If the picture of a motion was to go beyond these simplest rhythmical movements, if persons in action were really to be shown, it would be necessary to have a much larger number of pictures in instantaneous illumination. The wheel principle would have to be given up and a long strip with pictures would be needed. That presupposed a correspondingly long set of exposures and this demand could not be realized as long as the pictures were taken on glass plates. But in that period experiments were undertaken on many sides to substitute a more flexible transparent material for the glass. Translucent papers, gelatine, celluloid, and other substances were tried. It is well known that the invention which was decisive was the film which Eastman in Rochester produced. With it came the great mechanical improvement, the use of the two rollers. One roller holds the long strip of film which is slowly wound over the second, the device familiar to every amateur photographer today. With film photography was gained the possibility not only of securing a much larger number of pictures than Marey or Anschuetz made with their circular arrangements, but of having these pictures pass before the eye illumined by quickly succeeding flashlights for any length of time. Moreover, instead of the quick illumination the passing pictures might be constantly lighted. In that case slits must pass by in the opposite direction so that each picture is seen for a moment only, as if it were at rest. This idea is perfectly realized in Edison's machine. In Edison's kinetoscope a strip of celluloid film forty-five feet in length with a series of pictures each three-quarters of an inch long moved continuously over a series of rolls. The pictures passed a magnifying lens, but between the lens and the picture was a revolving shutter which moved with a speed carefully adjusted to the film. The opening in the shutter was opposite the lens at the moment when the film had moved on three-quarters of an inch. Hence the eye saw not the passing of the pictures but one picture after another at the same spot. Pretty little scenes could now be acted in half a minute's time, as more than six hundred pictures could be used. The first instrument was built in 1890, and soon after the Chicago World's Fair it was used for entertainment all over the world. The wheel of Anschuetz had been widespread too; yet it was considered only as a half-scientific apparatus. With Edison's kinetoscope the moving pictures had become a means for popular amusement and entertainment, and the appetite of commercialism was whetted. At once efforts to improve on the Edison machine were starting everywhere, and the adjustment to the needs of the wide public was in the foreground.