20.  How we See.—­Now we want to know how we see with the eye.  This is not very easy to understand, but we can learn something about it.  Let us make a little experiment.  Here is a glass lens.  If we hold it before a window and place a piece of smooth white paper behind it, we can see a picture of the houses and trees and fences, and other things out-of-doors.  The picture made by the lens looks exactly like the view out-of-doors, except that it is upside down.  This is one of the curious things that a lens does.  The lens of the eye acts just like a glass lens.  It makes a picture of everything we see, upon the ends of the nerves of sight which are spread out at the back of the eyeball.  The nerves of sight tell their nerves in the brain about the picture, just as the nerves of feeling tell their cells when they are touched with a pin; and this is how we see.

21. Did you ever look through a spyglass or an opera-glass?  If so, you know you must make the tube longer or shorter according as you look at things near by or far away.  The eye also has to be changed a little when we look from near to distant objects.  Look out of the window at a tree a long way off.  Now place a lead pencil between the eyes and the tree.  You can scarcely see the pencil while you look sharply at the tree, and if you look at the pencil you cannot see the tree distinctly.

22. There is a little muscle in the eye which makes the change needed to enable us to see objects close by as well as those which are farther away.  When people grow old the little muscles cannot do this so well, and hence old people have to put on glasses to see objects near by, as in reading.  Children should not try to wear old persons’ glasses, as this is likely to injure their eyes.

23.  How to Keep the Eyes Healthy.—­(1.) Never continue the use of the eyes at fine work, such as reading or fancy-work, after they have become very tired.

(2.) Do not try to read or to use the eyes with a poor light—­in the twilight, for instance, before the gas or lamps are lighted.

(3.) In reading or studying, do not sit with the light from either a lamp or a window shining directly upon the face.  Have the light come from behind and shine over the left shoulder if possible.

(4.) Never expose the eyes to a sudden, bright light by looking at the sun or at a lamp on first awaking in the morning, or by passing quickly from a dark room into a lighted one.

(5.) Do not read when lying down, or when riding on a street car or railway train.

(6.) If any object gets into the eye have it removed as soon as possible.

(7.) A great many persons hurt their eyes by using various kinds of eye-washes.  Never use anything of this kind unless told to do so by a good physician.

24.  How we Smell.—­If we wish to smell anything very strongly, we sniff or suddenly draw the air up through the nose.  We do this to bring more air to the nerves of smell, which are placed at the upper part of the inside of the nose.