CHAP, iv.] HEARING. 1003 



best studied by means of musical sounds, since with these owing 

 to their very nature the stimulation is more uniform than with 

 noises. With almost any note, the sensation diminishes and 

 finally disappears if the sound be maintained long enough ; but 

 the exhaustion comes on more rapidly with high than with low 

 notes, especially with very high ones. If a sounding tuning- 

 fork be held up to one ear, and then, just as the sound becomes 

 inaudible be transferred to the other ear, the sound may be dis- 

 tinctly heard ; the fresh uritired sensory apparatus of the one 

 side is sensitive to the vibrations which the tired apparatus of 

 the other side can no longer feel. Or, if the tuning-fork which 

 the tired ear can no longer hear, be replaced by one vibrating 

 at the moment so far as can be arranged with the same intensity 

 as it, but of distinctly different pitch, this will be heard ; the 

 first tuning-fork only tired certain parts of the sensory appara- 

 tus, those affected by vibrations of a certain period characteristic 

 of the pitch of that tuning-fork, but left untired the parts of the 

 sensory apparatus responding to the vibration of other periods, 

 such as those of the second tuning-fork. Again, the quality of 

 a note struck. on a musical instrument depends as we have seen 

 on the presence of partial tones, having certain relations to the 

 fundamental tone. Now, if immediately before striking a note 

 on an instrument, choosing especially an instrument whose notes 

 are ' rich ' by virtue of the number or prominence of the partial 

 tones, we cause one of the partial tones of the note to be sounded 

 powerfully in the ear, the note when subsequently struck does 

 not possess its full quality; it appears 'thin' or 'poor.' This 

 is because the previous sounding of the partial tone has tired 

 the particular part of the auditory apparatus with which we 

 hear the partial tone, and in the whole sensation of the subse- 

 quent full note the constituent sensation corresponding to that 

 particular partial tone is absent or at least is below its normal 

 intensity. Thus we have in auditory sensations something 

 analogous to the " negative image " of visual sensations. 



We do not in hearing experience a sensation analogous to 

 the visual sensation of white light, a simultaneous stimulation 

 of the apparatus by vibrations of all kinds, and cannot therefore 

 experience an auditory sensation corresponding to the visual 

 sensation of black; the nearest approach perhaps to such a psy- 

 chological condition is that in which \ve are placed upon the 

 sudden cessation of powerful and varied music; at such times 

 we seem to be the subject of a "silence whirh can In- heard. " 



626. As in the case of visual sensations, so likewise in 

 the case of auditory sensations the duration of the sensation is 

 longer than that of the action of the stimulus, the auditory 

 sensation lasts after the waves of sound have ceased to fall upon 

 the ear. Hence when two sensations follow each other within 

 a sufficiently short interval, they are fused into one. Since a 



