The project would soon head off in more productive directions, largely impelled by a trip of Edison's to Europe and the Exposition Universelle in Paris, for which he departed August 2 or 3, 1889. During his two months abroad, Edison visited with scientist-photographer Étienne-Jules Marey, who had devised a "chronophotographic gun"—the first portable motion picture camera—which used a strip of flexible film designed to capture sequential images at twelve frames per second. Upon his return to the United States, Edison filed another patent caveat, on November 2, which described a Kinetoscope based not just on a flexible filmstrip, but one in which the film was perforated to allow for its engagement by sprockets, making its mechanical conveyance much more smooth and reliable. The first motion picture system to employ a perforated image band was apparently the Théâtre Optique, patented by French inventor Charles-Émile Reynaud in 1888. Reynaud's system did not use photographic film, but images painted on gelatine frames. At the Exposition Universelle, Edison would have seen both the Théâtre Optique and the electrical tachyscope of German inventor Ottamar Anschütz. This disc-based projection device is often referred to as an important conceptual source for the development of the Kinetoscope. Its crucial innovation was to take advantage of the persistence of vision theory by using an intermittent light source to momentarily "freeze" the projection of each image; the goal was to facilitate the viewer's retention of many minutely different stages of a photographed activity, thus producing a highly effective illusion of constant motion. By late 1890, intermittent visibility would be integral to the Kinetoscope's design.