other; the heated glafs following each rod, will be finer, in
proportion to the length it is drawn to and the fmallnefs o f the
rod; in this manner you may procure threads o f glafs o f any degree
of finenefs. Direft the flame to the middle o f the thread,
and it will be inftatitly divided into two parts. When one o f
the threads is perfectly cool, place it at the extremity o f the
flame, by which it will be rendered round; and if the thread o f
glafs be very fine, an exceeding fmall globule will be formed
This thread may now be broke off from the rod, and a new one
may be again drawn out as before, by the affiftance o f the other
glafs rod.
The fmall ball is now to be feparated from the thread o f glafs.-;
this is eafily effected by the fharp edge o f a piece of flint. The
-ball fhould be placed in a groove o f paper, and another piece o f
paper be held over it, to prevent the ball from flying about and
being loft. A quantity o f globules ought to be prepared in this
manner; they are then to be cleaned, and afterwards placed in
the eavities o f the tripoli, by means o f a delicate pair o f nippers.
The globules are now to be melted a fecond time, in order to
render them completely fpherical; for this purpofe, bring one o f
the cavities near the extremity o f the flame, directing this towards
the tripoli, which muft be firft heated ; the cavity is- then to be
lowered, fo- that the flame may touch the glafs, which, when it is
red hot, will affume a perfefi globular form; it muft then be removed
from the flame, and kid b y ; when cold, it fhould be
cleaned, by rubbing between two pieces of white paper. Let it
-now be fet in a brafs eap, to try whether the figure be perfeft; I f
the objeft is not well defined, the globule muft be thrown away.
Though, if it be large, it may be expofed two or three times to
the
the flame. When a large globule is forming, it fhould be gently
agitated by (haking the tripoli, which will prevent i i . becoming
flat on one fide. By attending to' thefe direftions, the greater
part o f the globules will be round and fit for ufe. in damp weather,
notwithftanding every precaution, it will often- h-ppcn, that
out o f forty globules; four or five only will be fit for ufe.
Mr. Stephen Gray, of the Charter-Houfe, having obferved
fome irregular particles within a glafs globule, and finding that
they appeared diftinft and prodigioufly magnified when held clofe
to his eye, concluded that if he placed a globule o f water, in
which there were any particles more opake than the water, near
his eye, he fhould fee thofe particles diftinftly and highly magnified.
This idea, when realized, far exceeded his expeftation.
His method was, to take on a pin a fmall portion o f water which
he knew had in it fome minute animalcule; this he laid on the
end o f a fmall piece o f brafs wire, till there was formed fomewhat
more than an hemifphere of water ; on applying it then to the
eye, he found the animalcule moffi enormoufly magnified; for
thofe which were fcarce difcernible with his glafs globules, with
this appeared as large as ordinary fized peas. They cannot be
feen by day-light, except the room be darkened, but are- feen to
the greateft advantage by candle-light. Montucla obferves, that
when any objefts are- inclofed within this tranfparent globule,
the hinder part of it afts like a concave mirror, provided they be
fituated between that furface and the focus; and that by this
means they are magnified three times and a half more than they
would be in the ufual way. An extempore microfcope may be
formed, by taking up a fmall drop of water on the point o f a pin,,
and placing it over a fine hole made in a piece of metal; but as