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Of late years it has become very common to make the mirror of
glass and to cover the reflecting face with an exceedingly thin
film of silver, which can be polished by hand in a few minutes.
Such a mirror differs from our ordinary looking-glass in that the
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.
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