CH. XI] OPTICS OF MOVING PICTURES 409 



current and alternating current instruments. (See Chap. XIII 

 for using these instruments 662-674). 



OPTICS OF MOVING PICTURE PROJECTION 



576. For purposes of description the projection of the 

 individual pictures of a film can be considered apart from the 

 mechanism which moves the film. 



The projection of the film picture has much in common with that 

 of the ordinary lantern slide but it is somewhat more difficult. 



A theoretical treatment of the proper method of lighting the film 

 is found in 825. Briefly stated it is this: Light from the arc is 

 collected by the condenser so as to illuminate the film. This 

 illumination must be very intense and at the same time must be 

 evenly distributed over the entire area of the film. To secure this 

 result with the ordinary large condensers (4^2 in. in diameter) 

 requires the condenser to be quite a distance away from the film, 

 the crater of the arc to be of considerable size, and the projection 

 objective to be of fairly large diameter. 



Fig. 228 shows the optical arrangement most commonly used. 

 Light from the arc is collected by the condenser upon the 

 film at s, passing through the transparent parts of the film, it is 

 bent by the objective in such a way as to form a sharp image of 

 the film s, upon the screen. 



Only one picture of the film is seen at a time, the rest being 

 carried in the magazines or covered with shields. The picture 

 to be shown is just in front of the opening of the aperture plate. 

 Optically we are concerned only with the aperture plate and the 

 short section of film behind it. It is this short section of film which 

 must be evenly illuminated and projected upon the screen. 



Beyond the film is the objective (fig. 229). The objective should 

 be of good quality as it is the objective which determines the 

 sharpness of the screen picture. Moreover, the objective must 

 not be of too small diameter, for if it is too small there is danger 

 that the screen image will not be evenly lighted although the 

 illumination of the film may be perfectly even. The focal length 



