448 SCIENCE OF HOME AND COMMUNITY 



thin mixture over smooth surfaces, where it is allowed to 

 dry. It is then coated with the sensitive emulsion. The 

 sheet is then cut up into strips one and three eighths inches 

 wide, which is the standard width for movie cameras and 

 projectors. The materials of which the films were formerly 

 made were highly inflammable and their use was attended 

 with danger of fire. A kind of material has now been made, 

 however, which is practically non-inflammable. 



In order to have the film move steadily in the camera and 

 projector, perforations are made along either side of the film 

 by means of machines. The standard is sixty-four holes 

 per foot. The success of the picture depends to a great 

 extent upon the accuracy with which these perforations are 

 made. 



Moving-picture camera. The moving-picture camera 

 consists of three parts, the camera proper, a box for the un- 

 exposed film, and a box for the exposed film. By means of 

 a handle turned by hand, the film is unwound and brought 

 before the shutter; there it is exposed, and then wound up 

 in the other box. These boxes can easily be detached from 

 the camera; and when one film is used, the box can be re- 

 moved and another put in its place. The motion is given 

 to the film by turning a handle attached to the side of the 

 camera. This is so constructed that the film moves by 

 jerks, so that the exposed section of the film is still for a 

 fraction of a second while the picture is being taken. It 

 requires practice to be able to turn the handle at the right 

 speed. This should be done so as to make about sixteen 

 exposures a second. Sometimes the handle is turned by a 

 motor. 



When the end of a series of exposures is reached, the 

 operator pushes a film punch projecting from the camera, 

 which makes punch marks on the film. The number of 

 feet of film that has been used is indicated automatically 

 on a dial, so that the operator may know when to change 



