68 HERMANN VON HELMHOLTZ 



mission in nerve is by no means so enormous as that of light 

 or electricity may be .conjectured from du Bois' discovery that 

 electricity is developed in nerve during the propagation of a 

 message or stimulus; whence we must conclude that the 

 material particles of the nerve are altered in position during 

 the process. Transmission is in this case, as a matter of fact, 

 extremely slow, slower than that of sound. The reason why 

 the time occupied by this propagation seems to us so infini- 

 tesimal lies in the fact that we are unable to perceive more 

 quickly than our nervous system can act, and thus the intervals 

 required for its operations appear to us imperceptibly small. 

 The inaccuracy of our time-perceptions when based upon a 

 comparison between our perceptions by two different sense- 

 organs has recently been demonstrated in the most striking 

 manner. Astronomers vary in their estimation of the moment 

 at which a star crosses the web of their telescopes by more than 

 a whole second, while the estimates of any individual taken by 

 himself agree within one-tenth of a second if frequently re- 

 peated. Still more surprising is the difficulty of determining 

 whether the beats of two gently-ticking watches coincide, or 

 fall between each other, if held to either ear, while nothing 

 is easier than the same determination if both are held to the 

 same ear. I picture the matter to myself in this way: two 

 perceptions of different organs can only be estimated as regards 

 their time-relations, when there is a sufficient interval between 

 to reflect "now you have perceived one, but not as yet the 

 other". Our thought is not so rapid as we usually believe, 

 as I have proved from my experiment of taking an electric 

 shock at any point on my skin, and then trying to move my 

 hand as quickly as might be, measuring the time between 

 the shock and the first commencement of the movement. With 

 great attention, when the will is ready to act the instant it 

 receives the message, the message is only delayed about one- 

 tenth of a second in the brain, and is carried on with such 

 mechanical regularity to the motor nerve as a motor stimulus, 

 that I think the delay must here be caused only by the neces- 

 sary mechanical molecular processes. When, on the contrary, 

 the attention is fatigued, so that on receiving the message it 

 becomes necessary to think what is to be done, a much longer 

 and quite irregular interval is required. 



