342 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



these two conditions is that the amplitude of displacement 

 becomes rapidly greater as the wave passes forwards along 

 the basilar while it diminishes as the wave returns along 

 the Keissnerian. For any given series of waves therefore 

 due to the continuous sounding of a note of constant pitch, 

 and constant intensity, the maximum effect upon the organ 

 of Corti will be produced at the uppermost only of the 

 many levels at which waves pass each other on the two 

 membranes in corresponding phases. If, therefore, the 

 damping effect of the mechanism of the middle ear be of 

 proper extent then the effect upon the organ of Corti 

 already described will occur at this one uppermost only. 

 That is, though waves pass at many points it is only at 

 the uppermost of them that the disturbance will be 

 sufficiently great to bring the tectorial membrane down 

 upon the hairs of the hair-cells with sufficient force to 

 stimulate the nerve-ends. 



Equal intensity of sound in the case of two notes of 

 different pitch, does not correspond with equal amplitudes 

 of displacement of the air or of the tympanic membrane. 

 Nor does it correspond with equal displacements of the 

 basilar membrane. When two sounds — both pure tones — 

 of different pitch are said to be of equal intensity, that 

 means that the energy traversing unit area of a plane at 

 right angles to the direction of transmission in unit time 

 is equal in the two cases. This being the case two tones 

 of equal intensity but different pitch involve displacements 

 proportional to the wave-lengths. A very low note will 

 therefore involve a far larger displacement than a much 

 higher note of equal intensity. Still supposing the limits 

 of intensity above laid down, suppose two notes are 

 sounded in succession, a high one and a low one : and 

 suppose the intensity of the two to be at first equal : then 

 the higher note will produce a stimulation at one point of 



