RATE OF PROPAGATION OF NEGATIVE VARIATION. 219 



reaches the latter end. In ordinary experiments the 

 irritation occurs continuously, and the connection of 

 the other end of the nerve with the multiplier is also 

 continuous. But the time which elapses between the 

 commencement of irritation and the commencement of 

 negative variation is, even in the ca.se of the longest 

 nerves with which experiments can be tried, far too short 

 to allow of observation of this retardation. Bernstein 

 proceeded as follows : two projecting wires were fastened 

 to a wheel which turned at a constant speed. One of 

 these wires, at each revolution, closed an electric current 

 for a very brief time, and at regular intervals of time 

 repeatedly effected the irritation of one end of the 

 nerve. The second wire, on the other hand, for a very 

 brief time connected the other end of the nerve with a 

 multiplier. When irritation and connection with the 

 multiplier occurred simultaneously, no trace of negative 

 variation was observable; for, before the latter could pass 

 from the irritated point to the other end of the nerve, the 

 connection of the latter with the multiplier was again 

 interrupted. By altering the position of the wires it 

 was, however, possible to cause the connection of the 

 nerve with the multiplier to occur somewhat later than 

 the irritation. When this difference in time reached a 

 certain amount, negative variation intervened. From 

 the amount of this time, together with the length of the 

 passage between the point irritated and that at which 

 the current is diverted, it is evidently possible to calcu- 

 late the rate of propagation of the negative variation 

 within the nerve. Bernstein in this way determined 

 the rate at 25 m. per second. This value corresponds 

 as nearly with that found for the propagation of the 

 excitement in the nerves (24*8 m. ; see ch. vii. 3) as 



