58 PHYSIOLOGY OF MUSCLES AND NERVES. 



a push of but short duration and makes only a single 

 vibration, after which it retiurns to the position proper to 

 it when at rest. The degree of deflection must in this 

 case be proportionate to the strength of the current and 

 to the brevity of its duration. If, therefore, the strength 

 is known and remains constant, the time occupied by the 

 deflection maybe calculated from its extent. Such de- 

 flections are generally very slight. In order, therefore, 

 to measure them with certainty, an apparatus which was 

 first applied by the celebrated mathematician Gauss 

 is used. A small mirror o being connected with the 

 magnet, a graduated scale 8 s, which is reflected in 

 the mirror, is read by means of a magnifying glass. If 

 the scale is placed parallel to the mirror when the 

 magnet is at rest, and the magnifying glass is aj'ranged 

 at right angles to the direction of the mirror and of the 

 scale, it is evident that exactly the point a on the 

 scale which lies over the centre of the magnifying 

 glass will be seen reflected in the mirror. As soon as 

 the magnet with the mirror attached to it turns, the 

 reflection of a different point on the fixed scale, the 

 point c, is seen through the glass, and an observer 

 looking at the mirror through the lens sees the scale 

 apparently move in the same direction as that in 

 which the mirror, together with the magnet, turns. 

 From the extent of this change of position the angle 

 which the magnet describes in its deflection may be 

 calculated. 



4. This method, by which the duration of electric 

 currents may be measured with the greatest accuracy, 

 must now be applied to our task of examining the 

 duration of a muscle-pulsation. For this purpose we 

 must find some arrangement by which an electric 



