DIRECTION AND DISTANCE OF SOUNDS. 545 



tion of the effect of distance, it excites the idea of its originat- 

 ing at a distance; and this is also taken advantage of by ven- 

 triloquists. 



The experiments of Savart, already referred to, prove that 

 the effect of the action of sonorous undulations upon the nerve 

 of hearing, endures somewhat longer than the period during 

 which the undulations are passing through the ear. If, how- 

 ever, the impression of the same sound be very long continued, 

 or constantly repeated for a long time, then the sensation pro- 

 duced may continue for a very long time, more than twelve or 

 twenty-four hours even, after the original cause of the sound 

 has ceased. This must have been experienced by every one 

 who has travelled several days continuously ; for some time 

 after the journey, the rattling noises are heard when the ear 

 is not acted on by other sounds. 



We have here a proof that the perception of sound, as 

 sound, is not essentially connected with the existence of undu- 

 latory pulses ; and that the sensation of sound is a state of the 

 auditory nerve, which, though it may be excited by a succes- 

 sion of impulses, may also be produced by other causes. Even 

 if it be supposed that undulations excited by the impulse are 

 kept up in the auditory nerve for a certain time, they must be 

 undulations of the nervous principle itself, which, being ex- 

 cited, continue until the equilibrium is restored. 



Corresponding to the double vision of the same object with 

 the two eyes, is the double hearing with the two ears ; and 

 analogous to the double vision with one eye, dependent on 

 unequal refraction, is the double hearing of a single sound 

 with one ear, owing to the sound coming to the ear through 

 media of unequal conducting power. The first kind of double 

 hearing is very rare ; instances of it are recorded, however, by 

 Sauvages and Itard. The second kind, which depends on the 

 unequal conducting power of two media through which the 

 same sound is transmitted to the ear, may easily be experi- 

 enced. If a small bell be sounded in water, while the ears 

 are closed by plugs, and a solid conductor be interposed be- 

 tween the water and the ear, two sounds will be heard differ- 

 ing in tensity and tone ; one being conveyed to the ear through 

 the medium of the atmosphere, the other through the conduct- 

 ing-rod. 



The sense of vision may vary in its degree of perfection as 

 regards either the faculty of adjustment to different distances, 

 the power of distinguishing accurately the particles of the 

 retina affected, sensibility to light and darkness, or the per- 

 ception of the different shades of color. In the sense of hear- 

 ing, there is no parallel to the faculty by which the eye is 



