SEC. 2. TACTILE SENSATIONS. 



Sensations of Pressure. 



As with visual, so with tactile and indeed with all other sen- 

 sations, the intensity of the sensation maintains that general 

 relation to the intensity of the stimulus which we spoke of at 

 p. 521 as being formulated under Weber's law. We can distinguish 

 the difference of pressure between one and two grammes as readily 

 as we can that between ten and twenty or one hundred and two 

 hundred. 



When two sensations follow each other in the same spot at a 

 sufficiently short interval they are fused into one; thus, if the 

 finger be brought to bear lightly on a rotating card having a series 

 of holes in it, the holes cease to be felt as such when they follow 

 each other at a rapidity of about 1500 in a second. The vibrations 

 of a cord cease to be appreciable by touch when they reach the 

 same rapidity. When sensations are generated at points of the 

 skin too close together they become fused into one ; but to this 

 point we shall return presently. 



The sensation caused by pressure is at its maximum soon after 

 its beginning, and thenceforward diminishes. The more suddenly 

 the pressure is increased, the greater the sensation; and if the 

 increase be sufficiently gradual, even very great pressure may be 

 applied without giving rise to any sensation. A sensation in any 

 spot is increased by contrast when the surrounding areas are not 

 subject to pressure. Thus if the finger be dipped into mercury the 

 pressure will be felt most at the surface of the fluid ; and if the 

 finger be drawn up and down, the sensation caused will be that of 

 a ring moving along the finger. 



