426 COMPARATIVE ELECTRO-PHYSIOLOGY 



light applied for three seconds gave as its after-effect multiple 

 responses which lasted for ten minutes, the average period of 

 each oscillation being twenty seconds. I have also obtained 

 such multiple responses from the eye of vigorous bull-frogs, 

 a photographic record of one of these results being given in 

 fig. 252. We shall also see at the end of this chapter that 

 it is quite easy to detect the occurrence of these multiple 

 responses in the intact human eye under stimulus of light. 



As we have seen that multiple response is the expression 

 of energy previously absorbed, and held latent in the tissue 

 for a time, and since, as has been stated, the retina itself 

 exhibits multiple response, it is easy to see that this organ, 

 on the cessation of light, will show after-effects. 



In my experiments on the effects 

 of light on vegetable tissues I found, 

 as has already been said, three 

 different types of direct- and after- 

 effect The first of these related to 

 those highly excitable tissues which 

 under continuous stimulation gave 



normal responses ( I h). In 



FIG> ^ 52 'r^ h i 0to f a & hic Re ~ such cases, if the stimulus was 



cord of Multiple Response 



of Retina of Frog under stopped on the attainment of 

 uous Action of Light maximum pos i t ivity the immediate 



after-effect was an increase of positivity. The formula 

 was thus ( !-*). In the third type, with sub-tonic 

 tissues, the sequence, under continuous stimulation, was 

 ( + I ), and on the stoppage of stimulation at maximum 

 negativity this negativity became suddenly augmented. The 

 formula here was thus (H ). In the second or inter- 

 mediate type, again, the formula of the direct and after- 

 effects was either ( | ) or ( ). 



I shall now discuss in some detail the various types of 

 after-effects met with in the retina, corresponding, as I shall 

 be able to show, with those met with in vegetables tissues. 

 After-effects like those of Type I., had not hitherto been 

 noticed in the retina for the reason that their demonstration 



