Aerograph Air Bells
striking exhibitions in London, 1802, of a scene representing a witch's cave, a fine curtain of gauze let down in front, receiving images of lightning flash, ghosts, and moving figures. M. Fourtier's plan of a whitened lath rapidly moved up and down, or Mr. Bruce's plan of revolving the lath by a machine may in some cases give a better aerial screen than the device of Philipstal, but rapidly moving objects in darkened places may be a source of danger. Those interested should refer to Brewster's "Natural Magic,"# London, 1832, p. 80; or to Chambers' Journal, April 28th, 1849.
Aerograph. See Air Brush.
Agar-Agar (Ger. and Fr., Agar-Agar; Ital., Alga di Giava). This is vegetable gelatinous material obtained from species of white seaweeds (Fucus spinosus, or Gelidium cornetim), common on the coasts of Singapore and Straits Settlements. It has been suggested as a substitute for gelatine, but is liable to give emulsions which are full of transparent nodules. Rebikow has suggested a method of eliminating these nodules, but there appear to be no advantages which warrant its recommendation as a substitute for gelatine.
Agate Burnisher. Sometimes used for burnishing prints. In shape the agate burnisher used for this purpose is somewhat similar either to a short-handed spade or household meat-chopper. An agate burnisher is sometimes useful in smoothing over parts which have been spotted out or worked on with colour.
Agent. That which has the power of acting, or producing effects, upon anything else - e.g., light is said to be the agent which impresses the image upon a sensitive plate, and the developer the agent which makes such image apparent. In the strict or meta-physical sense no one of those antecedents which are essential to the result can be selected and called the agent.
Air Bells, or Air Bubbles. These annoying defects are liable to make their appearances in out of the way places where they are not wanted. They may occur in the glass support, and if of not large size, or in the face or other prominent part of the picture, may be ignored or touched out on the prints. Air bubbles in the emulsion itself sometimes occur, and give rise to small spots of bare glass, which may be touched out with a