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Types of Visual Aids and Their Uses 99
pictures in sequence on a strip of silk approximately three inches in width, and this strip of silk was pulled past the large opening by one person while another placed, one eye at the small hole to see these marvelous pictures of action. It was a far cry from those early attempts to produce the illusion of motion to the development of the motion-picture film which is used so extensively for education and entertainment today. However, the ancient and the new depend upon the same psychological phenomenon for the illusion.
Psychologists tell us that an image on the retina of the eye remains there approximately one-twelfth of a second after the object itself may disappear from view. This is known as "persistence of vision." If we can arrange, therefore, to remove one picture and substitute another similar picture within the period during which vision persists, we can view the pictures with a feeling of continuity just as we do the motion picture today. The pictures are changed on the screen at the rate of sixteen times per second when silent films are used, and at the rate of twenty-four times per second when sound pictures are used. The result is a smooth continuity of the series of still pictures placed so closely together that the eye travels from one to the next without noticing the break or change. The ordinary motion-picture reel, therefore, is made up of a series of 16,000 separate and distinct still pictures which are closely related, and are projected on the screen within a period of from ten to fifteen minutes.
One of the first attempts at producing motion pictures was made by Leland Stanford late in the nineteenth century. He was interested in determining whether or not a certain horse actually raised all four feet from the ground at any time while it was traveling around the race track. Motion-picture cameras had not yet been invented, so several still cameras were placed side by side around a sector of the race track and separate pictures were photographed as the horse passed these points. The experiment was successful in answering the question, but a young engineer by the name of Isaacs was given the task of devising an apparatus which would produce a continuous record of the action of the horse. To him is credited much of the early development of the process for recording motion pictures. It was not until the last few years of the nineteenth century that pictures were produced which were even passably satisfactory for education or recreational use. It is interesting to find that the motion-picture apparatus invented by Edison was perfected for the purpose of recording and projecting pictures to be shown in conjunction with phonograph records — pictures of the recording artists.