MEMORY 683 



indifferent areas there was no effect detected in the galvano- 

 meter. But as soon as the exploring electrode rested on the 

 area B, the latent enhancement of excitability there showed 

 itself by a sudden responsive up-movement of the galvano- 

 meter. When the electrode again passed over B and reached 

 an indifferent area, A,, response disappeared. But when it 

 reached c, with its depressed excitability, there was another 

 responsive movement, this time in the reversed or down 

 direction. It is thus seen that the impress made by the 



FIG. 406. Revival of latent Image in Metal 



action of stimulus, though it remain latent and invisible, can 

 be revived by the impact of a fresh excitatory impulse 

 (fig. 406). 



Again, this revival of the latent image by a subsequent 

 stimulation maybe exemplified in a simpler and more striking 

 way. We take a card and coat it with some so-called phos- 

 phorescent material, such as luminous paint. This is kept 

 a long time in the dark, till the whole is reduced to a uniform 

 A condition. From a previous experiment we have deter- 

 mined what is that duration of exposure, T, to a given 



