e a tennis ball rebound accurately.
Any surface that is smooth enough will act like a mirror, although
naturally, if it lets most of the light go through, it will not
reflect as well as if it sends all the light back. A pane of glass is
very smooth, and you can see yourself in it, especially if there is
not much light coming through the glass from the other side to mix up
with your reflection. But if the pane of glass is silvered so that no
light can get through, you have a real mirror; most of the light that
leaves your face is reflected to your eyes again.

WHY SMOOTH OR WET THINGS ARE SHINY. When a surface is very smooth,
we say it is shiny or glossy. Even black shoes, if they are polished,
become smooth enough to reflect much of the light that strikes them;
of course the parts where the light is being reflected do not look
black but white, as any one who has tried to paint or draw a picture
of polished shoes knows. Anything wet is likely to be shiny, because
the surface of water is usually smooth enough to reflect light rather
directly.

If a surface is uneven, like a pool with ripples on it, the light
reflects unevenly, and you see a distorted image; your face seems to
be rippling and moving in the water.

[Illustration: FIG. 64. How should the mirror be placed?]

_APPLICATION 34._ Some boys were playing war and were in a
ditch that they called a trench. They wanted to make a simple
periscope so that they could look out of the ditch at the
"enemy" without being in danger. They had an old stovepipe and
a mirror. Practically all of them agreed that if the mirror
were fixed in the top of the stovepipe and if they looked up
through the bottom, they would be able to see over the side
of the ditch. But they had an argument as to how the mirror
should be placed. Each drew a diagram to show how he thought
the mirror should be arranged, using dotted lines to show how
the light would come from the enemy to their eyes. Three of
the diagrams are shown in Figure 64.

The boy who drew the first said: "If you want to see the
enemy, the mirror's got to face him. Then it will reflect the
light down to your eyes."

The boy who drew the second said: "No, the light would just
go back to him again. The mirror must slant so that the light
that strikes it at a slant will be reflected to your eye at
the same slant."

"How could it get to your eye at