astronomical instrument, however, it dates from the
time of the great Italian scientist Galileo, who, with a very small and
imperfect telescope of his own invention, first observed the spots on
the sun, the mountains of the moon, and the chief four satellites of
Jupiter. A good pair of modern binoculars is superior to this early
instrument of Galileo's, and the history of telescope construction, from
that primitive instrument to the modern giant recently erected on Mount
Wilson, California, is an exciting chapter in human progress. But the
early instruments have only an historic interest: the era of modern
telescopes begins in the nineteenth century.

During the last century telescope construction underwent an
unprecedented development. An immense amount of interest was taken in
the construction of large telescopes, and the different countries of the
world entered on an exciting race to produce the most powerful possible
instruments. Besides this rivalry of different countries there was a
rivalry of methods. The telescope developed along two different lines,
and each of these two types has its partisans at the present day. These
types are known as _refractors_ and _reflectors_, and it is necessary to
mention, briefly, the principles employed in each. The _refractor_ is
the ordinary, familiar type of telescope. It consists, essentially, of a
large lens at one end of a tube, and a small lens, called the eye-piece,
at the other. The function of the large lens is to act as a sort of
gigantic eye. It collects a large amount of light, an amount
proportional to its size, and brings this light to a focus within the
tube of the telescope. It thus produces a small but bright image, and
the eye-piece magnifies this image. In the _reflector_, instead of a
large lens at the top of the tube, a large mirror is placed at the
bottom. This mirror is so shaped as to reflect the light that falls on
it to a focus, whence the light is again led to an eye-piece. Thus the
refractor and the reflector differ chiefly in their manner of gathering
light. The powerfulness of the telescope depends on the size of the
light-gatherer. A telescope with a lens four inches in diameter is four
times as powerful as the one with a lens two inches in diameter, for the
amount of light gathered obviously depends on the _area_ of the lens,
and the area varies as the _square_ of the diameter.

The largest telescopes at present in existence are _reflectors_. It is