ADVANCED EXPERIMENTAL PHYSIOLOGY 225 



II. METHOD ADOPTED FOR TESTING TACTILE SPATIAL 

 DISCRIMINATION. 



If the skin be touched simultaneously by a pair of compasses the 

 points of which are separated, the distance that these must be apart in 

 order to appreciate the doubleness of the contact varies in different 

 parts of the skin. Special instruments possessing two points, the 

 distance between which can be adjusted, are called aesthesiometers. 



EXPERIMENT IV. Using either a pair of compasses (the points 

 of which are guarded with small pieces of cork) or some form of 

 aesthesiometer, note the smallest distance apart the two points must be 

 in order that the two contacts may be appreciated, in the case of the 

 tip of the tongue, tip of the middle finger, the palm of the hand, the 

 forehead, the back of the hand, and the back. 



SENSATIONS DERIVED FROM THE CONTRACTIONS OF MUSCLE 

 AND FROM THE ACCESSORY STRUCTURES OF MOVEMENT. 



Nerve endings exist in muscles, tendons and joints, and these are 

 liable to be affected by the contraction of muscle, and the tension of 

 the tissues adjoining thereby nitiated. It is convenient to speak of 

 the resulting sensations as brought about by the mediation of a 

 "muscular sense." 



EXPERIMENT I. Gun cartridges, filled with shot, are made up to 

 different weights. Test the power of discriminating a difference in 

 two weights when the absolute weights are small and where com- 

 paratively great. It will be found that when the weights are low 

 the perceptible differences between two weights is much smaller than 

 when the weights are great. 



This is in agreement with Weber's law, which lays down that the 

 just recognisable difference between two weights is not a constant 

 for any person, but a constant fraction of the weight lifted. Roughly 

 speaking, an increase of 10 per cent, on a weight is just recognisable. 



