I I I I I 



L^ lX^~b < r~P^\i\ 

 L - M i \ l i I i 



THE SENSES 101 



The duration of the sensation depends upon the degree 

 of stimulation of the peripheral tactile organs. When 

 the stimuli are strong, as in 

 the contact of the finger with 

 the rough edge of a toothed 

 wheel, the sensations last a 

 considerable time, and thus 

 stimuli, following one another 



at about 500 times per Second, FlG - 43. Relationship of Sensation 

 r r , . n to Stimulus, with weak and strong 



cause a fusion of sensation. On stimuli> stirauli represented by 



the Other hand, if a Violin String vertical lines the strength being 



is made to vibrate against the ^^ed by their height. Sensa- 



, ... tions represented by the curves. 



nnger, the stimuli are weak, and 



the resulting sensation of short duration, and hence stimuli 

 may follow one another with greater rapidity, say to 1200 

 per second, and still be distinguished as separate sensations. 



II. Temperature Sense. 



Heat, like light, is physically a form of vibration of the 

 ether. The temperature sense depends upon the fact that 

 when heat is withdrawn from the body we have one kind 

 of sensation which we call cold, and when heat is added to 

 our body another sensation which we call hot. This depends 

 upon the temperature of our body in relationship to the 

 surroundings, and not merely on the temperature of sur- 

 rounding bodies. If three basins of water are taken, one 

 very hot, one very cold, and one of medium temperature, and 

 if a hand be placed, one in the very hot and one in the very 

 cold water for a short time, and then transferred to the basin 

 with water at a medium temperature, the water will feel hot 

 upon the hand that has been in the cold water, and cold to 

 the hand that has been in the hot water. (Experiment.) 



The rate at which heat is abstracted or added is the 

 governing factor in causing the sensation ; a sudden change 

 of temperature stimulates far more powerfully than a slow 

 change. For this reason the thermal conductivity of sub- 

 stances in contact with the skin has an influence upon the 

 sensation. If a piece of iron and a piece of flannel side by 

 side be touched, the first will feel cold, the second will not, 



