CH. XIV] 



CONDENSERS FOR PROJECTION 



589 



819. The two-lens type of condenser. In choosing the 

 objective and other optical parts of the lantern one must first 

 consider the room in which the projection is to be done and then 

 choose an objective of such a focal length that the picture will be 

 of the desired size (Ch. XII, 635). After the objective is deter- 

 mined upon, it is necessary to select the condenser lenses of such 

 focal length that the best results may be obtained. There are two 

 factors which must be balanced in this choice. First; the closer 

 the light is to the condenser, that is the shorter is its focus, the 



FIG. 330. EFFECT OF AN EXTENDED SOURCE OF LIGHT. 



L' (w, x, y, z) Extended light source. 



Single Lens Condenser near the source. 



5 Lantern slide. 



a and b Points on the lantern slide. 



O Single lens objective. 



y', x' Image of the extended light source on the objective. 



/, a Screen image of the lantern slide. 



If the light source xy, is not too large, all of the light collected by the con- 

 denser gets through the objective. 



If the light source wz, is too large, the image w' z', will be larger than the 

 objective and much light will be lost by falling outside the objective. 



greater will be the amount of light which it will collect. Second; 

 the shorter the focus of the condenser, the greater will be its 

 spherical aberration. In order to get the minimum of spherical 

 aberration with two plano-convex lenses such as are generally used 

 for condensers it is necessary to turn them so that parallel or nearly 

 parallel light strikes the curved surfaces and the diverging light 

 from the source strikes the plane surface (fig. 321, 323). When the 

 parallel beam strikes the curved surface of the lens, all of the rays 

 come to a focus more nearly at the same point than when the 

 parallel beam strikes the plane surface of the lens (fig. 320, 321). 

 This requires that the curved surfaces of the lenses shall be turned 

 towards each other as in fig. 329. 



