coating of silver is put on the front surface, so that the light
does not pass through the glass. Moreover, the coating of silver
is so thin as to be almost transparent: in fact, the sun may be
seen through it by direct vision as a faint blue object. Silvered
glass reflectors made in this way are extensively manufactured in
London, and are far cheaper than refracting telescopes of
corresponding size. Their great drawback is the want of permanence
in the silver film. In the city the film will ordinarily tarnish
in a few months from the sulphurous vapors arising from gaslights
and other sources, and even in the country it is very difficult to
preserve the mirror from the contact of everything that will
injure it. In consequence, the possessor of such a telescope, if
he wishes to keep it in order, must always be prepared to resilver
and repolish it. To do this requires such careful manipulation and
management of the chemicals that it is hardly to be expected that
an amateur will take the trouble to keep his telescope in order,
unless he has a taste for chemistry as well as for astronomy.

The curiosity to see the heavenly bodies through great telescopes
is so wide-spread that we are apt to forget how much can be seen
and done with small ones. The fact is that a large proportion of
the astronomical observations of past times have been made with
what we should now regard as very small instruments, and a good
deal of the solid astronomical work of the present time is done
with meridian circles the apertures of which ordinarily range from
four to eight inches. One of the most conspicuous examples in
recent times of how a moderate-sized instrument may be utilized is
afforded by the discoveries of double stars made by Mr. S. W.
Burnham, of Chicago. Provided with a little six-inch telescope,
procured at his own expense from the Messrs. Clark, he has