f)^^ ! 



. 



CHAP, iv.] HEARING. 225 



different vibrations occurring together. While the fundamental 

 tone determines the pitch of a note, the quality of the note is 

 determined by the number and relative prominence of the partial 

 tones. 



If we compare auditory with visual sensations it is obvious 

 that loudness of sound corresponds to brightness or luminosity 

 of light ; in both cases the terms denote the intensity of the 

 sensation. We may perhaps compare the character of pitch, de- 

 pendent on the wave-length of the sound vibration, to the character 

 of colour dependent on the wave-length of the ray of light ; and 

 we may, in a general way, liken the auditory effect produced by a 

 sound of a particular quality to the visual effect produced by an 

 object which excites mixed colour sensations, owing to rays of 

 several different wave-lengths falling at the same time on the 

 retina ; on examination however it will be found that the 

 differences in these respects between the two sets of sensations 

 are more striking than the resemblances. 



842. In much the same way that rays of light of more than 

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

 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 produce a sensa- 

 tion 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 second, 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 colour 

 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 arc fixed wholly by the capacities of the retina and 



15 



