296 



NATURE 



\_yan. 30, 1879 



duration of the negative variation of nerves and mutcles. 

 Guillemin's method is applicable in every case where a 

 current passes several times in succession through a me- 

 tallic circuit, with duration always the same. The electric 

 condition of the circuit is investigated during a succes- 

 sion of very short intervals, beginning at the moment 

 when ciu-rent is complete. 



The apparatus used by Guillemin and Bernstein was 

 the galvanometer ; M. Marey preferred to use a frog's 

 foot, which, in the successive investigation, gives a move- 

 ment which can be graphically recorded, as often as there 

 is an electric current. 



The graphic method, by which each duration is trans- 

 formed into a length easily measured on the paper, is 

 easily applied in performing those experiments of which 

 we are about to explain the principle. 



Let the point o (Fig. 3) correspond to the moment of 

 electric excitation of a torpedo-nerve, and let the succes- 

 sive points I, 2, 3, &c., denote successive hundredths of 

 a second, which correspond to very short intervals during 

 which the torpedo apparatus is put in contact with a 

 metallic circuit passing through a frog's foot. In the two 

 first trials, i and 2, after the excitation of the electric 

 nerve, there are no signals recorded; the frog's foot 



Fig. I. 



remaining motionless shows that the discharge 01 the 

 torpedo has not reached it, because, in fact, the pheno- 

 menon has not yet had time to take place. But at the 

 instant 3 the frog moves, which is expressed in the dia- 

 gram by a vertical stroke ; at the instants 4, 5, 6, 7, 8, 9, 

 and 10, the frog receives shocks which are indicated on 

 the diagram by vertical lines ; and finally, at the instant 

 II, and those succeeding, the frog shows no action, 

 whence we conclude that the electric wave of the torpedo 

 was finished before these last trials ; and we see that, 

 according to the tracing, the wave began three-hun- 

 dredths of a second after the instant of nerve-excitation 

 and finished ten-hundredths after the same instant. 



It was exactly in the same way that M. Marey pro- 

 ceeded to measure the duration of the electric wave in 

 the torpedo. An arrangement easily fixed induced elec- 

 tric action in e (Fig. 4) at constant intervals. A metallic 

 contact^ susceptible of being displaced at will, allowed 

 him, during very short intervals of different lengths, to 

 complete the circuit made by the electric wave of the 

 torpedo to reach the frog's-foot-signal. Moreover, to 

 avoid confusion of the curves which were registered by 

 the successive experiments, he took care to change the 

 position of the style each time, so that the curves 

 appeared one under the other in order. 



Fig. 4 shows that the first appearance of the electric 



Fig. 2. 



wave took place at instant i ; that in a series of succes- 

 sive trials, each later than the preceding, after the ex- 

 citation of the nerve, the wave was indicated at the 

 instants 2, 3, 4, 5, and 6 ; and that at the 7th trial the 

 frog gave no signal. The wave, therefore, was com- 

 pleted. Finally, by bringing the instant of trial nearer to 

 that of nerve-excitation, the wave was retraced in experi- 

 ments 8, 9, 10, II, and 12 ; but in the 13th, occurring too 

 soon after the instant of nerve-excitation, it was shown 

 that the electric wave no longer existed. 



The approNimation of these measurements necessarily 



Fig 3. 



depends on the number of successive trials, and is more 

 delicate in proportion as they succeed each other more 

 frequently. 



3. Each electric wave presents a phase 0/ suddenly 

 increasing intensity^ followed by a phase of gradually 

 decreasing intensity. 



On examining the tracings of electric waves obtained 

 by the electric-magnetic signal, Me observe an apparent 

 contradiction between the indication, the wave-duration 

 furnished by this apparatus, and that which we hare just 

 seen determined by the frog' s foot. The waves traced 



by the signal of Deprez seem to measure not moie than 

 one-hundredth of a second ; by Guillemin's method, on 

 the contrary, their duration is much more considerable, 

 being seven-hundredths of a second. This apparent 

 contradiction results from the fact that in the torpedo the 

 waves have not sufficient energy during the whole of 

 their duration to act upon the signal, whereas, from 

 beginning to end of their course they can act upon the 

 frog' s muscle, which is much more sensitive. There are, 

 then, in every electric wave, phases of increasing intensity 

 and decreasing intensity which remain to be determined. 



M. Marey has endeavoured to obtain a tracing of these 

 phases of variable intensity by a modification of the 

 apparatus of M. Deprez. Instead of limiting the excur- 

 sion of the style between two fixed obstacles, he allowed 

 it an excursion which varies and is proportional to the 

 intensity of the currents acting upon it. 



With this object an india-rubber thread, bent over two 

 bridges, was stretched horizontally between the soft iron 

 bars of the armature (Fig. 5). The bars had a groove filed on 

 the top to receive two demi-cylinders of metal which were 

 soldered to the lower part of the armature. In this way the 

 nearer those parts are brought which are subjected to the 

 magnetic attraction the greater is the resistance. Thus if 

 we consider the armature in its different stages when 

 gradually lowered, first it meets the elastic thread with 

 the two demi-cylinders borne on its lower surface, and 

 then the extensibility of the thread is very great. But as 

 the thread is lowered more and more, it rests on points 

 more and more separated, and becomes less and less 

 extensible. Lower down the india-rubber thread stretched 



