548 PHYSIOLOGY 



WEBER'S LAW. The smallest increment or decrement of stimu- 

 lus which determines a perceptible difference of sensation must, 

 according to Weber's law, always bear the same ratio to the whole 

 stimulus. In measuring such differences it is best to apply the stimulus 

 successively to the same surface of the skin rather than simultaneously 

 to adjoining areas. The time interval between two successive stimuli 

 should not be more than five seconds and the duration of the stimuli 

 should be equal. Weber found that in the terminal phalanx of the 

 finger the minimal perceptible difference was about one-thirtieth, 

 but the ratio was not the same for all regions of the skin nor 

 for all individuals. The following represents the liminal difference 

 in various skin regions : 



Forehead, lips, and cheeks . . . l/30th to l/40th 

 Back of fore-arm, of leg, and of thigh ;\ 



back of hand, and first and second -l/lOth to l/20th 



phalanx of finger, &c. . . . J 

 All parts of the foot, surface of leg, and 



thigh. ..... more than l/10th 



THE SPATIAL QUALITY OF TOUCH. DISCRIMINATION. If 

 any part of the skin be stimulated the subject of the experiment 

 can tell at once the exact situation of the excited spot. If two points 

 be stimulated simultaneously excitation is perceived as double, i.e. as 

 proceeding from two points, provided the distance between the points 

 exceeds a certain amount, varying in different parts of the body. The 

 power of discrimination, i.e. of judging whether a stimulus is single or 

 double, can be tested by arming the points of a pair of compasses 

 with small pieces of cork and then seeing how far apart the points 

 must be when pressed on the skin in order that the stimulus may 

 be perceived as double. The following Table represents this distance 

 for various regions of the body : 



DISTANCE IN MM. 



Skin region mm. 



Tip of tongue . . . . . .1-1 



Volar surface of finger tip .... 2-3 



Dorsum of third phalanx .... 6-8 



Palm of hand . . . . .11-3 



Back of hand 31-0 



Back of neck ...... 54 '0 



Middle of back, upper arm, and thigh . . 67-1 



When touch spots are sought out for stimulation with the points 

 of a compass, the distance at which the excitation is perceived as 

 double is much diminished, as is shown by the following Table of 

 distances for the touch spots in millimetres : 



