PROFESSOR AT KONIGSBERG 71 



the one, perfected by Werner Siemens, the intervals of time 

 are measured in terms of intervals of space ; in the other, the 

 mechanical effect produced by a force of known intensity 

 during the interval is measured, and the time of action is 

 subsequently calculated from it. This second method, dis- 

 covered by Pouillet in 1844, consisted in the measurement of 

 very small intervals of time by the deflection of a galvanometer 

 needle after the passage of a very short electric current, and was 

 elaborated by Helmholtz for physiological purposes. He made 

 the electro-magnetic determinations by means of a mirror attached 

 to the magnet, as introduced by Gauss and Weber, and estab- 

 lished the constant factor necessary to convert the differences of 

 oscillation into the corresponding time-differences, by a strictly 

 accurate method. 



Starting from the simplest cases, he next attacked the more 

 complicated problem, whether there is any appreciable lost 

 time during the propagation of a message from the remote 

 ends of the sensory cutaneous nerves, or from the nerve- 

 endings in the sense-organs, to the brain, or from the brain 

 to the muscles via the motor nerve trunks. He first deter- 

 mined in the frog, by a long series of most delicate experi- 

 ments, that when the muscle or nerve of an animal is excited 

 by an instantaneous electrical shock, a short time, about 

 one-hundredth of a second, elapses, during which the elastic 

 tension of the muscle is not appreciably altered the so-called 

 latent period of excitation after which the muscular tension 

 gradually rises to a maximum, and then as gradually falls 

 again. If, further, two different points of a motor nerve 

 are excited by an instantaneous stimulus, and if the magni- 

 tude of the excitation is alike for both, the time-relations of 

 the corresponding contractions will also be the same, but 

 the total effect makes its appearance later when the stimulus 

 is applied to a more distant point of the nerve. The 

 transmission of excitation through nerve to muscle there- 

 fore occupies a measurable time, and its speed is actually 

 found to be more than ten times less than the velocity of 

 sound in air. ' Happily,' says Helmholtz, ' the distances our 

 sense-perceptions have to traverse before they reach the brain 

 are short, otherwise our consciousness would always lag far 

 behind the present, and even behind our perceptions of sound/ 



