44 OPTICAL PROJECTION 



heat being injurious. I believe they wear out chiefly by the 

 opening and shutting in turn at the creases, where leaks 

 nearly always first occur. Hence they keep better, if con- 

 venient, laid flat between their boards, than if folded up, 

 which makes extra creases to wear out. If they have to be 

 folded to go on a shelf, as is usually the case, they should be 

 folded once only as loosely as possible, the sides being tucked 

 properly in. The taps should be left open. Whenever bags 

 are stiif with cold, they should be softened by gentle warmth 

 before use ; the creases will rapidly wear through in a har- 

 dened bag. 



The very large and heavy taps used by some makers are 

 to be avoided, as are nozzles with a projecting rim on the end. 

 A moderate nozzle, with fluting all round, either the same 

 diameter or gently increasing back from the tip, is best. Most 

 taps are, however, smaller than they should be in the bore. 

 Few are more than j- inch in aperture, and they manifestly 

 ought to be the same as the rubber tubing. This is a point 

 really needing the attention of manufacturers. 



Lock taps are made, to prevent accidental or wanton 

 turning of the tap in transit. It would be much simpler to 

 have a plain lever on the plug, instead of a thumb-piece, the 

 lever being set parallel to the nozzle when the tap is closed. 

 Then it could be simply bound round with the nozzle by some 

 string, and would be safe. If there is no way of securing the 

 tap, it is best to take a piece of rubber tubing which strains 

 tightly on the nozzle, double over the end and tie tightly, so 

 as to make an outside stopper, which is slipped on. Such 

 precautions are only needed, of course, when full bags have 

 to be taken about. 



With a bag full of oxygen, the next step is to force this 

 out with some regularity of pressure, to do which the bag is 

 placed between a pair of pressure boards, the top one of 

 which is loaded with weights. In all forms of the lime-light 

 with only one bag filled with oxygen, pretty much the same 



