588 



CONDENSERS FOR PROJECTION 



[Cn. XIV 



has the advantage of simplicity and cheapness while the three- 

 lens type has the advantage that it has very little spherical aberra- 

 tion and at the same time it is possible to bring the lamp closer to 

 the first surface of the condenser, thus utilizing a greater proportion 

 of the light of the illuminant ( 8i8a). 

 c 



FIG. 329. ARRANGEMENT FOR INTERCHANGEABLE LANTERN-SLIDE AND 

 MOVING PICTURE PROJECTION WHEN THE CONDENSER HAS SPHERICAL 



ABERRATION. 



a Projection of lantern slide. 



b Projection of moving picture film. 



L Arc-lamp crater, the source of light. 



C Condenser of two plano-convex lenses. 



S In (a) the lantern slide near the condenser. 



S In (b) the moving picture film. 



Objective for projection. 



c' The image of the condenser face. 



818a. Types of condensers. In the development of projection apparatus 

 almost every form of condenser has been used from a single piano- or double- 

 convex lens to one composed of three or more lenses. In form, the lenses have 

 been plano-convex, double-convex, meniscus and parabolic. 



For artificial light the condenser is now almost always composed of two or 

 more lenses. 



First element (fig. 332). The first element may be composed of a single 

 plano-convex lens or a meniscus, or it may be composed of two lenses. A 

 meniscus and a plano-convex, or a meniscus and a double convex or finally of 

 two meniscus lenses. The first element in all cases collects the light from the 

 source and renders it more or less parallel. 



Second element, (fig. 332). The second element of the condenser may be 

 a plano-convex or a double convex lens or an achromatic combination. 



We have tried the different forms of condensers and have found those com- 

 posed of two plano-convex lenses, or those with two plano-convex lenses and a 

 meniscus next the light, most satisfactory (fig. i, 2). 



Finally there has been recently produced a parabolic condenser for projec- 

 tion with the microscope. This form eliminates almost all the spherical 

 aberration and is promising. 



