7* OPTICAL PROJECTION 



to play gently rather more than up the lime, which is to be 

 slowly turned round a minute or so to warm through. Then 

 turn on rather more hydrogen say about half and a very 

 little oxygen ; just enough to slightly diminish the flame, and 

 increase the heat, so that the lime is heated red-hot easily 

 The whole lime is again carefully subjected to this heat 

 which is a great security against cracking. More oxygen can 

 now be turned on ; then as much hydrogen as it is intended 

 to use (full, if the extreme light be desired), and finally the 

 oxygen is turned on until the best light is obtained. It will 

 be found that the merest hair's-breadth of adjustment will 

 make a difference, and the taps should, therefore, always be 

 kept nicely fitted, and lubricated with a very little tallow 

 (unsalted) or sperm oil. A powerful jet generally roars ' till 

 the oxygen is adjusted, but quiets when the two are in pro- 

 portion. 



The best light is not obtained till the jet has played upon 

 the same spot for half a minute or so ; after that it remains 

 very steady for a couple of minutes. Then, as the cavity 

 deepens, it deteriorates ; and if kept too long on a deep pit, the 

 flame may be reflected back again, and crack the condenser. 

 Some limes will stand many minutes ; and when a box is 

 found very good, it is well to keep it for particular occasions. 



With the bags in proportion that is, hydrogen about five 

 volumes to four of oxygen and equally full, the taps will 

 scarcely need any subsequent alteration ; but if the light does 

 go down a little, the necessary adjustment must be made 

 from time to time. At the close, or if anything goes wrong, 

 let the first thing be to turn off the oxygen ; and cultivate from 

 the first a fixed, invariable habit of doing this. Remember, 

 also, when the bags are half empty, to let down the boards 

 as already described. 



At the close of all, see that the jet is out and the bag- 

 taps turned off before removing the rubber supply-pipes from 

 the lantern nipples ; else possibly the light might remain long 



