THE SENSE OF PRESSURE. :VX\ 



a single grain added to it will arouse no difference of sensation, an increment 

 of 10 grains being necessary in order that one weight may appear heavier 

 than the other. This fact is the basis for Weber's law of the relation between 

 stimulus and sensation; this law may be formulated as follows: The amount 

 of stimulus necessary to provoke a perceptible increase of sensation always bears 

 the same ratio to the amount of stimulus already applied. This law is found 

 to be only approximately correct, especially when very small and very large 

 weights are compared. Fechner attempted to express more exactly the relation 

 between the intensity of stimulus and sensation in his "psycho-physical law," 

 thus : The intensity of sensation varies with the logarithm of the stimulus. In 

 other words, the sensation increases in arithmetical progression, while the 

 stimulus increases in geometrical progression. With moderate weights a 

 difference of pressure is perceptible when the ratio of increase is smaller than 

 when either very small or very large weights are used ; that is, sensitiveness 

 to pressure-change is keenest under moderate stimulation. 



It is said that the forehead, the lips, and the temples appreciate an increase 

 of ^ to ^ of the weight estimated, while the skin of the head, the fingers, 

 and the forearm requires an increase of ^V to -^ for its perception. In this 

 as in other kinds of sensation it is the difference, or variation of intensity, of 

 the sensation of which the mind takes particular cognizance. One touch- 

 sensation is more acutely perceived when contrasted with another than when 

 felt alone. Weber 1 found the discrimination of pressure-differences to be 

 finer when two weights were laid in rapid succession on the same skin-area 

 than when the weights were applied either simultaneously or successively to 

 different parts. If a finger be dipped in a cup of mercury or of water having 

 the same temperature as the skin, the pressure will be marked onlv at the 

 margin between the air and the fluid, and if the finger be moved up and 

 down it will seem as if a ring were being slid back and forth upon it. The 

 constant pressure of the mercury upon the submerged finger is not felt. The 

 fingers are particularly sensitive to intermittent variations of pressure — a 

 facility the use of which is manifest when the function of these parts is 

 considered. 



Two weights, in being tested, should press upon equal areas of skin ; 

 according to Weber, 2 if two equal weights have different superficial expanse, 

 that which touches the larger skin-surface, and thereby excites the greater 

 number of touch-nerves, will appear to be the heavier. The important part 

 played by judgment and mental inference in such experiments is shown l»y 

 the facts that when it is sought to compare weights by lifting them and with 

 the aid of sight, the smaller of two equal weights seems to l>e the heavier; 

 and of two objects having the same size and weight, that which appears to 

 be the smaller seems heavier. 3 The simultaneous excitement of other sen- 

 sations may modify that of pressure ; thus, when two coins of equal weight, 



1 '' Tastsinn und Gemeingefiihl," Wagners Handworterbuch der Physiologic, 1846. 



2 Quoted in Hermann's Handbuch der Physiologic, Bd. iii. '2, S. 336. 



3 Dressier : American Journal of Psychology, 1894, vol. vi. No. 3. 



