HERMANN VON HELMHOLTZ 



we will to move the forefinger, the commands of the 

 will appear to be instantaneously carried into effect. 

 This arises, however, from our limited appreciation 

 of shorter intervals of time than say the one-tenth of 

 a second, so that if less time elapsed between the 

 moment of touching the finger and the moment of 

 the sensation, the two events appear to be one. It 

 was necessary, therefore, to have some means of re- 

 cording the duration of short periods of time, such 

 as the short period assumed to exist between the 

 moment of irritating a nerve and the moment of 

 the contraction of the muscle supplied by it. 



Manifestly the subject could be investigated most 

 easily by using the muscle of the frog and the motor 

 nerve passing to it. Helmholtz made use of the 

 graphic method of recording the contraction of 

 muscle, already invented by him in the form of the 

 well-known myograph. Just about this time elec- 

 trical mechanisms had been introduced into practical 

 physiology by Du Bois Reymond, and Helmholtz 

 made good use of these appliances. He first em- 

 ployed, as a means of recording the beginning and the 

 end of the phenomenon, a method devised by Pouillet, 

 which consisted in noting the instant of the move- 

 ment of the needle of a galvanometer when an in- 

 stantaneous current was sent through the instrument, 

 taking it for granted that the duration of the current 

 itself was practically nothing. Suppose, then, that an 

 arrangement was made by which a nerve could be 

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