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.