Its efficiency as a telescope
depends entirely on the accuracy with which the requisite form has
been imparted to the mirror. The surface has to be hollowed out a
little, and this has to be done so truly that the slightest deviation
from good workmanship in this essential particular would be fatal to
efficient performance of the telescope.
[PLATE: WILLIAM HERSCHEL.]
The mirror that Herschel employed was composed of a mixture of two
parts of copper to one of tin; the alloy thus obtained is an
intensely hard material, very difficult to cast into the proper
shape, and very difficult to work afterwards. It possesses, however,
when polished, a lustre hardly inferior to that of silver itself.
Herschel has recorded hardly any particulars as to the actual process
by which he cast and figured his reflectors. We are however, told
that in later years, after his telescopes had become famous, he made
a considerable sum of money by the manufacture and sale of great
instruments. Perhaps this may be the reason why he never found it
expedient to publish any very explicit details as to the means by
which his remarkable successes were obtained.
[PLATE: CAROLINE HERSCHEL.]
Since Herschel's time many other astronomers, notably the late Earl
of Rosse, have experimented in the same direction, and succeeded in
making telescopes certainly far greater, and probably more perfect,
than any which Herschel appears to have constructed.
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