OPTICAL LANTERN.—An apparatus employed for projecting magnified images upon a white screen. The rays emanating from an artificial source of light are collected together and passed through a transparent positive picture, which by the divergence of the rays through a lens is thrown brilliantly illuminated and magnified on a screen. The practical requirements in an optical lantern are first a light-tight box or case containing the ilhtmin ant, a lens or condenser to concentrate the light and throw the rays evenly and uniformly through the transparent slide, and a lens for mag nifying the image. If an oil lamp be used a silver reflector will also be necessary. A large variety of lanterns are now manufactured.
The cheapest form is the ordinary colza or sperm oil lantern. Paraffin oil is now generally used, burnt with a combination of from two to five wicks. This is usually accomplished by placing the long wicks parallel with or sloping towards each other, so that by a proper regulation of the ventilator the flames are made to converge and unite in one bril liant light.* The accompanying illus tration depicts a very convenient form of oil lantern.t It will be noticed that by an ingenious arrangement the oil reservoir is kept as far from the flame and heat as possible, thus doing away with the obnoxious smells so prevalent with this form of lantern.
The limelight is the illum inant most generally in use.
There are four different systems, the oxycalcium or spirit jet ; (2) the blow through ; (3) the mixed ; (4) the ether-oxygen light. All these depend for their illuminating qualities on the incandescence of a cake of lime by the impact upon its surface of a jet of oxygen passed through or mixed with a hydrogen or other flame to maintain combustion, because oxygen alone will not maintain a flame.
The limelight was first discovered about the year 1826, by Lieut. Drummond, R.E., but the modern methods of construction differ considerably. / The best limes to be obtained are of the kind known as " hard " or Nottingham limes.
For all these four systenis of limelight, the manufacture of oxygen is necessary. This will
be found treated upon under Oxygen (q.v.) It can now be purchased ready made in almost every town in strong steel cylinders. The oxycalcium or spirit jet requires spirits of wine as the flame maintainer, a reservoir containing spirits being attached to the jet in such a manner as to keep up a constant supply of spirit in a small lamp placed in front of the lime support. The spirit lamp is first lit, and then the oxygen turned on and is thrown by the jet through the spirit flame on to the lime, which it lights up with extreme brillance.
The blow-through or safety is similar in its action to the oxycalcium, except that instead of the spirit lamp the flame of a jet of hydrogen is used. This is the most commonly used form of lime jet. If properly constructed it should illuminate a picture at least is feet in diameter. The hydrogen gas can be supplied from the nearest household source by means of an indiarubber tube.
It is carried to a point just in front of the lime, and the oxygen tube is carried out in front of it and brought round to an angle so as to cross the lines of flame and blow the oxygen through the hydrogen flame as it emerges from the nozzle. This sys tem is a perfectly safe one. The others are all attended with more or less danger. It must be remem bered that hydrogen and oxygen when mixed together form an ex plosive vapor of enormous power.
In the safety jet, there is absolutely no danger of explosion, for the reason that no mixture of the gases takes place until they meet the lime cylinder.
The third system is the oxy-hydrogen or mixed jet. This gives a light of very powerful intensity. In it the gases are mixed together in a small channel below the nozzle, and they come out together on to the lime. In the hands of an inexperienced person, this method would be a very dangerous one, for if the pressure on the gas bag were to be removed for even a single instant, the mixed gas or flame would be sucked back into the bag, and a tremendous explosion would be the result. To prevent this disaster, safety valves have been used.