1000 AUDITORY SENSATIONS. [BOOK in. 



while a 4 tone ' is a single series of simple vibrations, a 4 note ' 

 may be and generally is a number of series of different vibra- 

 tions occurring together. While the fundamental tone deter- 

 mines the pitch of a note, the quality of the note is determined 

 by the number and relative prominence of the partial tones. 



621. In much the same way that rays of light of more 

 than or of less than a certain wave-length are incapable of ex- 

 citing the retina, our vision being limited to the range of the 

 visible spectrum, waves of sound of more than or of less than 

 a certain wave-length are unable to affect the ear so as to pro- 

 duce a sensation of sound. Vibrations having a recurrence 

 below about 30 a second are unable to produce a sensation of 

 sound; the note of the 16-feet organ pipe, 33 vibrations a sec- 

 ond, gives us the sensation of a droning sound ; a tone of 40 

 vibrations is quite distinct. Some authors however place the 

 limit at 24 or even 15 a second. If waves of long wave-length 

 are powerful enough we may feel them by the sense of touch, 

 though not by that of hearing. What we have just said applies 

 to vibrations which are simple, such as give rise to a pure tone ; 

 if the fundamental tone is accompanied by partial tones we 

 may hear one or other of these, and are thus apt to say we hear 

 the former when in reality we only hear the latter. As regards 

 the limit of high notes, it is possible to hear a note caused by 

 vibrations as rapid as 40,000 a second ; at least some persons 

 have this power, though the limit for most persons is far lower, 

 about 16,000. Some persons hear low sounds more easily than 

 high ones, and vice versa. This may be so pronounced as to 

 justify the subjects being spoken of as deaf to low or high tones 

 respectively, a condition which may be compared in a general 

 way to color blindness. The range in different animals differs 

 very widely, the high notes of the instrument known as Galton's 

 whistle, though inaudible to man, are distinctly heard by some 

 other animals, for instance, cats. 



The limitations which are thus imposed on our hearing do 

 not wholly correspond to the limitations of our vision. In the 

 latter case the limits are fixed wholly by the capacities of the 

 retina and 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 probable, 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 



