r than a small stone.

[Illustration: FIG. 77.--The lens is placed in such a position that
the image is about the same size as the object.]

Many familiar illustrations of lenses, or curved refracting surfaces,
and their work, are known to all of us. Fish globes magnify the fish
that swim within. Bottles can be so shaped that they make the olives,
pickles, and peaches that they contain appear larger than they really
are. The fruit in bottles frequently seems too large to have gone
through the neck of the bottle. The deception is due to refraction,
and the material and shape of the bottle furnish a sufficient
explanation.

By using combinations of two or more lenses of various kinds, it is
possible to have an image of almost any desired size, and in
practically any desired position.

116. The Human Eye. In Section 114, we obtained on a movable screen,
by means of a simple lens, an image of a candle. The human eye
possesses a most wonderful lens and screen (Fig. 78); the lens is
called the crystalline lens, and the screen is called the retina. Rays
of light pass from the object through the pupil _P_, go through the
crystalline lens _L_, where they are refracted, and then pass onward
to the retina _R_, where they form a distinct image of the object.

[Illustration: FIG. 78.--The eye.]

We learned in Section 114 that a change in the position of the object
necessitated a change in the position of the screen, and that every
time the object was moved the position of the screen had to be altered
before a clear image of the object could be obtained. The retina of
the eye cannot be moved backward and forward, as the screen was, and
the crystalline lens is permanently located directly back of the iris.
How, then, does it happen that we can see clearly both near and
distant objects; that the printed page which is held in the hand is
visible at one second, and that the church spire on the distant
horizon is visible the instant the eyes are raised from the book? How
is it possible to obtain on an immovable screen by means of a simple
lens two distinct images of objects at widely varying distances?

The answer to these questions is that the crystalline lens changes
shape according to need. The lens is attached to the eye by means of
small muscles, _m_, and it is by the action of these muscles that the
lens is able to become small and thick, or large and thin; that is, to
become more or less curved. When we look at near objects