CH. XIV] IMAGE FORMATION WITH MOVING PICTURES 597 



By reference to fig. 334, it can be seen that if the objective has a 

 small diameter, or if an iris diaphragm D, with a small opening is 

 present, only light from the central zones from b to e, is permitted 

 to pass. Increasing the diameter of the objective or diaphragm 

 opening has practically the same effect as increasing the diameter 

 of the condenser. As the diaphragm is opened the effect is striking, 

 it is as if there were three layers of light upon the screen: First, 

 the bright spot in the center of the screen increases in size until it 

 covers the entire opening of the aperture plate, then the light has 

 the appearance of folding over on itself and the second layer spreads 

 over the picture starting from the edges. During this stage the 

 illumination is uneven, there being a dark spot in the center of the 

 field (dark ghost). The second layer of light reaches the center 

 and goes beyond so there is a layer of light which starts at the 

 center and spreads out towards the periphery of the field. In this 

 stage the illumination is brighter at the center of the field than at 

 the edges, there being a bright spot in the center (light ghost). 

 With a larger aperture yet the third layer spreads over the entire 

 field. For this reason one is more likely to secure an evenly 

 illuminated field having no shadows in the center if an objective 

 with large lenses is used than if one with small lenses is used. 



830. Advantage in using a large diameter objective. The 



difficulty of lighting a picture evenly when using an objective of 

 small diameter is often very great and requires a good deal of 

 rather careful adjusting to eliminate a shadow in the center of the 

 field and to get rid of the reddish brown corners at the same time. 

 It is necessary to try various distances from the lamp-house to the 

 aperture plate, different positions of the arc with respect to the 

 condensers and it will perhaps be necessary to try condensers of 

 different focal lengths. It will generally be found more satisfactory 

 and convenient to have an objective of large diameter which will 

 allow quite a range of adjustment either side of the very best 

 without materially damaging the result. 



, 831. Special condenser for moving pictures. If the con- 

 denser of a moving picture outfit were designed especially for that 



