NEW CONCEPTIONS IN SCIENCE 



tions. There is no difficulty in weighing even frac- 

 tions between, say, twenty-four and twenty-five 

 ounces. The report of well-made machines is al- 

 ways identical, and they never weary. 



But before a given sensation, whether of light or 

 heat, touch or smell, taste or sound, can affect our 

 organs of perception, it must gain a certain intensi- 

 ty, and between varying degrees of this intensity of 

 sensation we can, by natural means, measure only 

 proportionate quantities. 



This limit of capabilities seems to be, at least with 

 regard to primitive sensations, rigidly set. Thus, 

 for example, if a sensation occur too rapidly, as 

 when a note is struck more than ten or eleven 

 times a second, it appears to us continuous. A 

 series of dots less than a thousandth of an inch 

 apart appear to us as a continuous line. These 

 limits appear to be fixed in the nature of nervous 

 action itself, because, for example, although we can 

 by mechanical means produce a muscular contrac- 

 tion of much greater frequency, we cannot count 

 out loud more than ten or eleven a second ; and this 

 is equally true if we try to count silently. It is the 

 limit of speed of mental action. 



The rate at which a sensation travels along a 

 nerve up to the brain or back again is compara- 

 tively very slow. Light and electricity, under or- 

 dinary circumstances, travel a hundred and eighty - 



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