298 PHYSIOLOGICAL PHYSICS. [Chap. xxiv. 



Thus, to return to our illustration, the period of vibra- 

 tion of one of the pieces of cork may be counted from 

 the moment the advancing wave reaches it in its 

 position of rest, to the moment when, the wave having 

 advanced and passed, it has returned to the same 

 position in readiness for the next wave. Or, again, 

 its period may be counted from the moment it has 

 reached the crest of one wave to the moment when it 

 reaches the crest of the next, supposing it to be 

 vibrating through a regular series of waves. By the 

 same illustration the PHASE OP VIBRATION will be 

 represented by the position occupied by a piece of 

 cork in the wave. Thus the phase of each particle in 

 the wave will be different. 



The amplitude of a vibration is the distance 

 from the middle position of the particle to one of its 

 extreme positions. Thus, for one of the pieces of 

 cork it is the distance between its point of rest and 

 one extreme (the crest of the wave), or the other 

 extreme (the trough of the wave). 



The frequency of vibration is determined by the 

 number of vibrations per second of time. The fre- 

 quency is related to the period. Thus if the number 

 of vibrations be 150 per second, the length of each 

 period is T ^th of a second. 



The wave length is the distance through which 

 the change of form has been propagated during the 

 complete period of vibration of a particle. The 

 longer this period, the greater will be the wave length; 

 the shorter the period, the shorter will be the wave 

 length. Thus, with our illustration, the wave length 

 is measured by the distance to which the wave has 

 advanced between the moment when one piece of 

 cork began the ascent of one wave of a series to the 

 moment when it begins the ascent of the succeeding 

 wave of the series. The faster the vibrations are the 

 shorter will be the wave length. 



