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.