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.