CHAP, iv.] HEARING. 227 



necessary for the development of a visual sensation. In like 

 manner, in order that a distinct sensation of a musical sound may 

 bs developed, several, or at least more than one wave of sound must 

 fall on the ear. The various observers are not agreed as to the 

 lower limit of the number of vibrations necessary in order that the 

 affection of consciousness may take the form of a definite musical 

 sound; some place it at five, others higher, while it has~t>e<Dn 

 asserted that two vibrations are sufficient. When the vibrations 

 are thus limited in number, the sound even though it is recognized 

 as a musical sound, is not clearly appreciated; its pitch is not 

 distinctly recognized. In such a case the recognition may be 

 made more full and certain by increasing the number of vibrations ; 

 in order that we may appreciate the pitch of a sound the ear must 

 receive a larger number of vibrations than are necessary merely 

 to enable us to recognize that the sound is a definite one. Con- 

 versely 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 sensations ; we might 

 say this of sensations in general, for similar results are observed in 

 the case of all sensations. 



845. As we said above ( 840) 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 irre- 

 gularity of a noise, to the varied character of the constituent 

 molecular movements, we have a very great range in distinguishing 



