sidewise while you let the other go free.
The eyes of insects.
The eyes of insects are very curious. You remember what Itold you about compound flowers. Now, as in a compound flower there are a great many flowers together, so it is with the eyes of insects. The eye of a common fly is made up of thousands of eyes; so, when he looks at any thing, there are thousands of very little images of it made by the light in these eyes, and the nerves tell the fly’s mind, in his little brain, about them. These eyes are so exceedingly small that you can not see them without a microscope. How fine, then, must be the nerves that go from them to the fly’s brain! Your eye is a very wonderful instrument, but God has put thousands of them just as wonderful into the head of the fly that buzzes about you. It is as easy for him to make little eyes as large ones, and he can make a multitude as easily as one.
Questions.—By what senses does the mind learn the most? What is the white of the eye? What is it filled with? What is there in the front part of the eye? What is said about the dark chamber of the eyeball? What is just inside of the front window of the eye? What is the pupil of the eye? What is the iris? How is it arranged? What is said of its color? What is its chief use? Tell about this. What is said about its being made nicely? What is said about the shape of its opening in different animals? What is said about the cat’s pupil in different lights? Tell about the images made in the eye. What is said about the nerve of the eye? How is it that, with two eyes, you do not see double? Why do you have two eyes? What is said about the eyes of insects?
Questions.—By what senses does the mind learn the most? What is the white of the eye? What is it filled with? What is there in the front part of the eye? What is said about the dark chamber of the eyeball? What is just inside of the front window of the eye? What is the pupil of the eye? What is the iris? How is it arranged? What is said of its color? What is its chief use? Tell about this. What is said about its being made nicely? What is said about the shape of its opening in different animals? What is said about the cat’s pupil in different lights? Tell about the images made in the eye. What is said about the nerve of the eye? How is it that, with two eyes, you do not see double? Why do you have two eyes? What is said about the eyes of insects?