it
is to the object.

[Footnote 4: The following explanation may be omitted by any children
who are not interested in it. Let such children skip to the foot of
page 156.]

[Illustration: FIG. 80. How an image is formed on the retina of the
eye.]

[Illustration: FIG. 81. A simpler diagram showing how an image is
formed in the eye.]

[Illustration: FIG. 82. A diagram showing how a reading glass causes
things to look larger by making the image on the retina larger.]

[Illustration: FIG. 83. Diagram showing how a reading glass enlarges
the image on the retina. More lines are drawn in than in Figure 82.]

You can understand magnification best by looking at Figures 80, 81,
82, and 83.

In Figure 80 there are a candle flame, the lens of an eye, and the
retina on which the image is being formed.

Figure 81 is the same as Figure 80, with all the lines left out except
the outside ones that go to the lens. It is shown in this way merely
for the sake of simplicity. All the lines really belong in this
diagram as in the first. In both diagrams the size of the image on the
retina is the distance between the point where the top line touches it
and the point where the bottom line touches it.

In order to make anything look larger, we must make the image on the
retina larger. A magnifying glass, or convex lens, if put in the right
place, will do this. In the next diagram, Figure 82, we shall include
the magnifying glass, leaving out all lines except the two outside
ones shown in Figure 81.

You will notice that the magnifying glass starts to bend the lines
together, and that the lens in the eye bends them farther together;
so they cross sooner, and the image is larger. Figure 83 shows more of
the lines drawn in.

[Illustration: FIG. 84. Diagram of a microscope.]

The two important points to notice are these: First, the magnifying
glass is too close to the eye for the light to be brought to a focus
before it reaches the eye; the light is bent toward a focus, but it
reaches the eye before the focus is formed. The focus is formed for
the first time on the retina itself. Second, the magnifying glass
bends the light on its way to your eye so that the light crosses
sooner in your eye and spreads out farther before it comes to a focus.
This forms the larger image, as you see in the simple diagram, Figure
82.

[Illustration: FIG. 85. This is the way a concave mirror forms a
magnified image.]

[Illustration: FIG. 86. The conc