SENSE OF HEARING. 907 



the other hand, where sounds cannot thus be perceived, there will be good rea- 

 son to believe that the nerve is diseased. 



908. A single impulse communicated to the Auditory nerve, in any of the 

 foregoing modes, seems to be sufficient to excite the momentary sensation of 

 sound ; but most frequently a series of such impulses is concerned, there being 

 but few sounds which do not partake, in a greater or less degree, of the charac- 

 ter of a tone. Any continuous sound or tone is dependent upon a succession of 

 impulses; and its acuteness or depth is governed by the rapidity with which 

 these succeed one another. It is not difficult to ascertain, by experiment, what 

 number of such impulses or undulations are required, to give every tone which 

 the ear can appreciate. Thus, if a circular plate, with a number of apertures 

 at regular intervals, be made to revolve over the top of a pipe through which 

 air is propelled, a succession of short puffs will be allowed to issue from this; 

 and, if the revolution be sufficiently rapid, these impulses will unite into a defi- 

 nite tone. In the same manner, if a spring be fixed near the edge of a re- 

 volving toothed wheel, in such a manner as to be caught by every tooth as it 

 passes, a succession of clicks will be heard ; and these, too, if the revolution of 

 the wheel be sufficiently rapid, will produce a tone. The number of apertures 

 in the plate which pass the orifice of the pipe in a given time, or the number 

 of teeth which pass the spring, being known, it is easy to see that this must be 

 the number of impulses required to produce the given tone. Each impulse 

 produces a double vibration forwards and backwards (as seen when a string is 

 put in vibration, by pulling it out of the straight line); hence the number of 

 impulses is always half that of the single vibrations. The maximum and mini- 

 mum of the intervals of successive pulses, still appreciable by the ear as deter- 

 minate sounds, have also been determined by M. Savart, more satisfactorily and 

 more accurately than had previously been done. If their intensity be great, 

 sounds are still audible which result from the succession of 24,000 impulses in 

 a second; and this, probably, is not the extreme limit to the acuteness of sounds 

 perceptible by the ear. From some observations of Dr. Wollaston's, it seems 

 probable that the ears of different individuals are differently constituted in this 

 respect some not being able to hear very acute tones produced by Insects, or 

 even Birds, which are distinctly audible to others. Again, the sound resulting 

 from 16 impulses per second, is not, as has been usually supposed, the lowest 

 appreciable note; on the contrary, M. Savart has succeeded in rendering tones 

 distinguishable which are produced by only 7 or 8 impulses in a second; and 

 continuous sounds of a still deeper tone could be heard, if the individual pulses 

 were sufficiently prolonged. In regard, however, to the precise time during which 

 a sonorous impression remains upon the ear, it is difficult to procure exact in- 

 formation, since it departs more gradually than do visual impressions from 

 the eye. This is certain, however that it is much longer than the interval 

 between the successive pulses in the production of tones ; since it was found by 

 M. Savart, that one or even several teeth might be removed from the toothed 

 wheel, without a perceptible break in its sound showing that, when the tone 

 was once established, the impression of it remained during an intermission of 

 some length. 



909. The Ear, like the Eye, may vary considerably as regards general acute- 

 ness, amongst different individuals; and its power may be much increased by 

 practice. A part of this increase depends, however, as in other instances, upon 

 the greater attention which its fainter indications receive ; but a part, also, upon 

 an increased use of the organ. The power of hearing very faint sounds is as 

 different from the power of distinguishing musical tones, as the' power of dis- 

 cerning very minute objects, or of seeing with very faint degrees of light, is 

 from that of distinguishing colors. Many persons are altogether destitute of 

 what is termed a musical ear; whilst others are endowed with it in a degree 



