406 MOVING PICTURE APPARATUS [Cn. XI 



The standard aperture plate has an opening 23.08 mm. wide x 

 17.31 mm. high (29/32 in. x 87/128 in.) with rounded corners. 

 When the picture is in focus on the screen the edges of the aperture 

 plate are also in focus at the same time ( S7oa). 



571. The objective. The objective forms the image of the 

 film picture upon the screen. It is in design exactly like an objec- 

 tive for the magic lantern but is of shorter focus. 



It is better to have the lenses of large diameter (see 830). 



Moving picture objectives with lenses 45 mm. (iK in.) and 

 65 mm. (2^2 in.) in diameter are on the market. The objectives 

 45 mm. (1^4 in.) in diameter will answer but those of 65 mm. (2^2 

 in.) are to be preferred. The larger objectives will give with less 

 trouble a screen image without shadows. (See 829, 830). 



One must select an objective of suitable focal length to give a 

 proper sized screen image for the auditorium to be used. This is 

 dealt with more fully in 635. In most rooms a screen image of 

 suitable size will be obtained with an objective of between 12.5 to 

 13.5 cm. focus (5 to 5 y in.) when the moving picture machine is at 

 the back of the room. 



572. The film mechanism. This consists in the proper gears 

 and sprocket wheels for moving the film, and for turning the shut- 

 ter. The mechanism is complex; differs in different makes of 

 machines, and no attempt will be made here to describe it in detail. 



573. The shutter which cuts off the light during the time 

 when the film is in motion is located either just beyond the aperture 

 plate and hence before the objective (fig. 225), or just beyond the 

 objective (fig. 226, 227). When located between the aperture 

 plate and the objective, it is called an inside shutter and when 

 located beyond the objective it is called an outside shutter. 



570a. Standard aperture. As there was some lack of uniformity in the 

 size of the opening of the aperture plate, the Gundlach-Manhattan Optical 

 Co. has selected a size for a standard as follows : The aperture has an opening 

 of 23.08 mm. long and 17.31 mm. high (29/32 x 87/128 inch). This standard 

 has been adopted by the Nicholas Power Co., the Edison Co., and the Precision 

 Machine Co. No doubt the other makers of machines will adopt the standard 

 in due time. Moving Picture World, Vol. 20, April 1 1, p. 210, April 25, p. 512. 



