374 



PRACTICAL PHYSIOLOGY 



intensified by being placed on a black blackground this rate will no 

 longer be sufficient. This may be shown in the following manner. 



EXPERIMENT V. Take a disc like that shown in Fig. 244 with black 

 and blue semicircular rings, and yellow and black backgrounds. On 



FIG. 244. 



rotating this disc it will be observed that the flicker persists much 

 larger in the outer blue and black ring than in the inner blue and black 

 rinse. 



FIG. 245. 



Fechner showed that certain colour effects may be produced by slow 

 rotation of discs which consist of black sectors of increasing size on a 

 white ground. They may also be seen in a disc showing black circular 

 lines of different circumferences on a white semicircular area, the other 

 half of the disc being black. Such a disc is shown in Fig. 245. 



