be moved forward and back
until they form a sharp focus on the screen. Even the lens in your eye
has muscles that make it flatter and rounder, so that it can make a
clear image on the sensitive retina in the back of your eye. The lens
in the eyes of elderly people often becomes too hard to be regulated
in this way, and so they have to wear one kind of glasses to see
things near them clearly and another kind to see things far away.

The kind of lens we have been talking about is the _convex_ lens.
"Convex" means bulging out in the middle. There are other kinds of
lenses, some flat on one side and bulging out on the other, some
hollowed out toward the middle instead of bulging, and so on. But the
only lens that most people make much use of (except opticians) is the
convex lens that bulges out toward the center. The convex lens makes a
clear image and it is the only kind of lens that will do this.

[Illustration: FIG. 78. Lenses of different kinds.]

WHY YOU CAN SET FIRE TO PAPER WITH A MAGNIFYING GLASS. A convex lens
brings light to a focus, and it also brings radiant heat to a focus.
And that is why you can set fire to things by holding a convex lens in
the sunlight so that the light and heat are focused on something that
will burn. All the sun's radiant heat that strikes the lens is brought
practically to one point, and all the light which is absorbed at this
point is changed to heat. When so much heat is concentrated at one
point, that point becomes hot enough to catch fire.

_APPLICATION 36._ Explain why there is a lens in a
moving-picture machine; why a convex lens will burn your hand
if you hold it between your hand and the sun; why the front
of a good camera is made so that it can be moved closer to the
plate or farther away from it, according to the distance of
the object you are photographing; why there is a lens in your
eye.

INFERENCE EXERCISE

Explain the following:

211. Cut glass ware sparkles.

212. An unpainted floor becomes much dirtier and is harder to
clean than a painted one.

213. If you sprinkle wet tea leaves on a rug before sweeping
it, not so much dust will be raised.

214. Food leaves a spoon when the spoon is struck sharply upon
the edge of a stewpan.

215. An image is formed on the photographic plate of a camera.

216. Ripples in a pool distort the image seen in it.

217. Cream rises to the top of a bott