The Optical Magic Lantern Journal (April 1891)

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102 The Optical Magio Lantern Journal and Photograpio Enlarger. Electric Incandescent Lamps for Optical Lanterns. By H. PaDGHAM (GREENWICH.) Now that many of our halls and private houses have the electric light fitted, and that still more will have it in future, it has become possible and convenient to use it for optical projection. It does away with all gas bags, cylinders, and explosions. In some few cases an arc light could be used, but asarule the hall or room would be fitted and the leads.arranged for incandescent lamps, and the lead would not be large enough to carry the current necessary for an arc lamp. I enclose a sketch of a lamp manufactured by the Edison Swan Company for optical work, and sold by them at ten shillings each. The sketch is half-size, and it will be seen that the filament is coiled round and round so as to get the light concentrated in the focus of the condenser. v 5 re ay Q 2 9 Ny] Asiwe have comparatively little heat radiating in this form of light, we can bring it as close to the condenser as the bulb of the lamp will allow, one great advantage being that we do not require a chimney. In the sketch I have suggested an arrangement of .a reflector and a sheet metal cover enclosing the lamp and constituting the body of the lantern; a diaphragm is also shown to cut off any stray rays from the reflector. The above mentioned lamps are constructed for an .| SO as to counteract any mistake in the exposure. ‘| impossible to tell any one just how to do it, as it is | E. M. F. of 105 volts 3°3 ampéres, and give a light of too candles. The lamps in general use for house lighting are run on a circuit of 100 volts E.M.F., so that it is merely necessary to take the lamp nearest our lantern out of the holder and insert a plug constructed exactly the same as the bottom of the lamp taken out, and having two wires of sufficient size to carry 3°3 ampéres (without heating) attached to it, and long enough to join on to the lamp in the lantern. All other lamps in the room must of course be switched out if provided with separate switches, but when, as is generally the case, no switches are provided, they should all be taken out of their holders, or placed in thick brown-paper bags while the exhibition is on. 20: Flame Extinguishers and Method of Testing. TESTED flame extinguishers consist of tubes filled with finely granulated pumice stone, which allows the gas to pass through, but through which no flame can pass, even when it is an explosive mixture of gas. These flame extinquishers are in their manufacture tested thus—a small bag is filled with a mixture of oxygen and hydrogen, the gases are lighted, and the pressure is taken off the bag; the mixed gases explode if it does not pass to the stock in the bag, where exerything is made easy for it to do so; they may be considered to confer great additional safety. The usé of these extinguishers will no doubt prevent many accidents ; they must be used in pairs. Of course, they obstruct the free passage of the gas, and require about one-fourth more weight on the gas bag than without them ; they also require now and then to be opened and inspected to see that they are full. 20; Hints on Enlarging. By A. R. DRESSER. CHAPTER VI. | THE last point we come to is the developing, and this is the most serious of all, for unless one knows how to develop properly good results in their enlargements cannot be obtained. If you can always strike the right exposure when making enlargements in bromide, so much the better, as good results are then certain ; but as you may have to enlarge from a variety of negatives, and not all of the same density, it is hard to do so, and so you must learn how to develop It is a sort of knack, and can only be learnt by trials and experience, but I will give hints as to exposure and developers that may be of use. When exposing, always give enough exposure—that is rather over than under expose, as over exposure can be corrected in the developing if the over exposure has not been too great, but under exposure can never be made good. The only trauble to be found will be when enlarging from a very thin negative ; then you must strike about the right exposure, as if you over