DEVELOPMENT OF CONVEX AND CONCAVE LENS 



341 



If the picture changes before the shutter 

 covers it, streaks appear going down. If 

 the shutter uncovers the picture before it 

 is completely changed and comes to a 

 stop, streaks and flashes of white are seen 

 going up. Each picture must come to a 

 complete stop before the shutter un- 

 covers it. 



With direct current a three-blade shutter 

 is used. While the picture is standing still, 

 the two other blades pass over it. With al- 

 ternating current a two-blade shutter is 

 used, one blade for the picture to move 

 behind when changing, and the , other to 

 pass over the face of the picture while 

 it is standing still. 



The reason for having one or two 

 blades pass over the face of the picture is 

 to prevent flickering. 



On each side of the film is a margin - 3 % 

 of an inch wide in which are perforations 

 for sprocket wheels which feed the pictures 

 to the machine. 



Moving pictures were first made on very 

 short films not over 75 feet long. They 

 were mostly used for comic pictures. To- 

 day films are several hundred feet in length 

 and are used for many educational pur- 

 poses. 



The Taking of Motion Pictures. Pic- 

 tures of moving objects are taken by a cam- 

 era which takes about sixty pictures per 



fc- 



FIG. 219. A part of a film for a moving picture. 

 Each picture is slightly different and if seen in 

 rapid succession the automobile appears to be 

 moving around the street corner. 



FIG. 219, 



