CHAPTER XI.
SEEING.
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 about there 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.