Photoreceftion 



429 



ending. The impulses therefore are reflections of the activity of individual 

 sensory units. The first record shows that after a short latent period, there 





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is a prolonged series of nerve impulses of high frequency for slightly more 

 than a fifth of a second and then of a lower but rather constant frequency 

 (Fig. 133). At a lower intensity (second record) the latent period is longer 



