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 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.