PHYSIOLOGY. 35 



ity remaining the parts of the image received by the former 

 appear dark, and those received by the latter are more light." 



XL IV. Different sensations do not always imply a different 

 kind of action in the bodies producing them ; for sometimes dif- 

 ferent sensations arise merely from a different degree of force in 

 the same kind of action, as is manifest in the case of heat and 

 cold. 



XLV. To sensation from impression, a certain duration of 

 impression is necessary. 



XLVI. The mind's resting for some time upon one sensa- 

 tion, is called ATTENTION. This, like the duration (XLV.), is 

 necessary to give an impression its full effect. 



XL VI I. The mind seems to be determined to attention by 

 the force of impression ; by the pleasure or pain arising from 

 it ; by the degree of emotion or passion produced by these ; 

 and, lastly, by the emotions being more or less related to the 

 person feeling. 



XL VIII. If the force and .duration of impression, and the 

 attention of mind, are all in the due degree, the sensation often 

 remains for some time after the impression or action of the ex- 

 ternal body has ceased. " This has been noticed by philoso- 

 phers ; and is, by Sir Isaac Newton, illustrated by the following 

 common experiment : If I give a circular motion to the end 

 of a stick made red hot in the fire, it appears as one circular 

 fire. There certainly is a succession of sensations ; but each of 

 the impressions remains so long that it is succeeded by another, 

 so that we only observe one single circle of fire." 



XL IX. The mind admits of, or can attend to, one sensa- 

 tion only at one time. " This, I think, is universally agreed 

 upon by metaphysicians ; they have demonstrated it, and I 

 leave it to their proof. Thus, if a number of impressions quick- 

 ly succeed one another, the first received is very often obliter- 

 ated by the succeeding ; and when our bodies are exposed to 

 two impressions in the different parts, we attend only to one of 

 them to the strongest. So if my eyes are occupied upon any 

 object that draws my attention, I can give very little heed to 

 what is said ; and if I am very attentive in hearing, and my at- 

 tention is fixed to that, I am insensible to impressions made 

 upon my eyes or other organs. So it is an old observation in 



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