40 ANIMAL ELECTRICITY. LECTURE II. 



have seen have been taken with a galvanometer of 

 this character ; others have been taken with a fully- 

 damped or dead-beat instrument, in which the magnet 

 comes slowly to its position of rest without overshoot 

 or oscillations. 



Each instrument has its advantages and dis- 

 advantages. The former is more sensitive, and 

 best adapted to show effects of short duration, 

 the latter to show more prolonged effects uncom- 

 plicated by oscillation ; and for after-effects in the 

 nerve they supplement each other. The time of 

 an oscillation to and fro, or " oeriod " of the partially- 

 damped magnet Is, say, eight seconds, and the range 

 of a swing is, say, double that of the next swing, i.e.y 

 the ''decrement" is 2. A swing, caused by an 

 electrical change lasting four seconds and ending 

 abruptly, will be followed by an instrumental after- 

 deflection In the opposite direction of, say, ^, 

 or about 07 of the original swing. And if the 

 change, Instead of ending abruptly, either goes on 

 of the same character, or gives way to an opposed 

 after-change, we shall have the after-oscillation either 

 checked and reduced, or helped on and augmented. 

 If this reduction or augmentation is very pronounced, 

 we shall be justified In saying that within the four 

 seconds Immediately after an excitation has ceased, 

 the after-state of the nerve has been like or unlike 

 Its state during the excitation. It would not do to 

 lay too much stress on slight alter^itions, but coarse 



