CHAP. XIV.] EXPERIMENTS OF WEBER. . 429 



exact impressions from the vibrations of the air, and so to be a means whereby 

 the animal may be informed of the distance and figure of the neighbouring 

 objects, which reflect or otherwise modify the undulations of the surrounding 

 medium. It is very probable that hearing also may be concerned in this 

 power. 



In men, as compared with animals, the sense of touch is exten- 

 sively diffused; but very interesting differences in its intensity are 

 observable in different parts of the surface, which have been espe- 

 cially illustrated by the experiments of Weber. 



These consisted in placing the two points of a pair of compasses, 

 blunted with sealing-wax, at different distances asunder, and in 

 various directions, upon different parts of the skin of an individual, 

 who was not permitted to see the bodies in contact with him. It 

 was then found, that the smallest distance at which the contact 

 can be distinguished to be double, varies in different parts between 

 the thirty-sixth of an inch and three inches ; and this seems a 

 happy criterion of the acuteness of the sense. We recognize a 

 double impression on very sensible parts of the skin, though the 

 points are very near each other ; while, in parts of obtuse sensi- 

 bility, the impression is of a single point, although they may be, 

 in reality, far asunder. 



In many parts we perceive the distance and situation of two 

 points more distinctly when placed transversely than when placed 

 longitudinally, and vice versa. For example, in the middle of the 

 arm or fore-arm, points are separately felt at a distance of two 

 inches, if placed crosswise ; but scarcely so at a distance of three, 

 if directed lengthwise to the limb. 



Two points at a fixed distance apart feel as if more widely sepa- 

 rated when placed on a very sensitive part, than when touching a 

 surface of blunter sensibility. This may be easily shewn by draw- 

 ing them over regions differently endowed ; they will seem to open 

 as they approach parts acutely sensible, and vice versa. 



If contact be more forcibly made by one of the points than by 

 the other, the feebler ceases to be distinguished : the stronger im- 

 pression having a tendency to obscure the weaker, in proportion to 

 its excess of intensity. 



Two points at a fixed distance are distinguished more clearly 

 when brought into contact with surfaces varying in structure and 

 use, than when applied to the same surface, as, for example, on 

 the internal and external surface of the lips, or the front and back 

 of the finger. 



Of the extremities, the least sensitive parts are the middle regions 



