ON THE SENSES IN GENEBAL. 511 



sensibility than that of sight. Thus, in the first place, hearing is 

 accomplished by the direct intervention and contact of a material 

 body the atmosphere ; for sonorous impulses cannot be produced 

 in a vacuum, and we hear no sound from a bell rung under an 

 exhausted receiver. Secondly, the nature of the impressions pro- 

 duced by sound is such that we can often describe them by the 

 same terms which are applied to ordinary sensations. Thus, we 

 speak of sounds as sharp and dull, piercing, smooth, or rough ; and 

 we feel the impulse of a sudden and violent explosive sound, like 

 that of a blow upon the tympanum. 



By this sense, therefore, we distinguish the quality, intensity, 

 pitch, duration, and direction of sonorous impulses. The delicacy 

 with which these distinctions are appreciated varies considerably 

 in different individuals ; and in different kinds of animals there is 

 reason to believe that the diversity is much greater, some of them 

 being almost insensible to sounds which are readily perceived by 

 others. In man, the number and variety of tones which can usually 

 be discriminated is very great ; and this sense, accordingly, in the 

 complication and finish of its apparatus, and the perfection and deli- 

 cacy of its action, must be regarded as second only to that of vision. 



ON THE SENSES IN GENERAL. There are several facts connected 

 with the operation of the senses, both general and special, which 

 are common to all of them, and which still remain to be considered. 

 In the first place, an impression of any kind, made upon a sensi- 

 tive organ, remains for a time after the removal of its exciting cause. 

 We have already noticed this in regard to the senses of taste, smell, 

 and sight, but it is equally true of the hearing and the touch. 

 Thus, if the skin be touched with a piece of ice, the acute sensa- 

 tion remains for a few seconds, whether the ice be removed or not. 

 For the higher order of the special senses, the time during which 

 this secondary impression remains is a shorter one. In the case of 

 hearing, however, it has been measured with a tolerable approach 

 to accuracy ; for it has been found that, if the sonorous undulations 

 follow each other with a greater rapidity than sixteen times per 

 second, they become fused together into a continuous sound, pro- 

 ducing upon the ear the impression of a musical note. The varying 

 pitch of the note depends upon the rapidity with which the vibra- 

 tions succeed each other. When the succession of vibrations is 

 very rapid, a high note is the result, and when comparatively slow, 

 a low note is produced ; but when the number of impulses falls 



