CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 595 



s w s f and s y s.' In the same way light from t, reaches t', between 

 the limiting rays txt' and tzt'. 



The objective 0, will bring an image of the condenser face to a 

 focus somewhere between it and the screen. In fig. 333 the image 

 of the condenser face is at the point a' b'. With the magic lantern 

 the condenser face and the lantern slide being so close together the 

 image of the condenser face is nearly in focus on the screen. 



FIG. 335. THE DOTTED LINES SHOW THE MARGINAL RAYS REMOVED BY THE 



SLIDE-CARRIER. 

 s, t Film. 

 O Objective. 



FIG. 336. SMALL CONDENSER FOR MOVING PICTURES. 



This is exactly comparable to lantern projection except that the condenser 

 and the object are smaller. 



827. Image formation when using a point source and a 

 condenser with no spherical aberration. The crater image in this 

 case would be focused at o, and only the rays a s y s', and b t x t', 

 would be used (fig. 333). 



828. Image formation with a point source and a condenser 

 having spherical aberration. The condenser must have either no 

 spherical aberration at all or just the right amount. Fig. 334 

 represents a condenser having the right amount of spherical aberra- 

 tion. Consider the effect of each zone of the condenser in illum- 

 inating the film. The center zone from d to e, lights most of the 

 center of the film. With this zone onlv, the illumination would 



