RELATION OF SENSATION TO STIMULUS 



539 



0-25 gramme. It is evident that the larger the area of the skin 

 stimulated the greater will be the minimal weight required, since this 

 has to be distributed over all the nerve-endings contained in the 

 area of skin which is subjected to pressure, and the larger the area the 

 smaller will be the stimulus applied to each nerve-ending. The follow- 

 ing figures were obtained by von Frey on different regions of the skin : 



Stimulated surface 21'2 mm. 2 



Volar side of wrist (Subject K) 

 Ball of thumb (Subject K) 

 Volar side of wrist (Subject F) 



Stimulated surface 35 mm. 2 



Ball of thumb (Subject F) 

 Tip of finger (Subject F) . 

 Volar sid'j of wrist (Subject F) 



Rate of loading 1/7 gnu. per second. 

 Threshold value of stimulus per mm." 



0-024 - 0-038 grm. 

 >0-189 - 0-039 

 . >0-236 - 0-055 



Rate of loading 3 grm. per second 

 Threshold value of stimulus. 



0-200 - 0-045 grm. 

 0-170 - 0-028 

 0-640 - 0-028 



It will be noticed that when the excited surface is small much 

 greater variations are found from spot to spot in the size of the minimum 

 stimulus. This is probably connected with the fact that the sense- 

 organs for pressure are distributed in points or spots over the surface 

 of the skin, so that when the stimulated surface is small the threshold 

 value of the stimulus will be determined by the number of the tactile 

 spots which are included in the stimulated area. The minimal effective 

 stimuli in the case of the other senses have been determined as follows : 



(a) HEARING. A musical tone can be heard when the varia- 

 tions of pressure in the air amount to -00000059 mm. Hg. with an 

 amplitude of vibrations of -000000066 mm. It has been calculated 

 that the intensity of the work performed on the drum of the ear by 

 such a minimal tone represents an average of 5-1 X 10~ ' ergs. In 

 the case of noise the amount of energy required to produce a minimum 

 sensation is still smaller. A distinct sound was heard when a weight 

 6-7 milligrammes was allowed to fall a distance of 1-2 mm. on to an 

 iron plate at a distance of 500 mm. 



(6) Y ISION - The minimum intensity of light necessary to arouse 

 sensation in a dark-adapted eye is, according to Aubert, equal to about 

 one three-hundredth of the intensity of the light reflected from a 

 piece of white paper which is being lit by the light of the full moon. 

 The amount of energy involved in such a stimulus is much smaller 

 even than that determining a sensation of touch or hearing. 



WEBER'S LAW 



It is an interesting question how far the strength of sensation 

 may be regarded as an index to the strength of stimulus. Although 

 it is easy to measure in absolute terms the intensity of a stimulus, i.e. 

 of a purely physical process, there is no means by which we can 



