

PSYCHO-PHYSICS 207 



then slowly disappears, after which the second (perceived by 

 the other eye) shoots out suddenly in a direction athwart 

 the first. This alternation proceeds for a long time, and 

 produces the curious effect of two luminous blades crossing 

 and recrossing each other. These alternating after-images 

 persist for a very long period. The recurrent after-image 

 is very distinct at the beginning, and becomes fainter at 

 each repetition ; a time comes when it is difficult to tell 

 whether the image seen is the objective after-effect due 

 to strain caused by stimulus or merely an after-effect of 

 memory. In fact there is no line of demarcation between 

 the two. One simply merges into the other. 



The visual impressions and their recurrence often persist for 

 a very long time. It usually happens that owing to weariness 

 the recurrent images disappear ; but in some instances, long 

 after this apparent disappearance, they will spontaneously 

 reappear at the most unexpected moments. In one instance 

 the recurrence was observed in a dream about three weeks 

 after the impression was made. It thus appears that in addition 

 to the images impressed on the retina of which we are conscious, 

 there are many others which are imprinted without our 

 knowledge. We fail to notice them because our attention 

 is directed to something else. But at a subsequent period, 

 when the mind is in a passive state, these impressions may 

 suddenly revive owing to the phenomenon of recurrence. This 

 observation may afford an explanation of some of the pheno- 

 mena connected with ocular phantoms and hallucinations.' 



He then investigates certain other phenomena connected 

 with ' Memory/ 



Of that mental revival of past experience -which we call 

 memory, we may notice two different types. One is the spon- 

 taneous and recurrent revival of some strong impression from 

 which we cannot escape : in the second case the primary im- 

 pression has faded away, and it is only after an effort that we 

 succeed in reviving the latent image. As regards spontaneous or 

 recurrent revival of impression, I have shown elsewhere that in 

 living tissues a very intense stimulus gives rise not to a single, but 

 to multiple or repeated responses. Since an intense excitation is 



