LIMELIGHT An illuminant for use in optical lanterns, etc., in which a spot on the surface of a cylinder of lime is heated to incandescence by a gas flame fed with oxygen under pressure. Oxyhydrogen limelight has a photometric value of 16.6, as compared with that of a multiple-wick lamp 1, and of a 50-ampere arc lamp 16o. When an ordinary flame, as of hydrogen, coal-gas, alcohol, etc., plays upon a piece of lime, the latter becomes dull red-hot, but when the combustion is forced and made more complete by meari% of a supply of oxygen under pressure, the heat of the flame is increased to such an extent that a small part is raised to such a high temperature that it emits a blinding white light. In the past, the combustible gas for limelight has been supplied in a number of different ways. The best known of these, hydrogen, was made by acting with dilute sulphuric acid on scrap zinc ; the gas was stored in a collapsible bag which, when required, was connected to the limelight jet. By other systems, special forms of jets were necessary ; for example, in one was a simple spirit lamp through the flame of which a jet of oxygen was forced ; in another was a vessel containing ether, through which was passed a current of oxygen, which thereby became laden or saturated with particles of the ether and was then burnt at a nipple. All such arrangements are, or should be, obsolete, as by the modern system of supplying gases in steel cylinders (see Gas") a convenient and safe supply of a combustible gas is easily secured. These cylinders are periodically tested and annealed, and their use is attended by a minimum of risk, which is not the case in the oxy-alcohol and oxy-ether systems, especially the latter.
Limelight jets are of two main kinds—the blow-through and the mixed. In the former, A, the gases do not mingle until they reach the point of combustion ; in the latter, B, the gases mingle in a mixing chamber at the base of the actual jet. The more intimately the gases mingle, the greater is the heat of the flame produced, and therefore the high-power jet is always of the mixed type, the blow-through jet being practically restricted to the use of the beginner. The jet is supported on a vert ical pin projecting from a sheet-metal tray which slides into the lantern body from the back. One nozzle n is connected by rubber tubing to the hydrogen supply, which in the case of a mixed jet is a cylinder, and in the case of a blow-through jet either a cylinder or an ordinary gas bracket. The other nozzle o is connected to the oxygen supply, which, nowadays, is always a cylinder. In the case of the blow-through jet, the nozzles are con nected to the gas supplies in such a way that the oxygen issues in a stream from the central nipple, whilst the cone of burning hydrogen surrounds it. The taps are generally stamped
with either " 0 " or " H," indicating the proper connections. The lime cylinder (see dotted lines C) is supported on a pin at an adjustable distance from the nipple, there being provision for rotating the lime by means of the rod D, so as to obviate the formation of deep pits in the lime while the jet is in use. When a flame plays upon a deeply-pitted lime, there is a risk of the lime cracking or of the flame being deflected upon the condenser. The distance of the lime from the nipple is a matter for experiment and varies with the class of jet ; it has a great influence upon the quality of the light. Simple jets are here illustrated, but very elaborate appliances are obtainable, these being fitted with screw-down adjustment valves and various mechanical devices for regulating the light as regards its height, distance from con denser, distance of lime from nipple, etc.
The following matters should receive attention when managing a limelight exhibition. An automatic regulator or screw-down adjustment valve regulator must be screwed into the valve of each cylinder (see " Cylinders, Gas "), it being necessary to reduce or check the pressure of the gas as it issues from the cylinder. High quality rubber tubing connections should be used, and if an automatic pressure regulator is not used, these tubes should not be tied on, in which case, also, the taps on the jets should be opened wide, and the whole of the adjust ment done from the cylinder valves. A lime (see " Cylinders, Lime ") is put in place, a small flame allowed to play upon it, and the lime turned occasionally so that it gets warm right through, the object being to prevent cracking when the oxyhydrogen flame is turned on. Then the hydrogen flame is increased slightly, and the oxygen gently turned on, adjusting the two taps and the distance of lime from nipple until the best effect is observed upon the screen. Push in the jet or withdraw it from the condenser until there is an even field of light ; if the jet is not central, there will be coloration or a shadow somewhere on the screen and the jet should be moved in the opposite direction to the defect until this is removed. Three minutes' experimenting will teach all that it is necessary to know in this matter. The adjustment of the light should preferably take place after a slide has been roughly focused and withdrawn. Always, when turning off the light, cut off the oxygen first, as otherwise there will be a slight pop in the hydrogen tube of a mixed jet.
It is advisable to retain a key on the valve stem of each cylinder so that in the case of a burst rubber tube or other similar accident the gas can be cut off immediately.