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thousands of years old in the ruins of Nineveh, Babylon, and Pompeii.
They are of the utmost importance in the science of astronomy. This slow
progress made in perfecting them shows the inherent difficulties that
exist in obtaining glass of the required purity. One of these is the
different specific gravities of the material used. Hence the lower part
of a pot of melted glass is of greater specific gravity than the top,
causing a tendency to cords or threads, an evil which science has yet
to learn to overcome. Not even the large bounty offered by the English
Government and the Board of Longitude has been successful in effecting
any important improvement in this branch of manufacture.
Munich enjoys the reputation of producing the
best lenses, and consequently the finest telescopes. Sir Isaac Newton,
Gregory, Dolland, Keir, and others adopted lenses made from flint- and
from crown-glass, it being necessary to use both in the construction
of achromatic telescopes, one possessing as small and the other as
great dispersive powers relative to the mean refractive powers as can
be procured. But the inherent defect of the lenses still remained.
M. Macquer remarks, "The correction of this fault
appears therefore to be very difficult." He had