684 COMPARATIVE ELECTRO-PHYSIOLOGY 



intensity of light, which will evoke a luminous or phosphor- 

 escent response. A stencilled pattern is now placed on the 

 prepared card, and the whole is exposed to light for the time T. 

 On now cutting off the light and removing the stencil a 

 luminous pattern is seen, which is the primary response. 

 This impression slowly fades out. But the cardboard now 

 contains a latent image, whose revival will be analogous to 

 that of memory. The stimulated areas which have now 

 ceased to respond are still, in virtue of previous stimulation, 

 in the B condition, which is more excitable than the indif- 

 ferent A. A feeble diffuse stimulus should now, by its 

 differential action, prove efficient to revive the latent image. 

 We now expose the whole card to diffuse stimulation of light, 

 of a duration shorter than T. The excitation of the indifferent 

 background will by this means be ineffective, whereas it will 

 be effective wherever the image proper has been impressed. 

 We shall, therefore, obtain a revival of the positive image- 

 that is to say, an image of the same kind as the original. 



An interesting case occurs here, showing the theoretical 

 possibility of obtaining a negative or reversed 'memory 

 image.' The possibility of this will be understood, from an 

 inspection of the characteristic curve. We saw that in the 

 region B the substance rises in excitability. But in the 

 region of D and E, where the maximum molecular distortion 

 has already been reached in consequence of over-stimulation, 

 fatigue changes are induced, by which the excitability is 

 depressed below the normal. It follows from what has 

 already been said that an impressed image of this character 

 will be revivable, under subsequent diffuse stimulation, but as 

 a negative, or reversed memory-image. 



I shall now describe a psycho-physiological version of this 

 experiment. Let the observer stare at the incandescent fila- 

 ment of an electric lamp, preferably with one eye, say the 

 right, the left being kept closed all the time. The right eye 

 is next closed, and is further covered by the hand. Multiple 

 after-images will now be seen for some time, till the impression 

 seems to have completely disappeared. No trace of the 



