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78 THE BOOK OF THE LANTERN'. cylinder first turn the 0 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 0 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 See that the tray upon which the jet is fixed is withdrawn so as to leave a space of about 4 inches between the light and the condensing lens.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