DEVELOPMENT OF CONVEX AND CONCAVE LENS 



339 



from the condensers. The first condenser nearest the light makes the 

 rays parallel and the condenser nearest to the lens brings the rays 

 of light to a point, so that they will meet inside the lens called the 

 objective. 



It is possible to tell by the screen whether the light behind the 

 objectives is adjusted properly. Numbers one and two show that 

 the light is not in the center, but too much to the right and left. 

 Numbers three and four show the results of the light being placed 

 too high or too low. Number five indicates that the light is too 

 near the condenser, and number six shows that the light is perfectly 

 adjusted, at the right distance and in the center of the condensers. 



2 



FIG. 217. 



Motion-picture Machines. A motion-picture film is a thin 

 ribbon of transparent pyroxylin plastic, or nitrocellulose, which is 

 highly inflammable: hence, the necessity of having the moving 

 picture machine in a fireproof booth. The photographs (1 by f 

 inches) on the film are arranged one after the other, and are 

 slightly different. There are sixteen of these pictures per foot. As a 

 foot of film is supposed to be run through a machine each second, 

 the audience sees sixteen different pictures per second. It requires 

 at least sixteen different pictures to make a complete change of 

 position to move the hand from one position to another, to lift 

 the arm, or to nod the head. 



The audience does not see the pictures moving. In fact, each 

 picture is made to stand still for a fraction of a second while the 



