124 SCIENTIFIC RECREATIONS. 



will be seen successively at the centre, if the plane mirrors are placed on 

 a concentric polygon, the " apotheme " 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. 1 2 8), (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, por- 

 trayed 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, 



Fig. 126. The Zootrope. 



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. In the evening, a lamp placed on a support ad hoc, in the centre 

 of the apparatus, suffices to light it up very clearly, and a number of persons 

 may conveniently assemble round it, and witness the effects produced. 



Besides the attractions offered by the animated scenes of the Praxinoscope, 

 the apparatus may also be made the object of useful applications in the 

 study of optics. It permits an object, a drawing, or a colour, to be substi- 

 tuted instantaneously in experiments on secondary or subjective images, etc., 

 on the contrast of colours or the persistence of impressions, etc. We can 

 also make what is called a synthesis of movements by placing before the 

 prism a series of diagrams of natural objects by means of photography. 



M. Reyaaud has already arranged an apparatus which exhibits in the 



