590 



CONDENSERS FOR PROJECTION 



[Cn. XIV 



In order that the combination of the two lenses of the condenser 

 shall have as little spherical aberration as possible they should be 

 of about equal focal length. If there is a difference in focal length, 

 the thicker lens, i. e., the one of shorter focal length, should be 

 placed next the radiant and the thinner lens, i. e., the one of greater 

 focal length, should be away from the radiant. The best lenses to 

 use in a given case can only be determined by experiment but as a 

 first trial we would suggest the following foci for condensers of 1 1.4 

 cm. (4^2 inches) diameter for magic lantern work and moving 

 picture projection. 



820. Table of condenser lenses. 



With these lenses the light from the first lens will be somewhat 

 diverging before it strikes the second lens. The best results are 

 obtained when the two lenses are as close together as they can be 

 put without touching. 



82 1 . The three-lens type of condenser. The three-lens type 

 of condenser illustrated in figure 332 is designed on a different 

 principle. Here the first combination, consisting of a meniscus 

 lens and a plano-convex or a double-convex lens (fig. ITT), is 

 designed to render the light from a point source parallel with but 

 a very small amount of spherical aberration. In order that the 

 light shall be focused at the center of the objective it is then neces- 

 sary that the last lens of the condenser shall bring the parallel beam 

 to a focus where it is wanted, that is, it must have a focus approxi- 

 mately equal to that of the objective. For long focus objectives 

 38-46 cm. (15-18 in.) this of course necessitates a rather thin lens 

 next to the lantern slide (fig. 332). 



