730 COMPARATIVE ELECTRO-PHYSIOLOGY 



by the polar action of currents. It has been shown that 

 the positive response is short-lived, whereas the negative is 

 relatively more persistent, its persistence increasing with the 

 intensity of the response. Now, by means of the Sensimeter 

 (fig. 405), we can apply a series of stimuli of measured 

 intensity in such a way as to induce the neutral sensation 

 which is neither pleasurable nor painful. The frequency of 

 this stimulation is so adjusted as to appear all but continuous. 

 If we now render the excited point moderately anode, and 

 thus reduce its excitability, the neutral will be converted to 

 the positive tone, and the sensation, moreover, will be rendered 

 strikingly discrete. If, next, the excitability of the stimu- 

 lated point be enhanced by the application of moderate 

 kathode, the neutral sensation will become converted into 

 painful, becoming, further, fused and continuous. 



It will thus be seen that in the determination of sensation 

 the internal plays as important a part as the external. By 

 the peculiar molecular disposition of the nerve, it is indeed 

 possible, as we have seen, to convert one quality of sensation 

 into another, and such dispositions are to a greater or less ex- 

 tent under the control of the will. It is not external circum- 

 stances, then, which are the dominant factor psychologically, 

 for the impression created by these is capable of indefinite 

 modification in any direction, by the action of habitual 

 induced dispositions. The reader will see for himself what 

 illimitable possibilities are opened up by the line of thought 

 here suggested. 



The last subject to be reviewed is the phenomenon ol 

 memory, which is an after-effect of stimulus. The after- 

 effect of strong stimulus is in general more persistent than 

 that of feeble. Similarly, the memory of a strong sensation 

 is more enduring than of a weak one. Very strong stimulus, 

 again, gives rise, as we have seen, to multiple responses. In 

 the retina these are perceived as multiple after-images, which 

 sometimes appear to be renewed spontaneously. This fact 

 will often be found a sufficient explanation of visual phantoms 

 and hallucinations. This, however, is not the usual method 



