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.

As optical glass is now made, the material is constantly stirred
with an iron rod during all the time it is melting in the furnace,
and after it has begun to cool, until it becomes so stiff that the
stirring has to cease. It is then placed, pot and all, in the
annealing furnace, where it is kept nearly at a melting heat for
three weeks or more, according to the size of the pot. When the
furnace has cooled off, the glass is taken out, and the pot is
broken from around it, leaving only the central mass of glass.
Having such a mass, there is no trouble in breaking it up into
pieces of all desirable purity, and sufficiently large for
moderate-sized telescopes. But when a great telescope of two feet
aperture or upward is to be constructed, very delicate and
laborious operations have to be undertaken. The outside of the
glass has first to be chipped off, because it is filled with
impurities from the material of the pot itself. But this is not
all. Veins of unequal density are always found extending through