1002 AUDITOBY SENSATIONS. [BOOK m. 



ited in number, the sound, even though it is recognized as a 

 musical sound, is not clearly appreciated ; its pitch is not dis- 

 tinctly recognized. In such a case the recognition may be 

 made more full and certain by increasing the number of vibra- 

 tions ; in order that we may appreciate the pitch of a sound the 

 ear must receive a larger number of vibrations than are neces- 

 sary merely to enable us to recognize that the sound is a definite 

 one. Conversely even when the vibrations are too few to give 

 rise to a sensation of a definite tone, consciousness is not wholly 

 unaffected, an auditory sensation is produced, though it cannot 

 be called one of tone. These facts indicate the complex nature 

 of the nervous processes which form the basis of auditory sen- 

 sations ; we might say this of sensations in general, for similar 

 results are observed in the case of all sensations. 



624. As we said above ( 619) noises are not sharply 

 defined from musical sounds, they differ only in being more com- 

 plex and less regular ; and what has just been said in respect to 

 musical sounds, holds good to a large extent for noises. We 

 readily distinguish, in noises, difference of loudness ; we may 

 also in many cases recognize a dominance of pitch, due to the 

 fact that among the multifarious vibrations certain groups of 

 vibrations are repeated periodically ; we distinguish a rumbling 

 noise in which vibrations of slow recurrence are prominent from 

 a harsh shrill noise in which rapid vibrations are similarly 

 prominent ; we also recognize qualities in noises, we distinguish 

 one noise from the other by the characters of the predominant 

 constituent vibrations. Owing to the fact to which we just now 

 referred, that in a musical sound the effect on consciousness is a 

 summation of the individual effects of the several vibrations, we 

 are more sensitive to a musical sound of not too short duration, 

 than to a noise involving an equal expenditure of energy. On 

 the other hand the limit of the number of movements necessary 

 to give rise to a sensation of noise is less than that required for 

 a musical sound ; a few vibrations insufficient in number to give 

 rise to the sensation of a tone are able to give rise to an auditory 

 sensation which we may call a noise, and probably one movement 

 of the tympanic membrane might if ample enough give rise to 

 such an auditory sensation. Moreover owing to the very irregu- 

 larity of a noise, to the varied character of the constituent molec- 

 ular movements, we have a very great range in distinguishing 

 various noises ; persons who have great difficulty in detecting 

 different notes can often readily recognize differences in noises. 



625. In treating of vision we dwelt at some length on the 

 phenomena of exhaustion which make their appearance when the 

 stimulus is continued. These occur in hearing also, and indeed 

 are indicated by such common phrases as " a deafening noise ; " 

 but they are not so prominent as in vision, and do not so dis- 

 tinctly serve as the basis for theoretical discussions. They are 





