A man represented with as many legs as a centipede would not give us any impression of running or movement; and a blur showing the positions taken successively by his legs would be equally futile. Therefore we are driven back to a _series_ of pictures, slightly different from one another; and in order that the pictures may not be blurred a screen must be interposed before the eye while the change from picture to picture is made. The shorter the period of change, and the greater the number of pictures presented to illustrate a single motion, the more realistic is the effect. These are the general principles which have to be observed in all mechanism for the production of an illusory effect of motion. The persistence of vision has led to the invention of many optical toys, the names of which, in common with the names of most apparatus connected with the living picture, are remarkable for their length. Of these toys we will select three for special notice. In 1833 Plateau of Ghent invented the _phenakistoscope_, “the thing that gives one a false impression of reality”--to interpret this formidable word. The phenakistoscope is a disc of card or metal round the edge of which are drawn a succession of pictures showing a man or animal in progressive positions. Between every two pictures a narrow slit is cut. The disc is mounted on an axle and revolved before a mirror, so that a person looking through the slits see one picture after another reflected in the mirror. The _zoetrope_, or Wheel of Life, which appeared first in 1860, is a modification of the same idea. In this instrument the pictures are arranged on the inner side of a hollow cylinder revolving on a vertical axis, its sides being perforated with slits above the pictures. As the slit in both cases caused distortion M. Reynaud, a Frenchman, produced in 1877 the _praxinoscope_, which differed from the zoetrope in that the pictures were not seen directly through slits, but were reflected by mirrors set half-way between the pictures and the axis of the cylinder, a mirror for every picture. Only at the moment when the mirror is at right angles to the line of sight would the picture be visible. M. Reynaud also devised a special lantern for projecting praxinoscope pictures on to a screen. These and other somewhat similar contrivances, though ingenious, had very distinct limitations. They depended for their success upon the inventiveness and accuracy of the artist, who was confined in his choice of subject; and could, owing to the construction of the apparatus, only represent a small series of actions, indefinitely repeated by the machine. And as a complete action had to be crowded into a few pictures, the changes of position were necessarily abrupt.