388 ON SENSATION, AND THE ORGANS OF THE SENSES. 



510. It is a general rule, with regard to all sensations, that their intensity 

 is much affected by habit; being greatly diminished by frequent and continual 

 repetition. This is not the case, however, with regard to those sensations to 

 which the attention is peculiarly directed ; for these lose none of their acute- 

 ness by frequent repetition; on the contrary, they become much more readily 

 cognizable by the mind. We have a good example of both facts, in the ef- 

 fects of sounds upon a sleeping person. If they are sounds which he has 

 been accustomed to hear, and to disregard, they may not awake him, however 

 loud they be : thus, the strokes of a forge-hammer, the firing of guns, the 

 shouts of a multitude, or the loudest music, may neither prevent the acces- 

 sion of sleep, nor arouse the already unconscious sleeper ; indeed, it oftener 

 happens that individuals are prevented from sleeping by the want of some 

 accustomed sound, or are awoke by its cessation. On the other hand, a very 

 slight sound, the nature of which excites the attention, is sufficient to prevent 

 sleep; thus, the buzz of a single musquito, in the stillness of the night, is 

 most effectual in dispelling repose ; and, in like manner, a person in a state 

 of the profoundest unconsciousness maybe aroused by a whisper, if the sound 

 be one to which he has been accustomed to pay regard. 



a. The following circumstance has been communicated to the Author by a Naval Officer 

 of high rank : When a young man he was serving as signal-lieutenant under Lord Hood ; 

 and being desirous of obtaining the favourable notice of his commander, he devoted him- 

 self to his duty with the greatest energy and perseverance, often remaining on deck nineteen 

 hours out of the twenty-four, with his attention continually on the stretch. During the few 

 hours which he spent in repose, his sleep was so profound, that no noise of an ordinary 

 kind, however loud, would awake him. But if the word "signal" was softly uttered in his 

 ear, he was instantly aroused. 



511. The general law, that Sensations, not attended to, are blunted by fre- 

 quent repetition, may perhaps be connected with certain other general facts, 

 which lie under the observation of every one. It is well known, that the 

 vividness of sensations depends rather on the degree of change which they 

 produce in the system, than on the absolute amount of the impressing cause; 

 and this is alike the case with regard to the special and the ordinary sensa- 

 tions. Thus, our sensations of heat and cold are entirely governed by the 

 previous condition of the parts affected ; as is shown by the well-known ex- 

 periment, of putting one hand in hot water, the other in cold, and then trans- 

 ferring both to tepid water, which will seem cool to one hand, and warm to 

 the other. Every one knows, too, how much more we are affected by a warm 

 day at the commencement of the summer, than by an equally hot day later 

 in the season. The same is the case in regard to light and sound, smell and 

 taste. A person going out of a totally dark room into one moderately bright, 

 is for the time painfully impressed by the light, but soon becomes habituated 

 to it ; whilst another, who enters it from a room brilliantly illuminated, will 

 consider it dark and gloomy. Those who are constantly exposed to very loud 

 noises, become almost unconscious of them, and are even undisturbed by them 

 in illness; and the medical student well knows, that even the elliuvia of the 

 dissecting-room are not perceived, when the organ of smell is habituated to 

 them, although an intermission of sufficient length would, in either instance, 

 occasion a renewal of the first unpleasant feelings, when the individual is 

 again subjected to the impression. 



512. Again, it is a well-known fact, that impressions made upon the organs 

 of sense continue for a time, after the cause of the impression has ceased. 

 It is in this manner that a musical tone, which seems perfectly continuous, 

 results from a series of consecutive vibrations, following each other with a 

 certain rapidity ; and that a line or circle of light is produced by a luminous 

 body moving with a certain, velocity. Now there is reason to believe that 



