

THE SENSES. 



This throws a doubt on the special office of the cochlea in auc 

 sensations. 



Persistence of Sonorous Impressions and the Production of Musical 

 Sounds. The effect produced by a sonorous vibration continues for a 

 short time after the cessation of its cause. Usually the interval between 

 two different impulses is sufficient to allow the first impression to dis- 

 appear before the second is received, and the ear distinguishes them in 

 succession. But if they follow each other at equal intervals, with a 

 certain rapidity, they produce the impression of a continuous sound ; 

 and this sound has a higher or lower pitch, according to the rapidity 

 of its vibrations. The numerical relation of musical notes thus pro- 

 duced has been studied by various means. One of these is the siren 

 of Savart, in which successive puffs of air are emitted through small 

 openings, with a rapidity which can be varied at will and registered 

 by an automatic index. In another method the shocks are given by 

 the points of a toothed wheel turning with known velocity against the 

 projecting edge of a card. The number of vibrations corresponding 

 to a particular note may also be registered by attaching to the extrem- 

 ity of a tuning-fork, a light stilet which traces upon the surface of a 

 revolving cylinder, an undulating line (Fig. 96, a) ; the number of un- 

 dulations in a given space indicating the frequency of vibration of the 

 tuning-fork. A simple vibration represents the movement in one direc- 

 tion ; a double vibration is the complete to-and-fro oscillation, which 

 brings the moving point back to its original position. 



By this means it is found that sonorous impulses, following each 

 other with a rapidity of less than sixteen times per second, are sepa- 

 rately distinguishable ; but above that frequency they are merged into 

 a continuous sensation. When the shocks are repeated at irregular 

 intervals, the only characters perceptible in the sound are its intensity 

 and quality. But if they succeed each other' at regular intervals, the 

 sound produced has a position in the musical scale, as a high or low 

 note. The more frequent the repetitions, the higher the note ; but a 

 limit is at last reached at which the ear fails to perceive the sound, 

 and an excessively high note is therefore inaudible. This is probably 

 due to the following reason : A sonorous vibration, to be perceptible, 

 must have a certain extent or amplitude ; that is, the particles of the 

 vibrating body must move to and fro, at each impulse, for a certain 

 distance in space. The intensity of a sonorous impression, accordingly, 

 depends on the amplitude of the vibrations, while its pitch or tone 

 depends on their frequency. But the more frequently a body vibrates 

 in a given time, the less extensive must be its movements, if their 

 velocity remain the same. Consequently, when these vibrations arrive 

 at a certain frequency, unless their velocity be increased in proportion, 

 their amplitude becomes so small that they make no impression on the 

 ear, and they are therefore inaudible. 



It is evident, however, that such a sound would be perceptible if the 

 sensibility of the auditory apparatus were increased to the requisite 



