eave to them the task of putting the
problem into a more positive shape.

V

MAKING AND USING A TELESCOPE

The impression is quite common that satisfactory views of the heavenly
bodies can be obtained only with very large telescopes, and that the
owner of a small one must stand at a great disadvantage alongside of
the fortunate possessor of a great one. This is not true to the extent
commonly supposed. Sir William Herschel would have been delighted to
view the moon through what we should now consider a very modest
instrument; and there are some objects, especially the moon, which
commonly present a more pleasing aspect through a small telescope than
through a large one. The numerous owners of small telescopes throughout
the country might find their instruments much more interesting than
they do if they only knew what objects were best suited to examination
with the means at their command. There are many others, not possessors
of telescopes, who would like to know how one can be acquired, and to
whom hints in this direction will be valuable. We shall therefore give
such information as we are able respecting the construction of a
telescope, and the more interesting celestial objects to which it may
be applied.

Whether the reader does or does not feel competent to undertake the
making of a telescope, it may be of interest to him to know how it is
done. First, as to the general principles involved, it is generally
known that the really vital parts of the telescope, which by their
combined action perform the office of magnifying the object looked at,
are two in number, the OBJECTIVE and the EYE-PIECE. The former brings
the rays of light which emanate from the object to the focus where the
image of the object is formed. The eye-piece enables the observer to
see this image to the best advantage.

The functions of the objective as well as those of the eye-piece may,
to a certain extent, each be performed by a single lens. Galileo and
his contemporaries made their telescopes in this way, because they knew
of no way in which two lenses could be made to do better than one. But
every one who has studied optics knows that white light passing through
a single lens is not all brought to the same focus, but that the blue
light will come to a focus nearer the objective than the red light.
There will, in fact, be a succession of images, blue, green, yellow,
and red, corresponding to the colors of the spectrum. It is imposs