248 PRACTICAL PHYSIOLOGY 



The rate necessary for the flickering sensation to pass into complete 

 fusion depends upon the intensity of the light. 



[ADVANCED EXPERIMENT. With a metronome, note the rate of revolu- 

 tion necessary to produce complete fusion in the outer ring. Darken the 

 room and observe whether the rate be altered. It will be found that 

 with diminished light a slower rate of revolution brings about fusion. 

 The converse is true up to a certain limit. 



The point at which flicker passes into fusion has been used as a 

 means of determining the condition of persistence of visual sensations. 

 It is to be noted that the flicker may be coarse or of a fine tremulous 

 character. The transition of this fine flicker into fusion should be 

 taken as the limiting sensation. 



The excitability of any portion of the retina is influenced by the 

 stimulation of that portion of the retina (temporal induction) and 

 changes are simultaneously induced in neighbouring regions of the 

 retina (spatial induction). These factors may be of very considerable 

 influence in determining the point at which flicker passes into fusion. 

 A 'physiological' state is brought about by a certain 'physical' stimulus, 

 and thereby the effect of the stimulus may be increased or diminished. 

 If then a succession of stimuli of say blue and black be presented to the 

 retina at a certain rate flicker will pass into fusion. But if the blue be 

 intensified by being placed on a black background this rate will 

 no longer be sufficient. This may be shown in the following 

 manner.] 



FIG. 217. 



[ADVANCED EXPERIMENT. Take a disc like that shown in Fig. 217 

 with black and blue semi-circular rings, and yellow and black back- 



