CH. XIV[ IMAGE FORMATION, INVERSION OF IMAGES 587 



source L, after passing through (a) will spread out over an angle 

 and strike the objective lens O. The purpose of the objective lens 

 O, being to collect all of the light from the point (a) on the slide and 

 to bring it together at the point (a') on the screen. This is exactly 

 similar to the case of image formation of a self-luminous or diffusely 

 reflecting surface described in (Ch. VII, 273, fig. 90) except that 

 the light from the point (a) does not spread out in all directions, 

 but only over the angle x' ay'. 

 c 



FIG. 328. INTERCHANGEABLE MAGIC LANTERN AND MOVING PICTURE 



PROJECTION WITH POINT SOURCE AND CONDENSER FREE FROM 



SPHERICAL ABERRATION. 



a Magic lantern arrangement. 



b Moving picture arrangement. 



L Crater of the arc lamp as a source of light. 



c Condenser. 



s,s The lantern slide in (a), and the film in (b). 



o, o Projection objectives. 



817. Simplicity of the Magic Lantern. From the above it is 

 seen that with the arc-light magic lantern the actual case is nearly 

 the same as the ideal case and the manipulation of the apparatus 

 is relatively simple. 



RELATION OF THE FOCAL LENGTH OF THE CONDENSER TO THE 

 FOCAL LENGTH OF THE PROJECTION OBJECTIVE 



818. Types of condensers. There are in general use two 

 main types of condenser; the two-lens type, as shown in fig. 331, 

 and the three-lens type, as shown in fig. 332. The two-lens type 



