Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
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


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