6o
OPTICAL PROJECTION
of fig. 30, except that the outer orifice was contracted to a
small aperture. It gave a fair light for those days, and is
used now sometimes, but the mixture almost at the very
orifice generally causes a whistling when much light is at¬
tempted. The jet next introduced, and still very often seen,
had a plain lime-spindle like fig. 31, but the two gas-tubes
were conducted side by side into a chamber, packed with
layers of gauze. This gauze is always getting rusty and
obstructed, to the detriment of the light, and actual increase
of the danger of mixing behind ; and it is unpleasant for both
operator and audience to be obliged to be continually opening
the lantern door to turn the lime round with the fingers.
For under the greater heat and stronger blast of the mixedjet,
even ‘ hard ’ limes are rapidly burnt into holes, or ‘ pitted ’ as
it is called ; and a fresh surface must be exposed to it every
few minutes, or the blast may be reflected back from the
concavity on to the condenser, and crack the lens. Hence a
cogged lime-turning movement was speedily introduced,
resembling fig. 38, by which the lime can be rotated from
the back of the lantern. This is the common trade jet
of the present day, and will answer fairly well for small
apertures, say up to 1 mm. diameter, if the gauze is removed,
or only one piece left in the chamber. The mixing-chamber
usually found is however too small for a really powerful light ;
and as the circular plate which supports the lime is simply