592 IMAGE FORMATION WITH MOVING PICTURES [Cn. XIV 



picture objective was 13.3 cm. e. f. (5^ in.) and the focal length 

 of the magic lantern objective to go with it is indicated. 



1. The dimensions of the aperture plate were 23 mm. x 17.3 

 mm., diagonal 28.0 mm. (|-| in. x H in., diagonal i^ in.) (For 

 new standard aperture see 5;oa). 



2. The moving picture is to be thrown on the screen with an 

 image either as wide or as high as the magic lantern picture or of 

 the same diagonal. 



Lantern slides have a maximum opening of 7.5 cm. wide, 7 cm. 

 high, diagonal 10.2 cm. (3 in. wide, 2^ in. high, 4 in. diagonal). 



FIG. 332. THREE-LENS CONDENSER. 



With a three-lens condenser the source of light is placed at the principal 

 focus of the first element of the condenser, the meniscus and the plano-convex 

 lens, the position of the lenses being as here shown. This gives a parallel 

 beam of light. The second element of the condenser should then be of a focal 

 length to cause the rays to cross at the center of the projection objective (fig. 2). 



That one dimension shall be the same with both the moving pic- 

 ture and the magic lantern, requires that the magic lantern objec- 

 tive have a focus which is from 3 to 4 times as long as the moving 

 picture objective, as is shown in the following table : 



