208 LIFE AND WORK OF SIR JAGAD1S C. ROSE 



liable to recur spontaneously, without the action of the will or 

 even in spite of it, it follows that any single impression, when 

 very intense, may become dominant and persist in automatic 

 recurrence. Instances of this are only too familiar. 



A more interesting form of memory is the revival of an 

 impression, the after-effect of which has faded out. Here 

 we find that when no tangible effect of the impression remains, 

 it may still be recalled by an effort or impulse of the will. It 

 is clear that such a revival of impression can only take place 

 by bringing about the original condition of excitation ; in other 

 words repeating the effect of original stimulus in its complete 

 absence. 



As a concrete example we may take the visual impression 

 of a bright cross against a dark background. Under primary 

 stimulus, it is clear that we have in the sensory field two areas 

 under differential excitation. The one the excited area 

 in the form of a cross ; the other outside this, remaining 

 unexcited. The image of the cross is therefore due to the 

 differential excitation of a definite region in the sensory field. 

 It is therefore obvious that in order to revive the picture we have 

 to reproduce, in the absence of the primary stimulus, the same 

 state of differential excitation as was originally induced. 



Bose next shows that by the shock of stimulus, the 

 surface acted on undergoes a molecular distortion from 

 which there is slow recovery ; but the recovery is never 

 quite complete. Traces are left of the impression made 

 by the stimulus. These, though invisible, remain latent, 

 and beyond ordinary means of detection. Under certain 

 conditions, however, this invisible script could once more 

 be rendered conspicuous. Bose was able to form impressions 

 on metallic surfaces, of which no sign whatever was visible 

 even under the microscope. But when the plate was 

 subjected to a diffused shock, these latent images were 

 found revived. Similarly all the impressions made on 

 the sensory surface by the localised action of stimulus 

 remain dormant as a latent memory-image. The localised 

 effect of this primary stimulus is to render the affected 

 part of the tissue more excitable or a better conductor 

 of excitation. Under the action of any form of diffuse 



