THE SENSES. 091 



Since about 3000 rods of Corti are present in the human ear, this 

 would give about 400 to each of the seven octaves which are within the 

 compass of the ear. Thus about 32 would go to each semi-tone., Weber 

 asserts that accomplished musicians can appreciate differences in pitch 

 as small as T ^th of a tone. Thus on the theory above advanced, the 

 delicacy of discrimination would, in this case, appear to have reached 

 its limits. 



Sounds. 



Any elastic body, e.g., air, a membrane, or a string performing a 

 certain number of regular vibrations in the second, gives rise to what 

 is termed a musical sound or tone. We must, however, distinguish be- 

 tween a musical sound and a mere noise ; the latter being due to irregular 

 vibrations. 



Musical sounds are distinguished from each other by three qualities. 

 1. Strength or intensity, which is due to the amplitude or length of the 

 vibrations. 2. Pitch, which depends upon the number of vibrations in 

 a second. 3. Quality, Color, or Timbre. It is by this property that 

 we distinguish the same note sounded on two instruments, e.g., a piano 

 and a flute. It has been proved by Helmholtz to depend on the number 

 of secondary tones, termed harmonics, which are present with the pre- 

 dominating or fundamental tone. 



It would appear that two impulses, which are equivalent to four single 

 or half vibrations, are sufficient to produce a definite note, audible as 

 such through the auditory nerve. 



The maximum and minimum of the intervals of successive impulses 

 still appreciable through the auditory nerve as determinate sounds, have 

 been determined by 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 produced by only fourteen or eighteen half vibrations, or 

 seven or eight impulses in a second ; and sounds still deeper might prob- 

 ably be heard, if the individual impulses could be sufficiently prolonged. 

 Direction. 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 



