[PLATE: LORD ROSSE'S TELESCOPE. From a photograph by W. Lawrence,
Upper Sackville Street, Dublin.]
The first duty that Lord Rosse had to undertake was the construction
of this tremendous mirror, six feet across, and about four or five
inches thick. The dimensions were far in excess of those which had
been contemplated in any previous attempt of the same kind. Herschel
had no doubt fashioned one mirror of four feet in diameter, and many
others of smaller dimensions, but the processes which he employed had
never been fully published, and it was obvious that, with a large
increase in dimensions, great additional difficulties had to be
encountered. Difficulties began at the very commencement of the
process, and were experienced in one form or another at every
subsequent stage. In the first place, the mere casting of a great
disc of this mixture of tin and copper, weighing something like three
or four tons, involved very troublesome problems. No doubt a casting
of this size, if the material had been, for example, iron, would have
offered no difficulties beyond those with which every practical
founder is well acquainted, and which he has to encounter daily in
the course of his ordinary work. But speculum metal is a material of
a very intractable description.
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