TOUCH. 559 



back of the forearm, where a weight of 2 milligr. (. 03 

 grain) pressing on an area of 9 sq. millim. (.0139 sq. inch) 

 can be felt. On the front of the forearm 3 milligr. (.036 

 grain) can be similarly felt, and on the front of the fore- 

 finger 5 to 15 milligr. (.07-0.23 grain). 



In order that the sense of touch may be excited neigh- 

 boring skin areas must be differently pressed; when we lay 

 the hand on a table this is secured by the inequalities of 

 the skin, which prevent end organs, lying near together, 

 from being equally compressed. When, however, the hand 

 is immersed in a liquid, as mercury, which fits into all its 

 inequalities and presses with practically the same weight 

 on all neighboring immersed areas, the sense of pressure is 

 only felt at a line along the surface, where the immersed 

 and non-immersed parts of the skin meet. 



It was in connection with the tactile sense that the facts 

 on which so-called psycho-physical law (p. 473) is based, 

 were first observed. The smallest perceptible difference of 

 pressure recognizable when touch alone is used, is about 

 J; i.e. we can just tell a weight of 20 grams (310 grains) 

 from one of 30 (465 grains) or of 40 grams (620 grains) 

 from one of 60 (930 grains); the change which can just be 

 recognized being thus the same fraction of that already act- 

 ing as a stimulus. The ratio only holds good, however, 

 for a certain mean range of pressures; and its existence for 

 any has lately been denied. The experimental difficulties in 

 determining the question are considerable; muscular sensa- 

 tions must be rigidly excluded; the time elapsing between 

 laying the different weights on the skin must always be 

 equal; the same region and area of the skin must be used; 

 the weights must have the same temperature; and fatigue of 

 the organs must be eliminated. Considerable individual 

 variations are also observed, the least perceptible difference 

 not being the same in all persons. 



The Localizing Power of the Sliin. When the eyes are 

 closed and a point of the skin is touched we can with some 

 accuracy indicate the region stimulated; although tactile 

 feelings are in general characters alike, they differ in some- 

 thing (local sign] besides intensity by which we can distin- 



