HERMANN VON HELMHOLTZ 



plain how it is that when we listen to the syren, as 

 its disc revolves faster and faster, our sensations go 

 on, not by leaps and bounds, but continuously. Since 

 the time when Helmholtz wrote, histological evidence 

 has accumulated, the rods and arches of Corti, the 

 fibres of the membrana basilaris, the hairs cells, and 

 even the nerve fibres in each auditory nerve, have been 

 counted, and it has been conclusively established that 

 there is in the cochlea a sufficient number of possible 

 vibratile masses to satisfy this theory. 



He then proceeded to examine the cause of con- 

 sonance and dissonance, and to apply his theory in ex- 

 planation. If we sound simultaneously two forks that 

 are in unison, the waves coincide and one sound is 

 heard ; but if the pitch of one of the forks is slightly 

 flattened, the waves do not coincide, and there are 

 maxima and minima. A rapid, rattling, beating sound 

 will then be heard, as if there were individual thuds 

 on the ear. If the forks are nearly the same in pitch, 

 the beat, as it is termed, will be heard as a rising 

 and falling in the intensity of the sound, and as the 

 difference in pitch between the forks is increased, 

 there is also an increase in the number of the beats. 

 If such beats are few in number, so as to be readily 

 counted, the sensation of waxing and waning is not 

 disagreeable ; but if they are sufficiently numerous, 

 it may be impossible to count them, and the sensation 

 is disagreeable. Such a sensation is that of disson- 

 ance. Helmholtz found the sensation to be most 

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