216 ELEMENTS OF GENERAL SCIENCE 



on a general photographic survey of the Pacific coast, 

 became interested in motion pictures. One of the debated 

 problems of that time was whether a horse in running ever 

 has all four feet off the ground at the same instant. Later, 

 at the University of Pennsylvania, a very elaborate studio 

 was constructed to take pictures of moving objects. Several 

 thousand views were made of birds, horses, wild animals, 

 and human beings in motion. These pictures were shown at 

 what was probably the first motion-picture theater, at the 

 World's Columbian Exposition held in Chicago, in 1893. 



Motion pictures depend upon the persistence of vision in 

 the eye ; that is, the image of a moving object does not dis- 

 appear at once, but remains for a time, the period of persist- 

 ence depending partly upon the condition of the light and 

 exposure. There are many familiar illustrations of this fact ; 

 for example, a ring of fire is produced by whirling a burning 

 stick, and the spokes of a wheel merge into a disk at a 

 certain speed. 



How are the pictures taken and projected upon the screen ? 

 The pictures of the original moving objects are obtained by 

 means of a camera of a special design, in which a photo- 

 graphic film is exposed by a revolving mechanism that makes 

 from 16 to 120 pictures per second. This film, when devel- 

 oped, forms the semitransparent negative, from which a roll 

 of positive pictures is printed. The positive film, which now 

 bears the small images, being printed on celluloid, is partially 

 transparent. When projected on the screen at the proper 

 rate (which varies from 16 to 120 per second) by means of 

 a suitable lantern, the rapid succession of still pictures gives 

 the sensation of motion. A rotating shutter prevents the 

 light from striking the screen while the next picture is being 

 moved into position. Colored effects are obtained by passing 

 the light through tinted and transparent shutters. Since the 

 celluloid is combustible, precaution is taken to make the 

 machine booth fireproof. 



