CH. I] CENTERING THE MAGIC LANTERN 43 



With the two-lens condenser used for lantern slides the lamp is 

 usually closer than the principal focal distance of the first lens, this 

 makes the beam between the lenses diverging, hence it is best to 

 have the two lenses as close together as possible to avoid loss of 

 light (fig. i). 



With this condenser and diverging light between the lenses the 

 only rule that can be given is to adjust the distance between the 

 lamp and the condenser until the best light is obtained on the 

 screen. If this brings the crater of the arc lamp within 8 to 10 cm. 

 (4 in.) of the first lens, then it will be necessary to substitute longer 

 focus lenses for either the first or the second condenser lens or for 

 both. In general, the first lens should be of about 15 cm. (6 in.) 

 focus and the second lens should have a somewhat shorter focal 

 length than the projection objective. For example, if the projec- 

 tion objective is of 38 cm. (15 in.) focus, the second lens of the con- 

 denser in the two-lens form should be of about 2 5-30 cm. (10-1 2 in.) 

 focus. This will bring the diverging cone to a focus near the center 

 of the objective. 



56. Condenser and projection objective. If the projection 

 objective and the condenser are properly proportioned the conden- 

 ser will focus the light near the center of the projection objective 

 when the lantern slide is in focus on the screen (fig. 1,2). 



If the condenser is of so short a focus that the light from the 

 condenser comes to a focus before reaching the objective the field is 

 restricted and bordered by a red margin (fig. 29). 



If, on the other hand, the condenser is of too long a focus for the 

 objective the light will not come to a focus by the time it reaches 

 the center of the objective (fig. 28). In this case the field will be 

 restricted and bordered by blue. 



OPTICAL TEST FOR CENTERING 



57. Optical test for centering the radiant and the condenser. 



If these are properly centered along one line, and the two are 

 separated a considerable distance when the lamp is burning, the 

 light spot on the face of the condenser looking toward the objective 



