2 T. L. Bolton and Dotuia L. IVithcy 



would probably be about the average for all observers. It 

 depends very much upon the method and purposes of the experi- 

 ments what threshold is got. The chief difficulty in discriminat- 

 ing weight by pressure sense alone is to exclude muscular reac- 

 tions and so make the discrimination depend solely upon pressure 

 sense. The problem has not been solved with- complete satisfac- 

 tion. Professor Jastrow's^ apparatus, which is a small scale so 

 arranged that when weights are laid upon the scale pan the end 

 of the scale beam presses the tip of the forefinger from below, is 

 probably the best, and yet it is i:ot entirely' satisfactory. One can 

 not be sure that the finger does not react downward against the 

 scale beam. When weights are lifted pressure is always present 

 in one form or another, so that in practice, even under experi- 

 mental conditions, these two senses are not separate or fully sep- 

 arable. The distinction between pressure sense and muscle sense 

 is, then, one of theory rather than of fact. The first problem set 

 for us was to show how pressure discriminations of weight were 

 affected by different amounts of participation of muscular reac- 

 tions in the act of discrimination. 



For this purpose a form of pressure balance, new in some re- 

 spects, was constructed. The points held in mind in plann.ing the 

 balance were first to apply the pressure to the palm of the hand, 

 second to permit the hand to rest in the most comfortable posi- 

 tion and to fix it so as to shut out as far as possible muscular re- 

 actions, third to permit of different degrees of hand reaction 

 against the pressure stimulus and to make the application of the 

 pressure to exactly the same area of the palm, fourth to attain 

 rapidity in the exchange of weights for different pressures, and 

 to avoid the use oi so many weights. 



The arm is laid upon a table with the palm, thumb upward, 

 against two upright pieces two inches apart set into a base board 

 that rests upon the table. Against the back of the hand a sup- 

 port is brought which can be adjusted to any prismatic form as 

 well as to anv thickness of the hand. The balance consist'? of two 



^Amer. Jour. Psych., vol. ITT, p. .')4 ff. See also S'lnford. R.rpcrimcntal 

 Psychology, pp. 417-18. 



176 



