quite well and strong; and how glad she was to see the leaves and a
hundred other things she had not seen before!

[Illustration: THE EYEBALL IN ITS SOCKET

The muscle from M to M, which helps to turn the eyeball, has
been cut away to show the optic nerve.]

Here we have a picture of the _eyeball_, as we call it. The little
bands fastened to it are the bands of muscle; and as soon as I say
_muscle_ you know what they are for--to move the eyeball about, up and
down and from side to side. There are muscles outside the eye as well
as inside. Coming out from the back of the eyeball is a pearly white
cord quite different from the muscle bands. This is what we call a
_nerve_. This nerve in your eye carries to your _brain_, or thinking
machine, picture-messages of whatever you look at.

The nerve in your eye gets messages of light much as the nerve deep in
your ear gets its messages of sound--from tiny waves in the air. The
light waves are smaller and faster even than the sound waves, and the
eye nerve is the only nerve that can get pictures of them. You know
that, for wireless messages, the receiving machines are not all alike
and cannot all take the same messages, if the messages are sent with
different sorts of electric waves; and neither can our receiving
machines. Some get messages of sight, and some of sound, and some of
touch, or taste, or smell.

Now shut your eyes as quickly as you can. How long did it take you? A
minute? No, not a quarter of a second. It is about the quickest thing
you can think of--"the twinkling of an eye." You shut your eyes "quick