CH. V] TROUBLES IN CHAPTER V 133 



source is large, one needs a good projection objective of large aper- 

 ture (see 855). 



218. Putting out the lamp. As this lamp is difficult to light 

 it should be kept burning during the entire exhibition. One can 

 shut the light from the screen by the objective shield (fig. 62). 



At the close of the exhibition, take the lamp from the lamp-house, 

 remove the rubber tube from the pressure apparatus to the tank 

 to relieve all pressure on the alcohol. Close the supply valve so 

 that no more alcohol can pass over to the stand-pipe. Close the 

 air-intake of the burner. Use a sponge well wet with water and 

 apply it to the burner as near the mantle as possible without touch- 

 ing the mantle. The sponge will naturally rest against the small 

 conducting pipe and the stand-pipe in this operation. This cools 

 the burner and the stand-pipe and stops the vaporization of the 

 alcohol. The flame then goes out as with any gas burner when the 

 supply of gas is cut off. 



219. Precautions. Remember that alcohol is very inflam- 

 mable, therefore special care should be exercised that none of it 

 overflows from the reservoir or leaks from poor joints. It is per- 

 fectly safe in burning through the burner, but any alcohol outside 

 the lamp is dangerous, for if it catches fire it cannot be extinguished 

 unless one has plenty of sand or non-inflammable dust to throw on 

 it and smother the flame, or one of the modern chemical fire extin- 

 guishers. 



TROUBLES IN CHAPTER V 



220. The prime difficulty with these relatively weak lights 

 is the dim screen pictures. That is, they will be dim in comparison 

 with the bright pictures obtainable with the direct current arc 

 light. 



Remember the conditions requisite for screen images with weak 

 lights ( 193). 



221. Smoking of the petroleum lamp or of the acetylene 

 burner. This shows that the wicks are not properly trimmed or 

 that they are turned up too high for the height of the chimney. 



