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MCINTOSH BATTERY AND OPTICAL CO., CHICAGO, ILL., U. S. A. 8 waiting not less than five minutes before the full effect is obtained. The light must not be turned so high as to smoke. When a full and perfectly white flame is produced, lower the hinged glass, replace the reflector and close the lantern door. HOW TO MANAGE AN EXHIBITION. The beginner should be provided with a list of all the apparatus and materials needed for an exhibition, and before starting out should see that nothing is omitted, otherwise he is liable to find something missing at the last moment which will interfere with his arrangements. To Locate the Lantern. —When the distance at which the lantern will cover a certain sized screen is known, then the whole ap- paratus may be placed without any hesitation; but if this is not known, or a different size objective is used, as is often necessary to accommodate the size of the hall, then the apparatus should be set up temporarily, and a trial made to see if the distance be correct. With many of the objectives used the size of the picture thrown will be just half the distance between the lantern and the screen, i. e., if the lantern be twenty feet away, the size of the picture will be ten feet. These objectives answer well for an illumination of from twelve to fifteen feet, but when it is required larger than this, as is often the case with the oxy-hydrogen light, then a larger objective should be used. Location of Gas-bag. —It should be located as near as possible to the lantern, taking care that it is placed so that the hose convey- ing the gas from the bag cannot be trodden upon. When the oxy- hydrogen light is used two bags will be needed. They may be placed between pressure-boards, one above the other, or they may be placed side by side, with a single board on each, while a narrower board reaches across both and receives the weights. Whichever arrange- ment is adopted, the weights should be so placed as to give an equal pressure on each bag. It is not necessary to carry heavy weights about for this purpose, as something may always be found—a keg or two of nails, a lot of window weights, kegs of white lead, or some rocks from a neighboring stone wall. One hundred and fifty pounds pressure will usually be sufficient when the bags are one above the other, but if placed side by riide, then the weight required will be about double. Attach the hose to the bags, and be careful that the proper con- nections are made with the jet. Never use the same hose for the lantern connections that has been used to connect retort with wash bottle in making gas. Always place each gas on the same side, i. e., the hydrogen on the left and the oxygen on the right. Blow through all the pipes to make certain that they are not obstructed. The Screen should be located as near the central part of the end of the hall as possible. It must be drawn perfectly smooth and stand parallel with the end of th objective. When placed higher than the lantern, it may be tilted forward a little at the top, while the lantern is raised by placing a block under the front legs to make the front