CH. IV] 



MANAGEMENT OF THE LIME LIGHT 



in 



169. Shield for cutting off the light from the screen. As it 



takes considerable time to start the lamp after it has been put out 

 it is not so easy to use the lime light intermittently as the arc lamp, 

 hence in a lecture or demonstration in which the lantern slides are 

 to be shown at several different times, it is best to leave the lamp 

 burning all the time. But the screen should not be lighted all the 

 time, and to avoid this the objective shield (fig. 62) may be used. 



FIG. 62. SHIELD FOR THE OBJECTIVE IN INTERMITTENT PROJECTION, 

 WHEN SLOW-LIGHTING RADIANTS ARE USED. 



S l Shield up to allow the light to pass from the objective to the screen. 



5 1 Shield down to cut the light off from the screen. This shield is especially 

 desirable when slides are to be shown at intervals, as in a demonstration lecture 

 with the lime light, a Nernst light, a kerosene light, or an alcohol light ( 169). 



Sometimes also to avoid using so much gas and burning out the 

 lime too quickly there are regulating valves, by which only a small 

 amount of the two gases is allowed to pass, without changing the 

 relative proportions. When these valves are opened again the full 

 amount needed and in the original proportions is allowed to flow 

 again. Even in this case there should be a shield before the objec- 

 tive to avoid lighting the screen. 



170. Proper lighting for the screen. The light on the screen 

 should be uniformly brilliant. This can be attained by following 

 the directions for centering and getting the proper distance of the 

 lamp from the condenser exactly as with the direct current arc 

 lamp ( 51-57). 



If there arc shadows on the screen make the proper change in the 

 position of the lamp, etc., as indicated in fig. 27-30, 83-93. 



