Page:Popular Science Monthly Volume 52.djvu/196

182 sensitized film, beneath an incandescent lamp, whereby the photographs are impressed or "printed" upon the sensitive surface. This second film in its turn is passed through the various photographic processes. When complete, it is wound upon a spool, and is then ready to be placed in the cinematograph or other machine used in exhibiting the pictures. Here, as already stated, the mechanical arrangements correspond to those employed in taking the negatives.



Thus the pictures, when displayed before an audience, are seen to flash out in the same rapid sequence in which the original scenes were presented to the "eye of the camera."

A homely illustration may aid the reader in arriving at a perfectly clear comprehension of this subject. Let us take the case of a man who is slowly walking past a high picket fence and gazing intently at some moving object on the other side of the fence. His view will be interrupted at regular intervals by the pickets as they successively encounter his line of sight. But if he proceed more quickly a seemingly continuous view of the object in question will be obtained, though rapid alternations in its brightness will be manifested. These effects are due to a well-known cause—viz., the persistence of luminous impressions upon the human retina. Thus, our observer's eye retains for a brief period its impression of each momentary glimpse that is afforded him under the conditions just described; and the successive visual images become merged into one