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.