SENSE OF TOUCH. 



pair of compasses be separated for that dis- 

 tance and placed simultaneously on these 

 parts (not too forcibly if the points be sharp), 

 the skin has the sensation of being touched by 

 one point only. The back of the hand can 

 distinguish points an inch apart, the palm of 

 the hand points less than half an inch, and the 

 end of the forefinger can distinguish points as 

 near together as one-twelfth of an inch. The 

 most sensitive part of the body in this respect 

 is the tip of the tongue, which can appreciate 

 the twenty-fourth part of an inch. 



Substances like sandstone, which have very 

 small prominences at a distance from one 

 another of less than the twenty-fourth part of 

 an inch, cannot have their projecting parts 

 distinguished by the most sensitive skin ; but 

 that they have such parts can be felt by the 

 appreciation of what we call roughness that 

 is, the capacity for producing a quantity of 

 friction which is great in proportion to the sur- 

 face and pressure. 



But there are some substances, like glass, 

 glazed pottery, and polished metal which, even 

 to the most sensitive parts of our touch-organ 

 the skin seem to have an absolutely smooth 

 and continuous surface quite free from projec- 

 tions with intervening spaces. 



The microscope once more shows that the 

 apparent smoothness and continuity are only 

 relative. Polished metal surfaces show the 



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