In
conjunction with the celebrated Fraunhofer he made disks of nine
or ten inches in diameter, which were employed by his colaborer in
constructing the telescopes which were so famous in their time. He
was long supposed to be in possession of some secret method of
avoiding the difficulties which his predecessors had met. It is
now believed that this secret, if one it was, consisted
principally in the constant stirring of the molten glass during
the process of manufacture. However this may be, it is a curious
historical fact that the most successful makers of these great
disks of glass have either been of the family of Guinand, or
successors, in the management of the family firm. It was Feil, a
son-in-law or near relative, who made the glass from which Clark
fabricated the lenses of the great telescope of the Lick
Observatory. His successor, Mantois, of Paris, carried the art to
a point of perfection never before approached. The transparency
and uniformity of his disks as well as the great size to which he
was able to carry them would suggest that he and his successors
have out-distanced all competitors in the process. He it was who
made the great 40-inch lens for the Yerkes Observatory.
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