DURATION OF PULSATION. 51 



gunpowder. The duration of the electric spark alone 

 yet remains unmeasured. This may, therefore, be re- 

 garded as really instantaneous, or at least as occupying 

 a time shorter than any measurable period. Some 

 observers have estimated its duration as less than 

 __j__ of a second. 



The most serviceable means of measuring very 

 short periods is by causing the process to be measured 

 to register itself on a rapidly moved surface, or by 

 using an electric current the action of which depends 

 on a magnet as regards its duration. Each of these 

 methods has been applied to muscle. 



Supposing a smooth surface, such as a glass plate, 

 -moved with great rapidity in its own plane, then a 

 pointed wire turned at right angles to the plate will 

 mark a straight line on the latter. If the plate has 

 been smoked this line will be visible. Supposing the 

 wire is attached to an instrument vibrating, like a 

 tuning fork, upward and downward, then the line 

 drawn by the pencil when the plate is moved will be 

 not straight but waved. As the number of the vibra- 

 tions may be told from the note which the vibrating 

 instrument emits, it is known that the distance be- 

 tween each two waves of the waved line obtained 

 represents a certain period of time. Assuming that the 

 instrument makes 250 vibrations in each second, it 

 is evident that the plate must have moved the dis- 

 tance between each two waves in -^-^ of a second. 

 Now, if it is possible to cause a muscle-pulsation to 

 register itself on the same plate, then from the distance 

 of the separate parts of the line thus registered, when 

 compared with the waves drawn by the vibrating 

 instrument, the duration of time may be accurately 



