200 THE HUMAN BODY. 



it by the greater or less force of the auditory impression. As 

 for noises of which we do not know the intensity at a given 

 distance, as thunder, we estimate it in the same way, but 

 with less certainty according as it is faint or loud. 



It is therefore to reasoning, founded on the sensation, that 

 we owe the ability to judge of the distance as well as the 

 intensity of sounds, and it is the same as to their direction. 

 When we hear a sound more distinctly with one ear than 

 with the other, we judge that it comes from the side on 

 which the impression is strongest, and the ability of the 

 organ to seize slight degrees in the intensity enables us to 

 tell in what position of the head the sound is most clearly 

 perceived. We are therefore led to place it in a certain 

 position in regard to the direction, and by this means we 

 acquire an idea of it within certain limits. Hence, if the 

 ears are both in the same situation relative to the sound, as 

 when it is in front of or behind us for example, we find it 

 impossible to distinguish in which direction it is without 

 turning the head. 



This uncertainty which we always feel in regard to the 

 exact distance and direction of sounds enables the ventrilo- 

 quist to produce what are wrongly supposed to be illusions 

 of hearing, but which are simply errors of judgment guided 

 by the imagination. The hollow, feeble voice of the ven- 

 triloquist seems to come from a great distance, from above 

 or from a certain depth below us, the sense of the words, the 

 expression of the voice, the varied tones and mimicry of the 

 juggler, do the rest. 



Duration of auditory impressions. Savart has demonstrated 

 that the duration of acoustic impressions is about the tenth 

 of a second. Thus when the vibrations of a body do not 

 exceed nine in a second, the ear perceives a series of distinct 

 impressions, but beyond ten or twelve the sensation becomes 

 continuous. 



Sensations of internal origin. As the eye may be the seat 

 of luminous impressions produced by other means than light, 

 so sounds and noises may be heard, without the ears having 

 been excited by sonorous waves. Ringing and humming 

 sensations may be produced in or imparted to them, under 



