60 OPTICAL PROJECTION 



of fig. 80, 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. 81, 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. 



. 38. Common Mixed Tap 



For under the greater heat and stronger blast of the mixed jet, 

 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. 88, 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 



