The next step leads us much further. In the fall of 1832 Stampfer in Germany and Plateau in France, independent of each other, at the same time designed a device by which pictures of objects in various phases of movement give the impression of continued motion. Both secured the effect by cutting fine slits in a black disk in the direction of the radius. When the disk is revolved around its center, these slits pass the eye of the observer. If he holds it before a mirror and on the rear side of the disk pictures are drawn corresponding to the various slits, the eye will see one picture after another in rapid succession at the same place. If these little pictures give us the various stages of a movement, for instance a wheel with its spokes in different positions, the whole series of impressions will be combined into the perception of a revolving wheel. Stampfer called them the stroboscopic disks, Plateau the phenakistoscope. The smaller the slits, the sharper the pictures. Uchatius in Vienna constructed an apparatus as early as 1853 to throw these pictures of the stroboscopic disks on the wall. Horner followed with the daedaleum, in which the disk was replaced by a hollow cylinder which had the pictures on the inside and holes to watch them from without while the cylinder was in rotation. From this was developed the popular toy which as the zooetrope or bioscope became familiar everywhere. It was a revolving black cylinder with vertical slits, on the inside of which paper strips with pictures of moving objects in successive phases were placed. The clowns sprang through the hoop and repeated this whole movement with every new revolution of the cylinder. In more complex instruments three sets of slits were arranged above one another. One set corresponded exactly to the distances of the pictures and the result was that the moving object appeared to remain on the same spot. The second brought the slits nearer together; then the pictures necessarily produced an effect as if the man were really moving forward while he performed his tricks. In the third set the slits were further distant from one another than the pictures, and the result was that the picture moved backward.