TOUCH, ETC. 19 



observers, are still the most careful and reliable on record. 1 

 This method consists in the application to the skin of two 

 fine, but blunt points, separated from each other by a known 

 distance. The individual experimented upon should be blind- 

 folded, and the points applied to the skin simultaneously. 

 By carefully adjusting the distance between the points, a 

 limit will be reached where the two impressions upon the 

 surface are appreciated as one ; i. e., by gradually approxi- 

 mating them, the subject will suddenly feel both points as 

 one, when, an instant before, with the points a little farther 

 removed from each other, he distinctly felt two impressions. 

 This gives a very accurate measure of the delicacy of the 

 tactile, as distinguished from the general sensibility of dif- 

 ferent parts, and it has lately been found a most important 

 guide in the investigation of diseases of the nervous system 

 attended with partial anaesthesia of the surface. Of course, 

 the instrument used may be very simple (a pair of ordinary 

 dividers will answer), but it is convenient to have some ready 

 means of ascertaining the distances between the points. 

 An instrument, consisting simply of a pair of dividers, with 

 a graduated bar giving a measure of the separation of the 

 points, is the best, as it combines simplicity, convenience of 

 use, and portability. 2 This instrument is called the sesthe- 

 siometer. 



The experiments of Weber were made upon his own per- 

 son, and of course do not show the variations that may occur 

 in different individuals in health, a point of considerable im- 

 portance in estimating the extent of anaesthesia in disease. 

 His observations also showed some slight variations with the 

 direction of the line of the two points, but these are not 

 important. Valentin repeated the experiments of Weber, 

 and, in addition, took the maximum, minimum, and mean, in 



1 WEBER, Ortsinn in der ffaut, in WAGNER, Handworterbuch der Physiologic, 

 Braunschweig, 1846, Bd. iii., zweite Abtheilung, S. 524, et seq. 



2 The instrument described above is made by Messrs. G. Tiemann & Co., of 

 New York. 



