THE RECEPTOR SYSTEM 179 



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 com- 

 pressed. 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 the so-called psychophysical law (p. 170) is based, were first 

 observed. The smallest perceptible difference of pressure recog- 

 nizable when touch alone is used, is about , 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 acting as a stimulus. The rate only holds good, how- 

 ever, for a certain mean range of pressure; it is not true for very 

 small or very great pressures. The experimental difficulties in 

 determining the question are considerable; muscular sensations 

 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 elimi- 

 nated. Considerable individual variations are also observed, the 

 least perceptible difference not being the same in all persons. 



The Localizing Power of the Skin. 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 something (local sign) 

 besides intensity by which we can distinguish them; some sensa- 

 tion quality must be present enabling us to tell from one another 

 two precisely similar contacts of an external object when ap- 

 plied, say, to the tips of the fore and ring fingers respectively. 

 The accuracy of the localizing power is not nearly so great as in 

 the eye and varies widely in different skin regions; it may be 

 measured by observing the least distance which must separate 

 two objects (as the blunted points of a pair of compasses) in order 

 that they may be felt as two. The following table illustrates 

 some of the differences observed: 



