78 THE BOOK OF THE LANTERN'. 



cylinder first turn the tap of the jet full on, and let it 

 remain so. This is a most important point, and for the 

 following reason : the supply of oxygen must be regulated 

 only from the tap on the bottle, for the pressure of gas is 

 so great that if we turn on the bottle tap and then attempt 

 to check its flow by moving the tap jet, the connecting 

 rubber tube will be blown off or possibly split up. There 

 is another advantage in keeping the hydrogen jet burning 

 for some time before the lantern is used. It warms the 

 glasses, and prevents that deposit of moisture upon them 

 which is otherwise always more or less apparent, especially 

 in a crowded room. Turn the H tap until there is a good 

 big flame from the jet, and now very, very gradually turn 

 the lever tap of the bottle. If you do this too quickly, the 

 sudden mixture of and H at the jet causes the light 

 to go out with an unpleasant crack. There is really no 

 danger, but the noise frightens nervous people, who are 

 prone to associate with lanterns generally the idea of being 

 blown skyward. The regulation of the two gases is a 

 matter which is soon learned by experience, and is governed 

 by the appearance of the disc of light obtained on the sheet. 

 Move the two taps until the best effect is obtained ; the 

 knack of doing so is very soon learned. 



Having seen that the gas jet is burning well and quietly, 

 which is one sign that all is at it should be, the next thing 

 is to see that the light is perfectly central with the optical 

 system. See that the tray upon which the jet is fixed is 

 withdrawn so as to leave a space of about 4 inches 

 between the light and the condensing lens. Raise or lower 

 the jet on its supporting rod, and move it from left to 

 right until the flare of light seen upon the sheet is as 



