ecessary for us to learn
about the eyeball, so let us notice some other parts about the eye.
First there are the eyelids. They are little folds of skin fringed with
hairs, which we can shut up so as to cover the eyeball and keep out the
light when we want to sleep or when we are in danger of getting dust or
smoke into the eye. The hairs placed along the edge of the lids help to
keep the dust out when the eyes are open.

~17. The Eyebrows.~--The row of hairs placed above the eye is called the
eyebrow. Like the eyelids, the eyebrows catch some substances which
might fall into the eye, and they also serve to turn off the
perspiration and keep it out of the eyes.

~18. The Tear Gland.~--Do you know where the tears come from? There is a
little gland snugly placed away in the socket of the eye just above the
eyeball, which makes tears in the same way that the salivary glands make
saliva. It is called the _tear gland_. The gland usually makes just
enough tears to keep the eye moist. There are times when it makes more
than enough, as when something gets into the eye, or when we suffer pain
or feel unhappy. Then the tears are carried off by means of a little
tube which runs down into the nose from the inner corner of the eye.
When the tears are formed so fast that they cannot all get away through
this tube, they pass over the edge of the lower eyelid and flow down the
cheek.

~19. Muscles of the Eyes.~--By means of little muscles which are
fastened to the eyeball, we are able to turn the eye in almost every
direction.

~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 oper