CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 593 



In the above case, with a moving picture objective of 13.3 cm. 

 focus, the focus of the magic lantern objective to use with the 

 moving picture objective is, for the same: 



Width of picture 42.1 cm. 1 6 y 2 in . 



Height of picture 53.1 cm. 21 in. 



Diagonal of picture 48.5 cm. 19 in. 



3. The same arc lamp, condenser, etc., are to be used inter- 

 changeably for either films or slides by simply pushing the appara- 

 tus sidewise. Usually the slide-carrier is mounted permanently 

 with the condenser so that the opening is not a circle of the diame- 

 ter of the condenser but a rectangle 7.5 cm. x 10 cm. (3 in. x 4 in.). 



4. Even illumination of the screen. If the light is not quite 

 uniform it is better to have the center the brighter rather than the 

 edge. 



824. Ideal case, moving pictures. The ideal case of projec- 

 tion (shown in fig. 328 a and b) is where the light is a point source 

 and the condenser has no spherical aberration. This is the case 

 which is usually figured, but it is not the best in practice if an 

 extended source is used. 



When changing over to moving picture films the lamp and 

 condenser are moved to the position b. The objective O, is still 

 45 cm. (18 in.) from the condenser face where the rays will cross 

 in the diaphragm plane, and the film is placed 13.3 cm. (5^ in.) 

 from the objective so that it will be in focus on the screen. 



825. Illumination of moving pictures, practical method. 



The method which has been found most successful in lighting 

 moving pictures is to focus the image of the crater not on the objec- 

 tive but on the aperture plate. This is because a moving picture 

 objective usually has a diameter greater than the diagonal of the 

 film (40 mm. to 65 mm. against 28.5 mm. diagonal; 1^2 in. to 2^in. 

 against i^s m - diagonal), hence the important point is to get the 

 light through the film ; the large objective will take in all the light 

 which can get through the film. 



Figures 333-335 show the effects of different methods of lighting. 

 In practice all three are used together, that is, the film is illum- 

 inated by the area of the condenser which is not covered by the 



