40 OPTICAL PROJECTION 



21. Gas Burners. A great deal of class demonstration may 

 be done, and views well exhibited on a disc of seven feet 

 diameter, with an Argand gas-burner ; and it is a very great 

 convenience thus to be able to get to work, or to decide the 

 details of a projection, with only a flexible tube from some 

 gas-supply (for supply see 27). A Silber, or Sugg's London 

 Argand, will give a light of about twenty candles with 

 London, and twenty- eight with cannel gas. This may be in- 

 creased to about thirty and thirty- eight candles by the use of 

 an adjusted reflector, as described under Oil Lamps, 19. A 

 Welsbach incandescent burner also gives pretty good results, 

 but is awkward on account of the great depth of the burner 

 beneath the radiant portion. It is rather superior to an 

 Argand in equality of intensity, an Argand flame being most 

 luminous at the edges, where the flame is seen edgeways. For 

 this latter reason an Argand burner is no use in microscopic 

 experiments, even on a small scale, there being two extra- 

 luminous edges to the flame, an inch apart. 



22. The Lime Light. The radiant in most common use 

 for public exhibitions will be the subject of separate chapters ; 

 but a few words may be added about 



23. The Electric Light. Of this three forms are available. 

 The arc, the ordinary incandescent filament, and the Nernst 

 lamp. 



The Arc, Lamp. This is the most powerful of all illumi- 

 nants for lantern work, and the radiant being extremely 

 small and concentrated, the greatest perfection of optical 

 projection is obtainable by means of it. 



Up to a few years ago the use of such lamps was chiefly 

 confined to large instruments such as described in Chapter XII., 

 and the lamps themselves were large and expensive ; but the 

 tremendous developments in electric lighting have caused a 

 corresponding increase in the application of the electric arc 

 to lantern work, and the result has been the introduction of 

 small arc lamps suitable for any lantern of ordinary size, and 



