436 ON SENSATION, AND THE ORGANS OF THE SENSES. 



another tube; the intensity of external sounds, and also that of the individual's 

 own voice, as heard by himself, is then much increased. 



568. Many facts prove, however, that the fluid of the labyrinth may be 

 thrown into vibration in other ways, than by the tympanic apparatus. Thus 

 in Osseous Fishes, it is only by the vibrations transmitted through the bones 

 of the head, that hearing can take place. There are many persons, again, 

 who can distinctly hear sounds which are thus transmitted to them ; although, 

 through some imperfection of the tympanic apparatus, they are almost insen- 

 sible to those which they receive in the ordinary way. It is evident, where 

 this is the case, that the nerve must be in a state fully capable of functional 

 activity ; and, on the other hand, where sounds cannot thus be perceived, 

 there will be good reason to believe that the nerve is diseased. 



569. 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 character of a tone. Any continuous sound or tone is dependent upon a 

 succession of such impulses ; and its acuteness or depth is governed by the 

 rapidity with which they succeed one another. 



a. It is not difficult to ascertain by experiment, what number of such impulses or undula- 

 tions 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 is sufficiently rapid, these impulses will unite into a definite tone. In 

 the same manner, if a spring be fixed near the edge of a revolving toothed wheel, in such 

 a mariner 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 is 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 minimum of the intervals of successive pulses, still appre- 

 ciable by the ear as determinate sounds, have also been determined by M. Savart, more satis- 

 factorily and more accurately than had previously been done. If their intensity is 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 indi- 

 viduals 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 distinguisha- 

 ble, which are produced by only 7 or 8 impulses inji 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 information, since it departs more gradually th:m do visual 

 impressions from the eye. This is certain, however, that it is much lotmer 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 percep- 

 tible break in its sound, showing that, when the tone was once established, the impression 

 of it remained during an intermission of some length. 



570. The Ear may, like the Eye, vary considerably, as regards general 

 acuteness 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 discerning very minuto objects, or of seeing with very faint degrees 

 of light, is from that of distinguishing colours. Many persons are altogether 



