satellites, when so near to the disc, are not seen in a straight line,
as the three shown in the figure happen to be. Of the three spots on the
disc, the faintest is a satellite, the neighbouring dark spot its
shadow, the other dark spot the shadow of the satellite close to the
planet's disc.




HALF-HOURS WITH THE TELESCOPE.




CHAPTER I.

A HALF-HOUR ON THE STRUCTURE OF THE TELESCOPE.


There are few instruments which yield more pleasure and instruction than
the Telescope. Even a small telescope--only an inch and a half or two
inches, perhaps, in aperture--will serve to supply profitable amusement
to those who know how to apply its powers. I have often seen with
pleasure the surprise with which the performance even of an opera-glass,
well steadied, and directed towards certain parts of the heavens, has
been witnessed by those who have supposed that nothing but an expensive
and colossal telescope could afford any views of interest. But a
well-constructed achromatic of two or three inches in aperture will not
merely supply amusement and instruction,--it may be made to do useful
work.