around the pupil is a colored ring which gives the eye its color. We say
a person has blue or brown or gray eyes according as this ring is blue
or brown or gray. This colored ring is a kind of curtain for the window
of the eye.

~11.~ If you observe the pupil closely, you will see that it is
sometimes larger and sometimes smaller. If you look at the light the
pupil is small; if you turn away from the light the pupil grows larger
at once. This is because the curtain closes when in a bright light and
opens in the darkness. It does this of itself without our thinking about
it. In this way the eye is protected from too strong a light, which
would do it great harm.

~12.~ If you look a little sidewise at the eyeball, you will see that
the curtain has something in front of it which is clear as glass. It is
about the shape of a watch crystal, only very much smaller. This is to
the eye what the glass is to the windows of a house. It closes the
opening in the front of the eyeball and yet lets the light shine in.

~13. The White of the Eye.~--The white of the eye is a tough, firm
membrane which encloses the eyeball and keeps it in a round shape.

~14. The Lens.~--Do you know what a lens is? Perhaps you do not know it
by this name, but you are familiar with the spectacles which people
sometimes wear to help their eyes. The glasses in the spectacle frames
are called lenses. Well, there is something in the eye almost exactly
like one of these lenses, only smaller. It is also called a _lens_. If
some one will get the eye of an ox for you, you can cut it open and find
this part. The lens is placed in the eyeball just behind the pupil. (See
picture.)