26o CHAP. XXIV. THE AFTER-EFFECT OF LIGHT 



further favoured by transverse conduction of excitation to 

 the distal side (p. 134). 



The character of the after-effect will presently be shown 

 to be modified by the duration of the antecedent stimula- 

 tion, the different phases of which will, for convenience, 

 be distinguished as pre-maximum, maximum, and post- 

 maximum. 



Determination of After-Effect by Electric 



Response 



Confining attention for the present to indications given 

 by the electric response, it is found that under continued 

 action of light the excitatory galvanometric negativity 

 increases to a maximum, after which there is a decline and 

 neutralisation. Fig. 149 gives the galvanographic record 

 of the electric response of the leaf-stalk of Bryophyllum 

 to light ; the up-curve represents increasing negativity 

 which, after attaining a maximum, undergoes neutralisa- 

 tion as seen in the down-curve. Fig. 150 exhibits the 

 various after-effects on sudden stoppage of light at three 

 different stages — before the attainment of maximum, at the 

 maximum, and after the maximum. Light is applied at 



arrow. 



Experiment 140. After-effect of pre-maximum stimula- 

 tion. — Continuous stimulation induces increasing galvano- 

 metric negativity. When stimulus is stopped at a, before 

 response reaches the maximum, the after-effect is a persist- 

 ence of excitatory galvanometric negativity, which carries 

 the response record higher up, after which recovery takes 

 place and the record returns to the zero-line of normal 

 equilibrium. The after-effect of pre-maximum stimulation 

 is thus a short-lived continuance of the response followed by 

 recovery (fig. 150). 



Experiment 141. After-effect at maximum. — In this case 

 the photic stimulation was continued till the attainment of 

 maximum, when light was suddenly removed at h. The 



