RESPONSE BY SENSATION 731 



of reviving memory-images. Long after every trace of the 

 primary stimulation has disappeared we can revive it by an 

 impulse of the will. Memory-impressions are often likened 

 to scars. Of this metaphor, however, it may be said, that 

 though, no doubt, when the blow is recent the smarting effect 

 will persist for some time, causing an ever-diminishing after- 

 sensation, yet, when the scar has healed, how could it, of 

 itself, reproduce the original sensation ? To do this, the 

 original excitation would require to be reproduced, in the 

 absence of the primary exciting cause. If, then, instead of 

 regarding it as a scar, we translate the original impression 

 into shades of light and darkness, we see that such a picture 

 was produced by different intensities of the primary stimulus 

 acting on the sensitive surface in other words, by means of 

 induced differential excitation. To bring back the picture we 

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

 same state of differential excitation as was at first induced 

 by it. 



Such a revival is possible, as already shown, under the 

 combined action of two different factors. It has been shown 

 that when an isotropic tissue is locally acted upon by stimulus, 

 the excitatory manifestation thus induced disappears after a 

 time. There is now nothing visible by which to distinguish 

 the stimulated from the unstimulated areas. In consequence 

 of this stimulation, however, there has been a transformation 

 of the molecular condition of the portions acted upon. The 

 tissue, which was originally isotropic, has now become an- 

 isotropic, by the impression of this latent image. On diffuse 

 stimulation, the differentially excitable structure will now 

 exhibit the latent image, by various forms of differential 

 excitation, of which some one particular manifestation will, 

 in the case of any given organ, be the most conspicuous. 



Thus, in a metallic plate containing latent positive and 

 negative chemical impressions, we shall obtain, on the appli- 

 cation of diffuse stimulus, corresponding positive and negative 

 galvanometric responses. In a phosphorescent plate, again, 

 a small area may be subjected to the action of light. On the 



