motion pictures on the screen, you would have a flicker of light and
shade, but no picture.

It is the lens that forms the pictures in your eye, on a photographic
plate or film, and on a moving-picture screen. And a lens is usually
just a piece of glass or something glassy, rounded out in such a way
as to make all the spreading light that reaches it from one point come
together in another point, as shown in Figure 69.

As you know, when light goes out from anything, as from a candle flame
or an incandescent lamp, or from the sun, it goes in all directions.
If the light from the point of a candle flame goes in all directions,
and if the light from the base of the flame also goes in all
directions, the light from the point will get all mixed up with the
light from the base, as shown in Figure 70. Naturally, if the light
from the point of the candle flame is mixed up with the light from
the base and the beams are all crisscross, you will not get a clear
picture of the flame.

[Illustration: FIG. 71. The reading glass is a lens which focuses the
light from the candle flame and forms an image.]

EXPERIMENT 47. Fasten a piece of paper against a wall and
place a lighted candle about 4 feet in front of it. Look at
the paper. Is there any picture of the candle flame on it?
Now hold a magnifying glass (reading glass) near the candle,
between the candle and the paper, so that the light will shine
through the lens on to the paper. (The magnifying glass is a
lens.) Move the lens slowly toward the paper until you get
a clear picture of the candle flame. Is it right side up or
upside down?

The lens has brought the light from the candle flame to a _focus_; all
the light that goes through the lens from one point of the flame has
been brought together at another point (Fig. 72). In the diagram you
see all the light from the _point_ of the candle flame spreading out
in every direction. But the part that goes through the lens is brought
together at one point, called the focus. Of course the same thing
happens to the light from the base of the candle flame (Fig. 73). Just
as before, all the light from the base of the flame is brought to a
focus. The light spreads out until it reaches the lens. Then the lens
bends it together again until it comes to a point.

[Illustration: FIG. 72. The light from the tip of the candle flame is
focused at one point.]

[Illustration: FIG. 73. And the light from