110 PHYSIOLOGY CHAP. 



they rest upon the motionless hand, as Weber first pointed out. 

 Our judgments are based less on tactile sensations than on the 

 complex kinaesthetic sensations by which muscular acts are 

 accompanied. 



According to Merkel's experiments, when weights of between 

 200 and 2000 grms. are estimated by counter-pressure on the scale- 

 pan of a balance, the liminal sensibility is about T \ th of the whole 

 weight if the finger remains at rest, while if the scale-pan is 

 compressed by voluntary movements it is about T Vth. The data 

 collected from various authors (Weber, Fechner, Jacoby, Gold- 

 scheider and Blecher, Langlois and Kichet), however, differ too 

 much for any positive value to attach to this experiment. 



According to G. E. Miiller and Schumann, in raising two 

 weights for purposes of comparison we generally employ the same 

 motor impulse for both weights, and our judgment is based 

 essentially upon the different rate at which they move, since from 

 previous experience we estimate the one that moves faster as 

 the lighter. 



According to Jacoby, the latent time of a movement is an 

 important factor in judging of weight. For a given weight a 

 given lost time corresponds with a certain intensity of innervation 

 effort, and if the effort remains constant the latent period is pro- 

 portional to the value of the weight. Another factor in the 

 discrimination of weights, according to Jacoby, is the facility 

 with which the movement can be stopped, which varies according 

 to the weight raised. 



The analysis of the factors in the judgment of weights made 

 by Z. Treves in his ergographic studies led him, on the other 

 hand, to hold that the object of the judgment is not so much the 

 weight in itself as the intensity of the effort, which is essentially 

 due to two factors, viz. the average muscular tension and its 

 duration. This element of judgment, however, is strictly dependent 

 on the central impulse of innervation, and varies indirectly to the 

 latter. So that the enormous variations and errors that generally 

 occur in this class of observations must be interpreted as the 

 indirect expression of the fluctuations of the motor impulse, which 

 is the expression of a neural act akin in its nature to attention, 

 and is highly unstable and insusceptible to direct control. With 

 the same weight the physical factors on which our judgment is 

 based may vary considerably with the variation of the impulse. 

 And the impulse, like all voluntary acts, fluctuates widely, even 

 when it is directed to a given end, with known conditions of 

 resistance. Treves proved this directly by showing that these 

 oscillations of impulse occur also in rhythmical movements with 

 a maximal voluntary impulse, and may, particularly in long-pro- 

 tracted work, have the effect of reducing the effort so much as to 

 mask the progressive muscular deterioration. 



