the base of the flame is
focused at another point.]

[Illustration: FIG. 74. The light from the tip and base (and from
every other point) of the flame is, of course, focused at the same
time. In this way an image of the flame is formed.]

But of course the light from the base of the flame is focused at the
same time as the light from the point; so what really happens is that
which is illustrated in Figure 74. In this diagram, we have drawn
unbroken lines to show the light from the point of the candle flame
and dotted lines to show the light from the base of the flame. This is
so that you can follow the light from each part and see where it
goes. Compare this diagram with the one where the light is shown all
crisscrossed (Fig. 70), and you will see why the lens makes an image,
while you have no image without it.

By looking at the last diagram (Fig. 74) you can also see how the
image happens to be upside down.

EXPERIMENT 48. Set up the candle and piece of paper as you did
for the last experiment, but move the magnifying glass back
and forth between the paper and the candle. Notice that there
is one place where the image of the candle is very clear. Does
the image become clearer or less clear if you move the lens
closer to the candle? if you move it farther from the candle?

The explanation is this: After the light comes together into a point,
it spreads out again beyond the point, as shown in Figure 75. So if
you hold the lens in such a way that the light comes to a focus before
it reaches the paper, the paper will catch the spreading light and you
will get a blur instead of a sharp image. It is as shown in Figure 76.

[Illustration: FIG. 75. The light spreads out again beyond the focus.]

[Illustration: FIG. 76. So if the light comes to a focus before it
reaches the paper, the image will be blurred.]

On the other hand, if you hold your lens in such a way that the light
has not yet come to a focus when it reaches the paper, naturally you
again have a blur of light instead of a point, and the image is not
sharp and definite (Fig. 77).

[Illustration: FIG. 77. Or if the light reaches the paper before it
comes to a focus, the image will be blurred.]

And that is why good cameras have the front part, in which the lens
is set, adjustable; you can move the lens back and forth until a
sharp image is formed on the plate. Motion-picture machines and
stereopticons likewise have lenses that can