440 PHYSIOLOGICAL PHYSICS. [Chap, xxxni. 



vibration by the regular rhythmic alternation. If it is 

 caused to move it will only be by a vibration of its 

 own period, for if, when it is moving in the direction of 

 condensation, it is met by a rarefaction due to the other 

 fork, its motion will be disordered and will cease, just as 

 the swing is stopped by its backward movement 

 being met by the forward impulse from the boy. 

 But when the oscillations of the two forks agree, 

 then the activity of the one will excite that of 

 the other. If a small coin be attached by wax to a 

 limb of the second fork, then the vibrations of the 

 first will no longer maintain those of the second, 

 because the vibrations of the two are no longer pre- 

 cisely alike, and the one will interfere with the other. 

 Sympathetic vibration may be produced with a piano- 

 forte. If the dampers be raised, and loud musical 

 sounds be uttered in the neighbourhood, a number of 

 strings may be heard humming. Each string has 

 picked out of the mass of sound the particular vibra- 

 tions to which it is tuned, and is sounding in harmony 

 with it. The strings have analysed the complex 

 sound, and the separate strings set into vibration 

 indicate its different elements. 



This phenomenon is of the utmost significance in 

 physiology, since it affords the basis of the theory of 

 the perception of musical sounds. The complex mass 

 of sound is conveyed to the inner ear, in which there 

 are supposed to be delicate structures which will 

 vibrate in harmony with particular vibrations, and 

 with none other. The ear, in this view, becomes an 

 analytical organ, and separates the complex sound 

 into its elements. An impression is thus made on the 

 nerve fibril in connection with each delicate vibrating 

 body, and the separate impulses, carried to the centre 

 in the brain, are fused again into a complex sensation. 

 The trained musician can, to some extent, become con- 

 scious of the analysis, and thus can distinguish in the 



