168 RESPONSE IN THE LIVING AND NON-LIVING 
stimulus is similar in both; and finally, even in abnor- 
malities—such 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 

Fic. 109.—Crrrain 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, 0). 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 0). 
In making a general retrospect, finally, of the effects 
