Blown LENSES, are only made use of in the single
microscope, and the common method of making them
has been to draw out a fine thread of the soft white
glass called crystal, and to convert the end of it into a
spherule by melting it at the flame of a candle. Mr
Nicholson observes that window glass affords excellent
spherules. A thin piece from the edge of a pane of
glass one-tenth of an inch broad was held perpendicu-
larly, and the flame of a candle was directed against
it by means of the blow-pipe, when it became soft, and
the lower end descended by its own weight to the dis-
tance of about two feet, where it remained suspended
by
by a thin thread of glass about of an inch in diameter. A part of this thread was applied endwise to the lower blue part of the flame of the candle without the blow-pipe, when the end became instantly white-hot, and formed a globule, which was gradually thrust towards the flame till it became sufficiently large. A number of these were made and examined, by viewing their focal images with a deep magnifier, when they appeared bright, perfect, and round.