LESSON XX. TOUCH. 



NOTE. In many experiments on touch, taste, and 

 smell, two students should work together. One (A) 

 should close his eyes, or be otherwise prevented from 

 seeing what is going on, the other (B) should apply the 

 tests to him, and record the results. At the end or mid- 

 way in the experiment they change roles. 



1. Threshold for two points (tactile spatial dis- 

 crimination). It will be sufficient for the student to 

 determine the threshold roughly, within a centimetre 

 in the case of the fore-arm. A sits with the fore-arm 

 bare, and extended on the table, palm uppermost; eyes 

 being shut. 



B has a pair of compasses and a millimetre scale ; he 

 separates the points 3-5 cm. and touches B's arm either 

 with one point or with both points. The two points must 

 be applied with equal pressure, and both at the same 

 moment, and they should be applied length-ways to the 

 arm. A says whether he feels one point, or two points, 

 or is doubtful whether it is one or two ; B writes down 

 after each observation whether he applied one point or 

 two points, if two their distance apart, and ^4's state- 

 ment. 



If A is right three times out of four with the single 

 point and the two points, B without telling A puts the 

 points of the compasses at 3-0 cm., and makes similar 

 observations; then with the points 2-5 cm. apart, and so 

 on. Probably A will be unable to distinguish two points 

 when they are 2-0 or 2-5 cm. apart. The minimal dis- 



