THE SENSE OF PRESSURE. 837 



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 J 1 ^ to -^j- of the weight estimated, while the skin of the head, the fingers, 

 and the forearm requires an increase of ^ to -fa 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 only 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 

 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 ; accord- 

 ing 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. This result, however, cannot 

 always, nor indeed usually, be verified. The simultaneous excitement of other 

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

 but one warm and the other cold, are laid upon the hand or the forehead, the 

 cold one appears to be much the heavier. 



There is a sensation of after-pressure depending for its strength on the 

 amount of the weight and the length of time this weight has been applied. 

 In fact, this after-sensation may produce a striking effect on consciousness, 

 a familiar example of which is the persistence of the sense of pressure of 

 the hat-band after the head-covering is removed. Even light weights leave 

 an after-sensation, and, in order to be perceived as separate, must be applied at 

 intervals of not less than -^^ to ^-J-Q- of a second. It is said that when the 

 finger is applied to the rim of a rotating wheel provided with blunt teeth, the 

 separate teeth are no longer felt, and the margin seems smooth, when the con- 

 tacts succeed each other at the rate of 500 to 600 in a second. 3 Vibrations of 



1 " Tastsinn und Gemeingefiihl," Wagner's Handwb'rterbuch der Physiologic, 1846. 



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



8 Landois and Stirling: Human Physiology, 1886. 



