I 12 



SCIENTIFIC RECREATIONS. 



of C. B. Dancer, of Manchester (fig. 1 1 3). 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 



Fig. in. Busold's chromatic top. 



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 

 A 



Fig. us. Rotating disc. 



by continuous motion. We must distinguish between the phenomena of 

 successive contrast and simultaneous contrast. 



Phenomena of successive contrast develop what are called accidental 

 images. If we fix our eyes for a considerable time on a coloured object, 



