CH. V] MAGIC LANTERN WITH MANTLE GAS LAMP 125 



papers. If the lamp is kept perfectly clean, and no oil is allowed to 

 remain on the outside, the disagreeable smell of partly oxidized oil 

 will be avoided. 



199. Amount of oil used. It takes about half a liter (one 

 pint) of kerosene per hour for the best lamps. 



200. Candle-power and size of screen. The candle-power of 

 the best petroleum lamps is not much above 100. While the older 

 lanternists used large screens (4 meters, 12 ft. square) it is better 

 to use, with this light, screens of small size, 2 to 3 meters square 

 (6-9 ft.), and to keep in mind the requirements for good images 

 with these feeble lights ( 193). 



201. Relative position of lamp and condenser. In general, 

 the middle of the flame should be in the axis of the condenser and 

 it should be at about the principal focal distance of the first ele- 

 ment of the condenser from it (fig. 64). One must get the best 

 possible position at any one time by experiment, i. e., by moving 

 the light a little closer or farther away than the focus of the con- 

 denser. For the two-lens condenser the lamp must be closer than 

 for the three-lens condenser ( 17, 55). 



202. The management of an exhibition is as described in 

 Chapter I, 21-41, and above, 193-194. 



MAGIC LANTERN WITH A MANTLE GAS LAMP 



203. Gas and gas lamps. The illuminating gas may be 

 drawn from the house lighting supply. 



The lamps are of two kinds, the vertical and the inverted or 

 reflex form (fig. 68-69). The burner is of the Bunsen type. It 

 heats the mantle to incandescence. While there is a very brilliant 

 light and a great deal of it, the source is very large, and cannot be 

 utilized so completely as the small source of the electric arc lamp 

 (see fig. i, 64). 



204. Position of the incandescent mantle. As this is the 

 source of illumination, the middle of the face next the condenser 

 should be on the horizontal axis (fig. 64). 



