318 SCIENTIFIC AMUSEMENTS. 



are apparently seen simultaneously, and the complete device becomes 

 visible, e.g., the bird inside the cage, or the mouse inside the trap. 

 A modification of the same principle is employed in another 

 class of toy illusions known as zoetropes and by various other fancy 

 names. One form of this instrument consists of a pasteboard or 

 metal hollow cylinder capable of revolution about its axis, which 

 is placed vertically. In the sides of the cylinder are pierced a 

 number of slits, so that an observer looking horizontally at the 

 cylinder whilst it is revolving sees alternately the outer surface of 

 the cylinder and the inner surface on the opposite side, the latter 

 becoming visible through the slits as they successively pass in 

 front of the eye. The outer surface should be painted black, so 

 as to be practically invisible ; whilst inside the cylinder are pasted 

 or painted a series of figures representing, for instance, a horse in 

 the various stages and attitudes of leaping over a hurdle, as many 

 figures being used as there are slits. When properly adjusted, the 



Fig. 159. Thaumatrope. 



effect on the eye of an observer looking through the slits whilst 

 the cylinder is in rapid rotation is that the succession of glimpses 

 obtained of the horse in the various successive stages of the leap 

 become blended one with the next, and so on, producing the same 

 sensation as that of viewing a moving object ; the horse appears 

 to rise and leap over the hurdle, the action being the more natural 

 and graceful the more accurately the various stages of the motion 

 are depicted relatively to one another. 



Fig. 160 represents another simple form of this kind of toy. A 

 circular plate of pasteboard, &c., is perforated at the centre with a 

 pinhole, so that by passing a pin through the hole and pressing 

 the pin into a long stick of wood, which serves as a handle, the 

 disc can be spun round in its own plane, the pin serving as axle. 

 A series of slits is cut in the edge of the disc, and corresponding 

 with each slit is painted one of the successive stages of the motion 

 intended to be depicted (e.g., two boys playing leap-frog). The toy 

 is held by the handle, so that the upper part of the disc is about 



