512 THE SPECIAL SENSES. 



below sixteen per second, we then begin to perceive the distinct 

 vibrations, and so lose the impression of a continuous note. 



All the senses, in the second place, become accustomed to a con- 

 tinued impression, so that they no longer perceive its existence. 

 Thus, if a perfectly uniform pressure be exerted upon any part of 

 the body, the compressing substance after a time fails to excite any 

 sensation in the skin, and we remain unconscious of its existence. 

 In order to attract our notice, it is then necessary to increase or 

 diminish the pressure ; while, so long as this remains uniform, no 

 effect is perceived. But if, after the skin has thus become accus- 

 tomed to its presence, the foreign body be suddenly removed, our 

 attention is then immediately excited, and we notice the absence of 

 an impression, in the same way as if it were a positive sensation. 



We all know how rapidly we become habituated to odors, whether 

 agreeable or disagreeable in their nature, in the confined air of a 

 close apartment; although, on first entering from without our 

 attention may have been attracted by them in a very decided 

 manner. A continuous and "uniform sound, also, like the steady 

 rumbling of carriages, or the monotonous hissing of boiling water, 

 becomes after a time inaudible to us; but as soon as the sound 

 ceases, we notice the alteration, and our attention is at once excited. 

 The senses, accordingly, receive their stimulus more from the varia- 

 tions and contrasts of external impressions, than from these impres- 

 sions themselves. 



Another important particular, in regard to the senses, is their 

 capacity for education. The proofs of this are too common and too 

 apparent to need more than a simple allusion. The touch may be 

 so trained that the blind may read words and sentences by its aid, 

 in raised letters, where an ordinary observer would hardly detect 

 anything more than a barely distinguishable inequality of surface. 

 The educated eye of the artist, or the naturalist, will distinguish 

 variations of color, size, and outline, altogether inappreciable to 

 ordinary vision ; and the senses of taste and smell, in those who are 

 in the habit of examining wines and perfumes, acquire a similar 

 superiority of discriminating power. 



In these instances, however, it is not probable that the organ of 

 sense itself becomes any more perfect in organization, or more 

 susceptible to sensitive impressions. The increased functional 

 power, developed by cultivation, depends rather upon the greater 

 delicacy of the perceptive and discriminative faculties. It is a mental 

 and not a physical superiority which gives the painter or the 



