THE SPIRIT-RAPPERS 



nervous action, or, as the larger-sounding phrase 

 goes nowadays, of psychic phenomena, are growing 

 clearer. The arguments that may be drawn from 

 recent progress in this direction will scarcely appeal 

 to the believing hosts ; they may appear a little ten- 

 uous before the prodigious successes of Mrs. Piper 

 and her kind. But inasmuch as certain recent ob- 

 servations seem to have escaped the attention they 

 might merit, it may be worth while here to set 

 them forth. 



One of the earliest steps towards a rational psy- 

 chology lay in Weber's discovery of the curious 

 limitations of our senses. There seems to be a 

 threshold of sensation that we knew well of old ; a 

 body must have a certain weight ere we can appre- 

 ciate its existence by lifting it. But what Weber 

 found was that there is a kind of a "difference- 

 threshold" as well. If, for example, the smallest 

 difference in weight we may detect with the hand 

 be, say, between twelve and thirteen ounces, we 

 may appreciate the difference between double these 

 weights, but not between twenty-four and twenty- 

 five, or twenty-five and twenty-six ounces. A sim- 

 ilar relation seems to hold true for all the senses; 

 this threshold of difference represents always a con- 

 stant quantity, varying slightly with individuals 

 and with the state of fatigue. With a machine a 

 pair of scales, for example we find no such limita- 



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