LANTERNS AND THEIR MANIPULATION 117 



must be allowed for the heat of the jets to drive it off. If the 

 time is at hand, this will be the next proceeding ; but if there 

 is some time to spare, it is best to turn off the gases at the 

 bags or cylinders (the oxygen first), only taking care to 

 re-light in time to get the condensers dry. Or a still better 

 plan is to use a 'cut-off jet' as described in the following 

 section. The final proceeding is to see that the slides are in 

 order and placed handy, and the reading lamp placed ready if 

 one is used ; when all may be left till required. 



It hardly need be remarked, that it is not enough to insert 

 the slides upside down. For diagrams, or diorarnic effects, 

 just as much care must be taken to have them the right way 

 as regards ' right and left,' and when exhibiting through a 

 transparent screen, this will be opposite to what it is when 

 an opaque screen is used. It is therefore most important to 

 have all the slides conspicuously marked, the same way, so 

 that they may be seen in the dim light of the exhibition hall. 



On a damp night, if there is no time to dry the slides, 

 there may be much trouble from dew upon them. To avoid 

 this, each slide should, if possible, be warmed iu turn on the 

 top of the lantern, for which a flat top is so useful. 



63. Cut-off Jets. It is very common for a lantern exhibition 

 to have an interval in the middle, and it is a matter of some 

 consequence to be able to start again with all the lighting 

 adjustments ready just as they were left. One lantern being 

 turned off by the dissolving-tap still leaves the other ' on,' 

 and we do not want to turn off the gas at the source, or to 

 alter the taps at the jet, which would cause a slight ' hitch ' 

 when the lantern was first used again. All such incon- 

 venience is avoided, and other obvious useful purposes served, 

 by having one of the jets fitted with its own 'cut-off,' as 

 invented by Mr. Andrew Pringle, which allows a single lantern 

 also to be cut off, but left ready for immediate use in the same 

 way. Fig. 65 shows this cut-off as added by Messrs. Newton 

 & Co. to one of the jets already recommended on p. 64. An 



