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 impossible to see these
different images clearly at the same time, because each of them
will render all the others indistinct.