58 rilYsIOLOGY OF MUSCLES AND NERVES. 



a push of but short duration and makes mily a single 

 vibration, after which it returns to the position prop. r to 

 it when at rest. Tin- degree of deflect ion mii>t in this 

 case be pm port ionat e 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 tin- 

 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 (ianss 

 is used. A small mirror o being connected with the 

 magnet, a graduated scale s 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 arranged 

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

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

 scale which lies over the centre of the magnify ing 

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

 the magnet with the mirror attached to it turns, thi- 

 n-flection 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 

 \\hii-h 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 In- 

 calculated. 



4. This method, by which the duration of electric 

 currents may be measured with the ^reate-t accuracy, 

 must now he applied to our task of examining the 

 duration of a muscle-pulsation. Kor this purpose we 

 must find sonic arrangement by which an electric 



