568 PHYSIOLOGY 



the fourth, /, is /, a, c, with the vibration frequencies -*, f, 

 4, 5, 6. The C major scale is therefore as follows : 

 CDEFGAB C 



, .e. 



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Different instruments are tuned to one normal note, i.e. to A 

 with 440 vibrations per second (this note varies somewhat in different 

 countries). Taking this as the normal, the vibration frequencies of the 

 various notes used in music are given in the following Table : 



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 frequencies 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 reinforced, 

 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 produced 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. 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 vibrations, and to each single vibration corresponds a simple 



