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PHYSIOLOGY 



COMBINATION TONES. If two tuning-forks, with an interval of one-fifth 

 between them, are sounded together, we may hear a weak lower tone, the 

 pitch of which is an octave below that of the lower fork. This is known as 

 a ' combination tone.' The combination tones are divided into two classes : 

 (1) ' difference tones,' in which the frequency is the difference of the frequen- 

 cies of the generating tones ; (2) ' summation tones,' which have a pitch 

 corresponding to the sum of the vibrations of the tone of which they are 

 composed. By means of appropriate resonators these tones can be rein- 

 forced, showing that they have an objective existence and are not produced 

 in the ear itself. 



Not only can the ear appreciate differences between different musical 

 instruments, dependent on the varying overtones present in the sound pro- 

 duced by each instrument but, when a number of these instruments are 

 sounded simultaneously, the ear can pick out from the compound sound the 

 notes due to the individual instrument, and a person with a trained ear can 

 with ease name notes composing any chord struck on an instrument such as 

 the piano. 



OHM'S LAW. This power of analysis, which is possessed by the ear, or 

 at any rate by the auditory apparatus, may be stated in the form of the law, 

 known as Ohm's law, which is as follows : 



" Every motion of the air which corresponds to a composite mass of 

 musical tones is capable of being analysed into a sum of simple pendular vibra- 

 tions, and to each single vibration corresponds a simple tone, sensible to the 

 ear and having a pitch determined by the periodic time of the corresponding 

 motion of the air." 



SOUND LOCALIZATION. Experiment shows that man and animals 

 can appreciate with fair accuracy the direction from which a sound 

 is coming. There has been considerable speculation as to how this 

 information is obtained, and although the subject has not been completely 

 elucidated it appears to have been established that the following factors are 

 important. 



(1) The intensity of the sounds entering the two ears. When a sound is 

 coming from one side the ear on that side receives the more powerful stimulus. 



(2) The relative intensities of the components of high and low pitch vary 

 with direction, because the notes of long wavelength (low pitch) will be 



