[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. The details of
these later methods are now well known, and have been extensively
practised. Many amateurs have thus been able to make telescopes by
following the instructions so clearly laid down by Lord Rosse and the
other authorities. Indeed, it would seem that any one who has a
little mechanical skill and a good deal of patience ought now to
experience no great difficulty in constructing a telescope quite as
powerful as that which first brought Herschel into fame. I should,
however, mention that in these modern days the material generally