We present here an apparatus based on a very different optical arrangement. In the _Praxinoscope_[12] (a name given by the inventor, Mr. Reynaud, to this new apparatus), the substitution of one object for another is accomplished without interruption in the vision, or solution of continuity, and consequently without a sensible reduction of light; in a word, the eye beholds _continuously_ an image which, nevertheless, is incessantly changing before it. The result was obtained in this manner. Having sought unsuccessfully by mechanical means to substitute one object for another without interrupting the continuity of the spectacle, the inventor was seized with the idea of producing this substitution, not with the objects themselves, but with their virtual images. He then contrived the arrangement which we will now describe. A plane mirror, AB (fig. 129), is placed at a certain distance from an object, CD, and the virtual image will be seen at C′D′. If we then turn the plane mirror and object towards the point, O, letting BE and DF be their new positions, the image will be at C″D″. _Its axis_, O, _will not be displaced_. In the positions, AB and CD, first occupied by the plane mirror and the object, we now place another mirror and object. Let us imagine the eye placed at M. Half of the first object will be seen at OD″, and half of the second at OC′. If we continue the rotation of the instrument, we shall soon have mirror No. 2 at TT′, and object No. 2 at SS′. At the same moment the image of object No. 2 will be seen entirely at C‴D″. Mirror No. 2 and its object will soon after be at BE and DF. If we then imagine another mirror and its corresponding object at AB and CD, the same succession of phenomena will be reproduced. This experiment therefore shows that a series of objects placed on the perimeter of a polygon will be seen successively at the centre, if the plane mirrors are placed on a concentric polygon, the “apothème” of which will be less by one-half, and which will be carried on by the same movement. In its practical form, M. Reynaud’s apparatus consists of a polygonal or simply circular box (fig. 128), (for the polygon may be replaced by a circle without the principle or result being changed), in the centre of which is placed a prism of exactly half a diameter less, the surface of which is covered with plane mirrors. A strip of cardboard bearing a number of designs of the same object, portrayed in different phases of action, is placed in the interior of the circular rim of the box, so that each position corresponds to a plate of the glass prism. A moderate movement of rotation given to the apparatus, which is raised on a central pivot, suffices to produce the substitution of the figures, and the animated object is reflected on the centre of the glass prism with remarkable brightness, clearness, and delicacy of movement. Constructed in this manner, the _Praxinoscope_ forms an optical toy both interesting and amusing.