, an extra glass lens strengthens it and enables
objects to be seen nearer and therefore apparently bigger. But to
apply a magnifying glass to distant objects is impossible. In order
to magnify distant objects, another function of lenses has also to
be employed, viz., their power of forming real images, the power on
which their use as burning-glasses depends: for the best focus is
an image of the sun. Although the object itself is inaccessible,
the image of it is by no means so, and to the image a magnifier can
be applied. This is exactly what is done in the telescope; the
object glass or large lens forms an image, which is then looked at
through a magnifying glass or eye-piece.

Of course the image is nothing like so big as the object. For
astronomical objects it is almost infinitely less; still it is an
exact representation at an accessible place, and no one expects a
telescope to show distant bodies as big as they really are. All it
does is to show them bigger than they could be seen without it.

But if the objects are not distant, the same principle may still be
applied, and two lenses may be used, one to form an image, the
other to magnify it; only if the object can be put where we please,
we can easily place it so that its image is already much bigger
than the object even before magnification by the eye lens. This is
the compound microscope, the invention of which soon followed the
telescope. In fact the two instruments shade off into one another,
so that the reading telescope or reading microscope of a laboratory
(for reading thermometers, and small divisions generally) goes by
either name at random.

The arrangement so far described depicts things on the retina the
unaccustomed way up. By using a concave glass instead of a convex,
and placing it so as to prevent any image being formed, except on
the retina direct, this inconvenience is avoided.

[Illustration: FIG. 38.--View of the half-moon in small telescope. The
darker regions, or plains, used to be called "seas."]

Such a thing as Galileo made may now be bought at a toy-shop for I
suppose half a crown, and yet what a potentiality lay in that "glazed
optic tube," as Milton called it. Away he went with it to Venice and
showed it to the Signoria, to their great astonishment. "Many noblemen
and senators," says Galil