532 OF SENSATION IN GENERAL. 



impression on the system as its first commencement; thus there are 

 persons, who have become so habituated to the sound of a waterfall or 

 even of a forge-hammer, that they cannot sleep anywhere but in its- 

 vicinity ; and it is well known that, when a person has gone to sleep 

 under the influence of some continuous or frequently-recurring sound 

 (such as the voice of a reader, the dropping of water, the tread of a 

 sentinel, &c.), the cessation of the sound will cause his awaking. 



935. The acuteness of particular sensations is influenced in a re- 

 markable degree, by the attention they receive from the mind. If the 

 mind be entirely inactive, as in profound sleep, no sensation whatever 

 is produced by very feeble impressions ; on the other hand, when the 

 mind is from any cause strongly directed upon them, impressions very 

 feeble in themselves produce sensations of even painful acuteness. It 

 is in this manner, that the habit of attending to sensations of any par- 

 ticular class increases their vividness ; so that they are at once per- 

 ceived by an individual on the watch for them, when they do not ex- 

 cite the observation of others. We may even, by a strong effort, direct 

 the mind into one particular channel, so as to receive only those sensa- 

 tions which have reference to it, and to be unconscious quoad all others. 

 Thus, the application of the mind to some particular train of thought 

 may prevent our being conscious of anything that is going on around or 

 within us, the conversation of friends, the striking of the clock, 

 the calls of hunger, &c. This abstraction may be altogether voluntary ; 

 .and the possession of the power of thus withdrawing the mind at will 

 from the influence of external disturbing causes, and of fixing it upon 

 any particular .train of ideas, is an extremely valuable one. But it may 

 also be involuntary, and may be a source of inconvenience from its 

 tendency to recur at improper times, producing the habitual state 

 which is known as absence of mind or reverie. 



936. It is desirable that we should make a distinction, between the 

 sensations themselves, and the ideas which are the immediate results of 

 those sensations, when they are perceived by the mind. These ideas 

 relate to the cause of the sensation, or the object by which the impres- 

 sion is made. Thus, the formation of the picture of an object, upon 

 the retina, produces a certain impression upon the optic nerve ; which, 

 being conveyed to the sensorium, excites a corresponding sensation, 

 with which, in all ordinary cases, we immediately connect an idea of 

 the nature of the object. So closely, indeed, is this idea usually re- 

 lated to the sensation, that we are not in the habit of making a distinc- 

 tion between them. Thus, I may say at this moment, " I see u book 

 on the table before me ;" the fact being, that I am conscious of a certain 

 picture, which conveys to my mind the ideas of a book a.nd of a table, 

 and of their relative positions ; these ideas being (in Man) the result of 

 experience and associations, in fact, originating in the immediate 

 application of the knowledge w.e have previously acquired, that a cer- 

 tain object, whose picture we see, is a book, another object a table, and 

 so on. We are liable to be deceived on this assumption ; as when, by 

 a clever imitation, a picture on a plane surface is made to represent an 

 object in relief, so perfectly as at once to excite the idea of the latter, 



