More complicated contrivances have also been constructed on the same principle, by which one may perceive the rotating disc through slits which turn at the same time. We will now describe the construction of some discs invented by Plateau under the name of the Phenakistoscope. These discs are made of strong cardboard, from six to ten inches in diameter (fig. 112), on which a certain number of figures (eight to twelve) are placed in circles at an equal distance from each other, presenting the successive phases of a periodical movement. This disc is placed on another opaque circle of rather larger diameter, which has on its margin as many openings as the first disc has figures. The two discs are placed one on the other, and are fixed in the centre by means of a screw at the anterior extremity of a small iron axis, the other end being fitted into a handle. To make use of this contrivance we place ourselves in front of the glass, towards which we turn the disc with the figures, placing the eye so as to see the figures through one of the holes of the large disc. Directly the apparatus begins to turn round, the figures seen in the glass appear to execute the particular movements which they represent in different positions. Let us designate by means of the figures 1, 2, 3, the different openings through which the eye successively looks, and point out by the same numbers the figures in the radiuses thus numbered. If the experimenter looks in the glass through opening 1, he will see first figure 1, which appears in the glass to pass before his eyes; then the rotation of the disc displaces opening 1, and the cardboard intervenes, until opening 2 appears; then figure 2 takes the place of figure 1, until it in turn disappears, and opening 3 presents figure 3 to view. If these figures were all similar, the spectator would have but a series of visual impressions, separate but alike, which by a sufficiently rapid rotation mingle together in one durable impression like a perfectly immovable object. If, on the contrary, the figures differ slightly from each other, the luminous sensations will also mingle in a single object, which will however appear to be modified in a continuous manner, conformably with the differences of successive images. With a difference of speed, we obtain a new series of phenomena. A most simple contrivance of this kind is a top of C. B. Dancer, of Manchester (fig. 113). It will be seen that the axis carries another disc, pierced with openings of different shapes, to the edge of which a thread is attached. This second disc is carried along by the friction of the axis, but its rotation is less rapid because of the great resistance offered by the air to the piece of thread which participates in the movement. If the lower disc has several differently-coloured sectors, they produce a very motley appearance, which seems to move sometimes by leaps, and sometimes by continuous motion. We must distinguish between