HERMANN VON HELMHOLTZ 



note, as it can be shown experimentally that as a 

 note rises in pitch the number of pressures also in- 

 creases. Helmholtz, for this and other purposes, 

 improved the syren of Cagniard-de-la-Tour, and 

 invented the well-known polyphonic syren now 

 found in every physical and physiological laboratory. 

 Another way of looking at this question of pitch is 

 to say that it depends on the duration of the indi- 

 vidual pressures. For example, the variation of pres- 

 sure involved in a tone of 256 vibrations per second 

 will last the -ai^th of a second, while that of its 

 octave will last only the -5x2^ of a second. It 

 is not necessary, therefore, to have a large number 

 of pressures per second to arouse a sensation of a 

 tone of a certain pitch ; a small number, possibly 

 only a few, if they come at a given rate, will be 

 quite sufficient. If now we suppose that there are 

 nerve-endings, say in the cochlea, so constructed as 

 to have each its own period of vibration, when 

 pressures come in at a certain rate, the structures 

 adapted to that rate will be thrown into action, 

 and we can conceive movements to be excited. 

 These movements will in some way stimulate the 

 nerve-ending, and a nervous impulse will be trans- 

 mitted to the brain, in which will arise, by some 

 molecular process, utterly unknown, the sensation of 

 a tone of that pitch. Again, on such a supposition, 

 it is easy to explain intensity or loudness. This will 

 evidently depend on the amplitude of movement of 

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