1 62 THE ART OF PROJECTING. 



near the light and between it and the screen, and focus 

 it in the way indicated on page 25. The incandescent 

 carbons will show beautifully and between them the 

 moving bluish arc. For this experiment the white 

 wall of a room, in other directions than where the 

 screen may be, will be found to be a good surface to 

 receive the image upon. The source of light is so 

 bright that the most distant place in the room will show 

 it plainly enough, and the more distant the image is 

 the larger it will be. 



The incandescent filament may be projected in a 

 similar manner, and will show as an inverted, glowing 

 loop. 



The common incandescent electric light does not give 

 light enough to enable one to use it in a lantern. Most 

 of them give a light of but fifteen or twenty candles. 

 Those that give more have a filament so long that 

 its use in a lantern is quite impracticable, not alone 

 on account of size of the bulb, but because the source 

 of the light is from so large an area that definition is 

 impossible. Lamps may be made, though, having the 

 luminous filament reduced to a small area, like a coil, 

 thus : 



When this can be done so that the luminous area 

 does not much exceed an inch in diameter, a very 

 good source of light is provided. But if common fila- 

 ments are made into this shape they must be supplied 

 with a much larger current than they are usually sup- 

 plied with, and they will not, therefore, last so long. 

 A filament about six inches long is intended to give 

 about sixteen candles' light or nearly three candles 



