30 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 



images are desired. When, however, the screen distance is 10 

 meters (30 ft.) or over, it is not usually necessary to focus for each 

 slide. 



If the screen distance is very great (20 meters; 65 ft. or more) 

 the operator cannot tell by his eye alone when the screen image is 

 perfectly sharp. In such a case he must have an assistant stand 

 near the screen to tell him when the image is sharp, or he can use 

 good opera-glasses and determine for himself. 



When the focus is once found for these long distances it is well 

 to mark in some way the exact position of the objective ; then in 

 future the operator can be sure of good screen images in the same 

 position provided the lantern has not been moved. 



39. Hints on running the lantern for a demonstration lecture. 



It frequently happens that in a demonstration lecture, slides are 

 to be shown at several different times. Ordinarily the arc lamp is 

 turned out during the intervals ; but to make sure that the desired 

 slide can be shown without delay, the arc lamp can be left burning 

 all the time, and to avoid lighting the screen a mask can be put in 

 front of the objective (fig. 14). A "push-through" carrier (fig. 6) 

 should be used, and the next slide to be shown put in one of the 

 compartments. The other compartment is left vacant, and this 

 empty compartment is put in front of the condenser. If the slide 

 were left in position all the time it might become over heated and 

 break. 



Whenever the slide is called for it is pushed into position and 

 the mask turned aside. This will bring the picture on the screen 

 almost instantly. 



A mask or shield for the objective is much more important for 

 the slow starting lights like the Nernst, than for the arc ( 146, 

 169, 202, 217). 



40. Collecting and arranging the lantern slides at the close 

 of an exhibition. After the exhibition is over be sure to remove 

 the last lantern slide from the slide-carrier. It not infrequently 

 happens that the last slide is left in the carrier, and the lecturer's 

 set is thus rendered incomplete. 



