696 THE SENSE OF HEARING. 



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 

 undulatory pulses ; and that the sensation of sound is a 

 state of the auditory nerve, which, though it may be 

 excited by a succession 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 excited, 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 analagous 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 experienced. If a small bell be 

 sounded in water, while the ears are closed by plugs, and 

 a solid conductor be interposed between the water and the 

 ear, two sounds will be heard differing in tensity and tone ; 

 one being conveyed to the ear through the medium of the 

 atmosphere, the other through the conducting-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 par- 

 ticles of the retina affected, sensibility to light and dark- 

 ness, or the perception of the different shades of colour. 

 In the sense of hearing, there is no parallel to the faculty 



