226 AUDITORY SENSATIONS. [BOOK in. 



cerebral centres ; radiant rays of longer wave-length than the 

 extreme visible red are able to get access to the retina through 

 the dioptric apparatus though they are unable to excite visual 

 impulses, or at least such visual impulses as can affect consciousness. 

 In the case of hearing, though the auditory epithelium is probably, 

 like the retinal structures, limited in its powers, narrower limits 

 are fixed by the subsidiary acoustic apparatus ; the tympanic 

 membrane, extensive as is its range compared with that of most 

 artificial membranes, cannot respond to all vibrations ; and hence 

 its powers fix the limits of hearing. The reason why we appreciate 

 high notes more readily than low ones is probably to be referred to 

 the tympanum rather than to the auditory epithelium. And the 

 condition of the tympanal apparatus as affected by disease will 

 determine the relative appreciation of low or high tones ; in 

 certain states of the tympanum the ear becomes unusually sensitive 

 to high notes ; an instance of this is seen in the paralysis of the 

 stapedius muscle due to injury or disease of the seventh nerve. 



843. We dwelt, in speaking of vision, on our power of appre- 

 ciating differences of brightness or luminosity ; we have a similar 

 power of appreciating differences in loudness ; and that relation 

 between differences in the intensity of the stimulus and differences 

 in the intensity of the sensation, which we spoke of as Weber's 

 law ( 747), holds good for hearing as well as for vision. 



The power of distinguishing difference of pitch, 'the power of 

 recognizing the difference between two notes of different pitch, 

 and the appreciation of the qualities of various musical sounds 

 which is built up on this, may in a general way be compared 

 to acuteness of colour vision. It is however, as we have said, very 

 different from that in many respects, and varies much more widely 

 than does that. As is well known the difference in this power 

 between different individuals, according as they have or have not 

 a ' musical ear,' is very great. Some persons even though fairly 

 sensitive to differences of loudness, are unable to distinguish two 

 notes differing considerably in wave-length. On the other hand 

 a well-trained ear can distinguish the difference of a single or even 

 of a half vibration a second, and that through a long range of notes. 

 As might be expected the power of appreciating difference of pitch 

 is not the same for all audible notes. The range of an ordinary 

 appreciation of tones lies between 40 and 4000 vibrations a second, 

 i.e. between the lowest bass C (Ci 33 vibrations) and the highest 

 treble C (C 5 4224 vibrations) of the piano ; tones above and below 

 these, even though audible, are distinguished from each other with 

 great difficulty. The power of recognizing, and being able to name, 

 a note when heard, is an extension of and based upon this power 

 of recognizing differences of pitch, though not by itself exactly the 

 same thing. 



844. We said, in speaking of vision ( 748) that, probably, 

 several undulations falling in succession upon the retina were 



