CENTERING THE MAGIC LANTERN 



[CH. I 



FIG. 19. CONCENTRIC CIRCLES ON THE 



FACE OF THE CONDENSER, SHOWING 



THE SIZE OF THE CIRCLE OF LIGHT 



WITH VARIOUS POSITIONS OF 



THE RADIANT. 



When the radiant is at the proper distance, the entire face of the condenser 

 is illuminated (/). 



As the radiant and condenser are separated the part illuminated becomes 

 smaller and smaller (2-4). (See also fig. 20). 



cylinder of approximately parallel rays between the two elements 

 of the condenser, and will fully light the face of the second element. 

 One can determine this easily by putting a sheet of white paper 

 over the face of the condenser which is toward the objective. If 

 the radiant is in the right place the entire face will be light. If the 

 radiant is too far off, only a part of the face will be illuminated 

 (fig. 19). If the radiant is too close the face will be lighted, 

 but the light will be diverging between the condenser lenses. 

 In this case a part of the light falls outside the second element and 

 is lost. There is liable also to be a defective screen image (fig. 28). 

 One can get the condenser at the right distance from the lamp by 

 first separating the lamp and condenser a considerable distance and 

 then gradually bringing them closer and closer together until the 

 condenser face is just filled with light. Sometimes the radiant is 

 put nearer than the principal focal distance on purpose, so as to 

 correct in part for the lack of proper proportion between the con- 

 denser and the objective ( 56). 



