le of milk.

218. Your eyes have to adjust themselves differently to see
things near by and to see things at a distance.

219. A vacuum cleaner does not wear out a carpet nearly as
quickly as a broom or a carpet sweeper does.

220. You can see a sunbeam in a dusty room.

SECTION 25. _Magnification._

Why is it that things look bigger under a magnifying glass
than under other kinds of glass?

How does a telescope show you the moon, stars, and planets?

How does a microscope make things look larger?

Everybody knows, of course, that a convex lens in the right position
makes things look larger. People use convex lenses to make print
look larger when they read, and for that reason such lenses are often
called _reading glasses_. For practical purposes it is not necessary
to understand how a convex lens magnifies; the important thing is the
fact that it does magnify. But you may be curious to know just how a
magnifying glass works.

First, you should realize that the image formed by a convex lens is
not always larger than the object. Repeat Experiment 41, but this time
move the lens from near the candle toward the paper, past the point
where it makes its first clear image. Keep moving the lens slowly
toward the paper until a second image is formed. Which image is larger
than the flame? Which is smaller?

[Illustration: FIG. 79. A section of the eye.]

The important point in this experiment is for you to see that if the
lens is nearer to the image on the paper than it is to the candle, the
image is smaller than the candle. That is why a photograph is usually
smaller than the thing photographed; it would be impossible to take
a picture of a house or a mountain if the lens in the camera gave a
_magnified_ image.

[4]Your eye is a small camera. It has a lens in the front; it is lined
with black; and at the back there is a sensitive part on which the
picture is formed. This sensitive part of the eye is called the
_retina_. It is in the back part of your eyeball and is made of many
very sensitive nerve endings. When the light strikes these nerve
endings, it sends an impulse through the nerves to the back part of
the brain; then you know that the image is formed. And, of course,
since your eyeball is small and many of the things you see are large,
the image on the retina must be much smaller than the object itself,
and this is because the lens is so much nearer to the retina than