100 ON THE PHYSIOLOGICAL CAUSES OF 



of the tone. If the beats are more rapid the tone grates 

 on the ear, or, if it is high, becomes cutting. A grating 

 tone is one interrupted by rapid breaks, like that of the 

 letter B, which is produced by interrupting the tone of 

 the voice by a tremour of the tongue or uvula. 1 



When the beats become more rapid, the ear finds a 

 continually-increasing difficulty when attempting to hear 

 them separately, even though there is a sensible rough- 

 ness of the tone. At last they become entirely undis- 

 tinguishable, and, like the separate puffs which compose 

 a tone, dissolve as it were into a continuous sensation 

 of tone. 2 



Hence, while every separate musical tone excites in 

 the auditory nerve a uniform sustained sensation, two 

 tones of different pitches mutually disturb one another, 

 and split up into separable beats, which excite a feeling 

 of discontinuity as disagreeable to the ear as similar 

 intermittent but rapidly repeated sources of excitement 

 are unpleasant to the other organs of sense ; for example, 

 flickering and glittering light to the eye, scratching with 

 a brush to the skin. This roughness of tone is the es- 

 sential character of dissonance. It is most unpleasant to 

 the ear when the two tones differ by about a semitone, in 

 which case, in the middle portions of the scale, from twenty 

 to forty beats ensue in a second. When the difference is 

 a whole tone, the roughness is less ; and when it reaches 

 a third it usually disappears, at least in the higher parts 

 of the scale. The (minor or major) third may in conse- 



1 The trill of the uvula is called the Northumbrian burr, and is not 

 known out of Northumberland, in England. In France it is called the 

 r grasseye or proven fal, and is the commonest Parisian sound of r. The 

 uvula trill is also very common in Germany, but it is quite unknown in 

 Italy. Tn. 



2 The transition of beats into a harsh dissonance was displayed by means 

 of two organ pipes, of which one was gradually put more and more out of 

 tune with the other. 



