430 COMPARATIVE ELECTRO-PHYSIOLOGY 



during the action of light presents unusual experimental 

 difficulties. 



It is a well-known fact that if, after looking at a bright 

 object for some time, we close the eyes, we see the image 

 repeated many times. The occurrence of these after-images 

 is somewhat different under a transitory flash of illumination, 

 and with more persistent exposure to light. As the visual 

 sensation is to be regarded as the excitatory effect, there 

 will owing to the fact that this excitation must reach its 

 maximum some time after the cessation of an instantaneous 

 exposure be a persistence of the excitatory after-effect, with 



FIG. 259. FIG. 260. FIG. 261. 



FIGS. 259, 260, 261. Parallel Records of Responses given by Plant and 

 Retinae, during and after Illumination, illustrative of Type III. 

 (+ -.) 



Fig- 259. Response of petiole of cauliflower. Light was here cut 



off on attainment of maximum negativity. 



Fig. 260. Response of retina of Ophiocephalus fish when de- 

 pressed. 

 Fig. 261. Response of isolated retina of fish as observed by 



Kiihne and Steiner. 



N.B. The true excitatory negative response was by these 

 observers described as the ' positive variation. ' 



its corresponding visual sensation. Th^s is not so, however, 

 when the eye is closed after looking at a bright object for a 

 considerable time. In this case there is a positive rebound- 

 opposite to the true excitatory negative effect with con- 

 comitant sensation of darkness. The next rebound is negative, 

 giving rise to a repetition of the original sensation ; and these 

 alternating phases may be repeated many times. With the eyes 

 closed, the negative or luminous phases are the more prominent. 

 The same phenomenon may be observed in a somewhat 

 different manner when, after staring at a bright object, we 



