DEVICES WHICH USE LIGHT 



115 



Courtesy of Lightolier Company 



FIG. 179. HOW TO USE LIGHT FOR THE PROTECTION OF THE EYES 



pensive camera has a speedier lens. Only the central 

 portion of the lens of a low-cost camera can be used 

 to give a sharp image. The more costly cameras con- 

 tain lenses that are so carefully ground that even if 

 the diaphragm or stop is opened much wider, they 

 produce sharp images. Thus the length of exposure is 

 shortened. 



Do moving pictures really move? Since it is pos- 

 sible to obtain small portable motion-picture projec- 

 tors, many persons have become interested in amateur 



motion-picture photography. There is no essential 

 difference between a motion-picture camera and a still 

 camera except that in the former, pictures are taken 

 automatically at the rate of sixteen per second on a 

 long narrow film. The film is moved into place back 

 of the lens by a spring and remains stationary % 2 of a 

 second during exposure, after which another portion 

 of the film comes into position for exposure. A nega- 

 tive is developed from the exposed film, and a posi- 

 tive print is made from the negative. The positive 



