1 68 RESPONSE IN THE LIVING AND NON-LIVING 



stimulus is similar in both ; and finally, even in abnor- 

 malitiessuch as reversal of response, preliminary 

 negative twitch on commencement, and terminal posi- 

 tive twitch on cessation of illumination, and decline 

 and reversal under continued action of light parallel 

 effects are noticed. 



We may notice here certain curious relations even 

 in these abnormal responses (fig. 109). If the equi- 

 librium position remain always constant, then it is easy 

 to understand how, when the rising curve has attained 



FIG. 109. CERTAIN AFTER-EFFECTS OF LIGHT 



its maximum, on the cessation of light, recovery should 

 proceed downwards, towards the equilibrium position 

 (fig. 109, a). One can also understand how, after 

 reversal by the continued action of light, there should 

 be a recovery upwards towards the old equilibrium 

 position (fig. 109, b). What is curious is that in certain 

 cases we get, on the stoppage of light, a preliminary 

 twitch away from the zero or equilibrium position, 

 upwards as in (c) (compare also fig. 107) and downwards 

 as in (d) (compare also fig. 108 b). 



In making a general retrospect, finally, of the effects 



