SENSE OF HEARING. 269 



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 information, 

 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. 



363. 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 minute objects, or of seeing with very faint degrees 

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

 destitute of what is termed a musical ear ; whilst others are endowed with it 

 in a degree, which is a source of great discomfort to them, since every dis- 

 cordant sound is a positive torment. The power of distinguishing the direc- 

 tion of sounds appears to be, in Man at least, for the most part acquired by 

 habit. It is some time before the infant seems to know any thing of the direc- 

 tion of noises which attract his attention. Now although there can be no 

 question that this perception is acquired by attention to certain variations in 

 the impression made upon the nerve, through the medium either of the tym- 

 panic apparatus or of the bones of the head, yet it is equally evident that 

 there can be nothing in these variations themselves adequate to excite the 

 idea, and that it must therefore be either intuitive or acquired by habit. This 

 is a consideration of some importance, in regard to the similar question as to 

 the sense of Visual direction. In some cases we are probably assisted by the 

 relative intensity of the sensations, communicated by the two ears respectively. 

 The idea of the distance of the sonorous body is another acquired perception, 

 depending principally upon the loudness or faintness of the sound, when we 

 have no other indications to guide us. In this respect there is a great simi- 

 larity between the perception of distance of an object, through the Eye, by 

 its size, and through ^he Ear by the intensity of its sound. When we know 

 the size of the object, or are acquainted with the usual intensity of its sound, 

 we can judge of its distance ; and vice versa, when we know its distance, we 

 can at once form an idea of its real from its apparent size, and of its real 

 Strength of tone from that which affects our ears. In this manner, the mind 

 may be affected with corresponding deceptions through both senses ; thus, in 

 the Phantasmagoria, the figure is gradually diminished while its distance re- 

 mains the same, and it appears to the spectators to recede, the illusion being 

 more complete, if its brightness be at the same time diminished ; and the 

 effect of a distant full military band gradually approaching, may be alike given 

 by a corresponding crescendo of concealed instruments. It is upon the com- 

 plete imitation of the conditions, which govern our ideas of the intensity and 

 direction, as well as of the character, of sounds, that the deceptions of the 

 Ventriloquist are founded. 



364. Some facts of much interest have lately been ascertained, in regard 

 to an occasional variation in the rapidity of the perception of sensory impres- 

 sions, received through the Eye and through the Ear. These facts are the 

 result of comparisons made amongst different astronomical observers, who may 



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