rs, however,
there seemed no insurmountable obstacle to an almost unlimited increase
of light-gathering capacity; and it was here, after some unproductive
experiments with fluid lenses, that Lord Oxmantown concentrated his
energies.

He had to rely entirely on his own invention, and to earn his own
experience. James Short had solved the problem of giving to metallic
surfaces a perfect parabolic figure (the only one by which parallel
incident rays can be brought to an exact focus); but so jealous was he
of his secret, that he caused all his tools to be burnt before his
death;[320] nor was anything known of the processes by which Herschel
had achieved his astonishing results. Moreover, Lord Oxmantown had no
skilled workmen to assist him. His implements, both animate and
inanimate, had to be formed by himself. Peasants taken from the plough
were educated by him into efficient mechanics and engineers. The
delicate and complex machinery needed in operations of such hairbreadth
nicety as his enterprise involved, the steam-engine which was to set it
in motion, at times the very crucibles in which his specula were cast,
issued from his own workshops.

In 1827 experiments on the composition of speculum-metal were set on
foot, and the first polishing-machine ever driven by steam-power was
contrived in 1828. But twelve arduous years of struggle with recurring
difficulties passed before success began to dawn. A material less
tractable than the alloy selected, of four chemical equivalents of
copper to one of tin,[321] can scarcely be conceived. It is harder than
steel, yet brittle as glass, crumbling into fragments with the slightest
inadvertence of handling or treatment;[322] and the precision of figure
requisite to secure good definition is almost beyond the power of
language to convey. The quantities involved are so small as not alone to
elude sight, but to confound imagination. Sir John Herschel tells us
that "the _total_ thickness to be abraded from the edge of a spherical
speculum 48 inches in diameter and 40 feet focus, to convert it into a
paraboloid, is only 1/21333 of an inch;"[323] yet upon this minute
difference of form depends the clearness of the image, and, as a
consequence, the entire efficiency of the instrument. "Almost infinite,"
indeed (in the phrase of the late Dr. Robinson), must be the exactitude
of the operation adapted to bring about so delicate a result.

At length, in 1839, two specula, each three feet