694 THE SENSE OF HEARING. 



until two only are left ; the sound produced by the impulse 

 of these two teeth has still the same definite value in the 

 scale of music. 



The maximum and minimum of the intervals of suc- 

 cessive impulses still appreciable through the auditory 

 nerve as determinate sounds, have been determined by 

 M. Savart. If their intensity is sufficiently great, sounds 

 are still audible which result from the succession of 48,000 

 half vibrations, or 24,000 impulses in a second ; and this, 

 probably, is not the extreme limit in acuteness of sounds 

 perceptible by the ear. For the opposite extreme, he has 

 succeeded in rendering sounds audible which were pro- 

 duced by only fourteen or eighteen half vibrations, or 

 seven or eight impulses in a second ; and sounds still 

 deeper might probably be heard, if the individual im- 

 pulses could be sufficiently prolonged. 



By removing one or several teeth from the toothed 

 wheel before mentioned, M. Savart was also enabled to 

 satisfy himself of the fact that in the case of the auditory 

 nerve, as in that of the optic nerve, the sensation continues 

 longer than the impression which causes it ; for the re- 

 moval of a tooth from the wheel produced no interruption of 

 the sound. The gradual cessation of the sensation of sound 

 renders it difficult, however, to determine its exact duration 

 beyond that of the impression of the sonorous impulses. 



The power of perceiving the direction of sounds is not a 

 faculty of the sense of hearing itself, but is an act of the 

 mind judging on experience previously acquired. From 

 the modifications which the sensation of sound undergoes 

 according to the direction in which the sound reaches us, 

 the mind infers the position of the sounding body. The 

 only true guide for this inference is the more intense 

 action of the sound upon one than upon the other ear. 

 But even here there is room for much deception, by the 

 influence of reflexion or resonance, and by the propagation 

 of sound from a distance, without loss of intensity, through 



