The scientific principle which controls the moving picture world of today was established with these early devices. Isolated pictures presented to the eye in rapid succession but separated by interruptions are perceived not as single impressions of different positions, but as a continuous movement. But the pictures of movements used so far were drawn by the pen of the artist. Life showed to him everywhere continuous movements; his imagination had to resolve them into various instantaneous positions. He drew the horse race for the zooetrope, but while the horses moved forward, nobody was able to say whether the various pictures of their legs really corresponded to the stages of the actual movements. Thus a true development of the stroboscopic effects appeared dependent upon the fixation of the successive stages. This was secured in the early seventies, but to make this progress possible the whole wonderful unfolding of the photographer's art was needed, from the early daguerreotype, which presupposed hours of exposure, to the instantaneous photograph which fixes the picture of the outer world in a small fraction of a second. We are not concerned here with this technical advance, with the perfection of the sensitive surface of the photographic plate. In 1872 the photographer's camera had reached a stage at which it was possible to take snapshot pictures. But this alone would not have allowed the photographing of a real movement with one camera, as the plates could not have been exchanged quickly enough to catch the various phases of a short motion. Here the work of Muybridge sets in. He had a black horse trot or gallop or walk before a white wall, passing twenty-four cameras. On the path of the horse were twenty-four threads which the horse broke one after another and each one released the spring which opened the shutter of an instrument. The movement of the horse was thus analyzed into twenty-four pictures of successive phases; and for the first time the human eye saw the actual positions of a horse's legs during the gallop or trot. It is not surprising that these pictures of Muybridge interested the French painters when he came to Paris, but fascinated still more the great student of animal movements, the physiologist Marey. He had contributed to science many an intricate apparatus for the registration of movement processes. "Marey's tambour" is still the most useful instrument in every physiological and psychological laboratory, whenever slight delicate movements are to be recorded. The movement of a bird's wings interested him especially, and at his suggestion Muybridge turned to the study of the flight of birds. Flying pigeons were photographed in different positions, each picture taken in a five-hundredth part of a second.