annealed; in which state, the plates are sold to the optician,
for cutting and grinding into disks.
Since the time of Guinand, of Brenette,
in Switzerland, and of Frauenhofer, and others who have done good
service to science by their meritorious labours and improvements,
the opticians of England have imported many large sized achromatic
Glasses from the Continent; although the British manufacturer, (who
only occasionally makes for home use,) still supplies eminent foreign
workers with Glass for optical purposes, some of which, no doubt,
finds its way back into England, as foreign, at a very advanced price.
Guinand's plan has already been published: the secret of his success is
considered not to have been in the novelty of materials or proportions,
but in agitating the liquid Glass while at the highest point of fusion,
then cooling down the entire contents of the pot in a mass, and, when
annealed and cool, by cleavage separating unstriated portions, afterwards
softening into clay moulds. Dr. Faraday is of opinion that the usual
materials of British Flint Glass are excellent; and that the necessary
improvement is chiefly mechanical, and not chemical. As a proof, the
very heavy Glass he has produced owes its freedom from striæ to
his plan of constant agitation, as detailed in a paper communicated to
the Royal Society, in the year 1829.
M. Bontemps, a scientific French
Glass-maker, has succeeded in making good Flint optical Glass also on
the principle of mechanical agitation; and was rewarded for his process
by the French Society of Arts, in 1840. In the year 1845, he published
the result of his experiments.
M. Bontemps operated in conjunction with
one of the sons of Guinand as follows. A long hollow cylinder or sheath
of fire-clay closed at one end, with a bore sufficiently large to admit
of a strong blowing-iron, was brought to a red heat, and introduced