526 TOUCH. 



in water at J04- , and the whole of the other hand in water at 

 102, the cooler water will be thought the warmer : and water, 

 borne by a forefinger, will seem to scald the whole hand. 

 Minute differences are appreciated by plunging the whole hand 

 successively into two vessels of hot water, which are impercep- 

 tible to a single finger. 



Differences of temperature and weight are best ascertained 

 when the perceptions are not simultaneous, but successive : just 

 as is the case with differences in objects of taste, smell, and hear- 

 ing. If an acid and a sweet substance are applied to the tongue 

 with pencils in rapid succession, they are nicely distinguished : 

 but not if applied together. It is the same if two vials of odorous 

 fluids are applied to the nostrils ; and two notes are always better 

 distinguished if struck in succession than together. Vision is no 

 exception, because, although we compare two lines best when 

 placed side by side, we in fact do not view them simultaneously, 

 but in rapid succession ; since nothing is seen accurately unless 

 its image falls on the retina at the extremity of the optic axis. 



Persons differ greatly in their power of estimating weight, and 

 practice increases it considerably. Men accustomed to estimate 

 weights by poising them will distinguish a difference of a thirtieth 

 part in two bodies. They use the same hand for each weight in 

 instant succession. The intervention of a few seconds does not 

 prevent accuracy. A true estimate may be made although the 

 second weight is poised twenty seconds after the first ; but an in- 

 terval of forty seconds prevents accuracy. The sense of sight is 

 more accurate, for a well-practised eye will distinguish a differ- 

 ence of a hundredth part in the length of two lines : and the ear 

 surpasses the eye, for a well-practised musical ear will distinguish 

 between two sounds differing only ^^, the number of vibrations 

 being calculated that are made by the sounding bodies in a given 

 time. If two lines differ only -jJ- r in length, the difference may be 

 perceived although the one is looked at fifty or sixty seconds 

 after the other. If they differ ^ 1 T , an interval of thirty-five se- 

 conds may elapse. If they differ ^, an interval of three seconds 

 is the longest compatible with accurate judgment.? 



Not only does touch appear too general an expression for the 



p De .Pulsu, Resorptione, Auditu et Tactu* Annotatwnes Anatomdcce et Physio- 

 logicte, auctore Henrico Ernesto Weber. Lipsiae, 1834. ; and Dr. Graves '.s 

 Analysis of it, in the Dublin Journal vf Medical Science, March 1836. 



