RESPONSE OF RETINA TO STIMULUS OF LIGHT 429 



That, for its explanation, however, it is not necessary to 

 postulate the two processes of assimilation and dissimilation is 

 clearly seen from the fact that I have obtained an exactly 

 similar effect in inorganic substances, 

 such as silver bromide, a record of whose 

 response is seen in fig. 258. 



We next turn to what has been 

 designated as Type III., in which the 

 sequence of responses, owing to the 

 depressed condition of the tissue, is 

 reversed, the formula here being 

 (H 1 ), while the direct and after- 

 effects are represented by ..-(4 ). 



^This result was already obtained in the 

 sub-tonic tissue of the petiole of 

 cauliflower, a record of this being given 

 in fig. 259. I was able to detect similar 

 effects in a retina of Ophiocephalus, in 

 which response had become reversed 

 under the sub-tonicity due to long 

 isolation (fig. 260). These results will 

 explain the somewhat anomalous re- 

 sponse which Kuhne and Steiner 

 obtained with the isolated retinae of 

 certain fishes (fig. 261). 



Returning now from the question 

 of multiple excitations during and after 

 the exposure to light in prepared speci- 

 mens, where results must be modified 

 to an unknown extent by the effects 

 of isolation and preparation to that 

 of the responses of the intact eye, 

 where alone we may expect to obtain 

 the truly normal effects, we may attack 



the problem by means of visual sensation itself. Multiple 

 excitation, as the after-effect of strong luminous stimulation, 

 is here somewhat easy of demonstration. But its exhibition 



FIG. 258. After-effect 

 of Light on Silver 

 Bromide 



The thick line represents 

 response during light 

 . (half a minute's ex- 

 posure), and dotted 

 line the recovery dur- 

 ing darkness. Note 

 the terminal positive 

 twitch. 



