72
that the oil in the burner may return into the body when fo placed
and filled; if, by being too full, any oil appears above the guard,
only move the lamp a little, and the oil will difappear; the lamp
may then be placed ereft, and the oil will flow to it’s proper level.
The oil mull be o f the fpermaceti kind, commonly called
chamber oil, which may generally be diftinguifhed by it’s pale-
neft, tranfparency, and inoffenfive fcent; all thofe oils which are
o f a red and brown colour, and o f an offenfive fcent, fhould be
carefully avoided, as their glutinous parts clog the lamp, and
the impurities in fuch oil not being inflammable, will accumulate
and remain in the form o f a cruft on the wick. Seal oil is nearly
as pale and fweet as chamber o i l; but being o f a heavy fluggifh
quality, is not proper for lamps with fine wicks.
Whenever bad oil has been ufed, on changing it, the wick mud
alfo be changed, becaufe after having imbibed the coarfe particles
in it’s capillary tubes, it will not draw up the fine oil.
T o obtain the greateft degree o f light, the wick fhould be trimmed
exaftly even, the flame will then be completely equal.
There will be a great advantage in keeping the lamp clean,
elpecially the burner and air tubes ; the negleft of cleanlineft in
lamps is too common: a candleftick is generally cleaned every
time it is ufed,, fo fhould a lamp f and if a candleftick is not to be
objefted to becaufe it does not give light after the candle is ex-
haufted, fo a lamp fhould not be thought ill of, if it does not
give light when it wants oil or cotton; but this laft has often happened,
becaufe the deficiency is left vifb'.e.
The glafs tubes are beft cleaned with a piece of wafh leather.
I f a fountain-lamp fis left partly filled with oil, it may be liable
to overflow; this happens by the contraction o f the air when
cold, and it’s expanfion by the warmth o f a room, the rays of
the fun, or the heat of the lamp when re-lighted: this accident
may be effectually prevented by keeping the refervoir filled, the
oil not being fubjeft to expanfion like air. On this account, thofe
with a common refervoir are beft adapted for microfcopic pur-
pofes.
T o examine opake Objects with the L ucernal
Microscope.
The microfcope is reprefented as mounted, and entirely ready
for this purpofe, in Fig. 1, Plate III..
T o render the ufe o f this inftrument eafy, it is ufually packed
with as many o f the parts together as poflible; it occupies on
this account rather more room, but is much left embarrafling to
the obferver, who has only three parts to put on after it is taken
out of it’s box, namely, the guide for the eye, the ftage, and the
tube with it’s magnifier.
But to be more particular, take out the wooden Aide A, then
lift out the. cover and the grey glafs from their refpeaive grooves
under the Hide A.
Put the end N of the guide for the eye L M N into it s place, fo
that it may Hand in the pofition which is reprefented in this figure.
K , Place