908 OF SENSATION, AND THE ORGANS OF THE SENSES. 



which is a source of great discomfort to them, since every discordant sound is 

 a positive torment. The power of distinguishing the direction 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 anything of the direction 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 impressions made upon the 

 nerve, through the medium either of the tympanic apparatus, or of the bones 

 of the head, yet it is equally evident, that there can be nothing in these varia- 

 tions 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 commu- 

 nicated 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 similarity between the perception of the distance of an 

 object, through the Eye by its size, and through the 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 being gradually diminished whilst its 

 distance remains the same, 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 complete 

 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. 



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

 an occasional variation in the rapidity of the perception of sensory impressions, 

 received through the Eye and through the Ear. These facts are the result of 

 comparisons made amongst different astronomical observers, who may be watch- 

 ing the same visual phenomenon, and timing their observations by the same 

 clock ; for it has been remarked, that some persons see the same occurrence a 

 third or even a half of a second earlier than others. There is no reason to suppose 

 from this, however, that there is any difference in the rate of transmission of 

 the sensory impressions in the two nerves. The fact seems rather to be that the 

 Sensorium does not readily perceive two different impressions with equal dis- 

 tinctness ; and that, when several impressions are made on the senses at the 

 same time, the mind takes cognizance of one only, or perceives them in succes- 

 sion. When, therefore, both sight and hearing are directed simultaneously to 

 two objects, the communication of the impression through one sense will neces- 

 sarily precede that made by the other. The interval between the two sensations 

 is greater in some persons than in others ; for some can receive and be conscious 

 of many impressions, seemingly at the same moment ; whilst in others a per- 

 ceptible space must elapse. 



911. Amongst other important offices of the power of Hearing, is that of 

 supplying the sensations by which the Voice is regulated. It is well known 

 that those who are born entirely deaf are also dumb; that is, they do not 

 spontaneously or imitatively form articulate sounds, though not the least de- 

 fect exist in their organs of voice. Hence it appears that the vocal muscles 

 are usually guided in their action by the sensations received through the Ears, 

 in the same manner as other muscles are guided by the sensations received 



