584 IMAGE FORMATION, INVERSION OF IMAGES [Cn. XIV 



IMAGE FORMATION WITH THE MAGIC LANTERN 



811. Ideal case. When using transparent lantern slides 

 with a magic lantern and a small source of light the ideal arrange- 

 ment is that shown in fig. 325. 



L, is a point source of light (crater of the arc light). The con- 

 denser C, focuses this light at the point O,in the optical center of 

 the objective. The slide-carrier S, is placed just in front of the 

 condenser. The objective O, is at the proper distance from S, to 

 formja real image of the slide on the screen. All of the rays of 

 light from S pass directly through the center of the objective O, and 



FIG. 325. LANTERN-SLIDE PROJECTION; No SPHERICAL ABERRATION. 



This shows an ideal case where there is a point source of light, and a con- 

 denser without spherical aberration. The light from the condenser crosses at 

 the center of the objective (O) and goes on without deviation to the image 

 screen. 



L Light source. 



C Condenser. 



S Lantern slide. 



O Projection objective. 



hence undergo practically no deviation. If the source of light L, 

 were really a point source, and the condenser C, had no spherical 

 aberration, the shadow of the lantern slide S,in the screen without 

 an objective would be just like the image which is projected by the 

 objective. 



812. Inversion of the image. In their passage from the 

 lantern slide to the screen the rays pass from the top of the slide 

 to the bottom of the screen, and from the bottom of the slide to the 

 top of the screen. In like manner the rays from the two sides of 

 the slide cross before reaching the screen (fig. i.) 



This crossing of the rays gives what is known as an inverted 

 image. 



