786. Light Sources. The first projections of photographic slides were lighted by Argand lamps for melted tallow, adapted for this use by F. Newton (1851). Paraffin (kerosene) lamps with fiat wicks (Marcey, 1874) then came into general use. These in turn were replaced by incandescent mantles (Auer von Welsbach, 1885), burning with coal gas or vapours of various liquid fuels (alcohol, petrol, paraffin). Meanwhile limelight had been used for high power projectors (R. Hare, 18o1 ; T. Drummond, 1826), and also the arc-lamp invented by Davy in 18o9 but not used on any scale until about 1877. Various improvements to the limelight jet enabled the coal gas to be replaced by certain vapours (ether, petrol) ; acetylene was for a time also used as a projection light, with special burners. At the present time, wherever electric current is available, only arc lamps or incan descent lamps are used, according to the power required. Numerous patterns of both kinds of lamps have been designed specially for pro As the optical conditions are the same as in enlarging with directed light (§ 757), the intrinsic brightness of the light has a much greater effect on the illumination of the projected image than its luminous intensity.
787. Paraffin Oil Lamps with Multiple Wicks. Though these lamps are now scarcely ever used, it may be well to describe briefly the method of use. The wicks are about 2 in. wide, and usually there are four of them placed parallel to the optical axis. Over them there is a combustion chamber, closed in front and behind by thin plates of tempered glass, and forming part of a high, telescopic chimney. Essential conditions for satisfactory working are absolute cleanliness of the lamp and the correct trimming of the wicks, the corners of which must be trimmed off. The lamp having been filled with oil, turn up the wicks slightly and light them. As the heat gradually develops, raise the wicks until the flames merge into one very bright and non-sooty flame. This starting of the lamp requires about 10 minutes. The flame must be watched from time to time by the sight-hole in the combustion chamber, as it produces dense smoke if it draws too high. The lamp must be emptied and cleaned after each time of use ; the carbonized portions of the wicks must be trimmed off exactly level with the burners ; and the chimney must be cleaned. The light can be improved by dissolving a little naphthalene in the paraffin, but the lamp then heats much more.
788. Lamps with Incandescent Mantles. Ordi nary incandescent gas burners do not usually give a sufficiently bright light for satisfactory projection. There are, however, various types
functioning with the vapours of alcohol, petrol, or paraffin. They produce these vapours under pressure, thus producing a much brighter light. In using these lamps, some of which are very noisy in operation, the maker's instructions must be very carefully followed, as a mistake in manipulation may easily lead to an accident.
After having been once used, mantles, as a rule, cannot be carried from one place to another, unless, after cooling, they are dipped in I per cent collodion and then allowed to dry. Such a collodion-treated mantle must be burned off before use, just like a new mantle.
789. Acetylene Burners. Acetylene lamps formed of small horizontal rows of three burners, placed at about to the optical axis, enable slides to be shown satisfactorily before audiences of about fifty people.
As portable generators are often dangerous in manipulation, cylinders of dissolved acety are to be preferred to them.
790. Oxy-acetylene A pastille of rare earths 3 held in the flame of an oxy-acetylene jet becomes incandescent. Its intrinsic bright ness is considerable as well as its luminous intensity. This is the most powerful light avail able if there is not electric supply of wattage sufficient for an arc, but it calls for considerable care in use, and for this reason a detailed des cription of its manipulation is given.
The axial tube of the jet is connected to the regulator valve of the oxygen cylinder (§ 791), and the side tube is connected to the regulator valve of the acetylene cylinder. The pastille is fixed in the clips of the holder so that one of its flat surfaces projects about in. beyond the spring belt. The pastille is then pushed aside, the acetylene tap is opened, the burner is lighted, and the flame is regulated to a length of in. The oxygen tap is now slowly opened until the little blue cone of the oxy-acetylene flame measures about in. The pastille is then brought back into the flame, and the blowpipe and pastille holder are adjusted so that the tip of the blowpipe is about in. from the pastille, opposite the lower third of its vertical diameter. In this position the pastille is at its maximum brightness.
If the blowpipe sizzles or crackles it is a sign that the supply of acetylene is insufficient. Too much acetylene may produce a deposit of soot on the pastille the brilliancy of which is then much reduced. A considerable excess of oxygen can produce a deposit of black copper oxide. In either case the pastille must be withdrawn and allowed to cool ; it must then be scraped to renew the surface.