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VIII. § 91.] INFLUENCE OF COMBINATION-TONES. 183
when the interval widens and when it contracts, so that the Octave, in simple tones, is a well-defined concord bounded on either side by decided discords. This result may be easily verified experimentally by taking two tuning-forks forming an Octave with each other, and throwing the interval slightly out of tune by causing a pellet of wax to adhere to a prong of one of them. On vigorously exciting the forks the beats will be distinctly heard1.
The Octave is the only interval which is defined by the beats of a combination-tone of the first order with one of the primary tones. For the next smoothest concord, that of the Fifth, we are obliged to have recourse to the second order. Thus, pro­ceeding by successive subtraction, we have :—
Primaries                   200                301
C. T. of 1st order                 101
C. Ts. of 2nd order 99                200
No. of beats per sec.                 2
1 As, however, this mode of treatment produces very percep­tible overtones, [§ 48, note.] the experimenter must be on his guard against attributing to combination-tone beats an effect which is really due to beats of overtones. There is special risk of being thus led astray in the case of the Octave, where the first-order combination-tone coincides in pitch with the lower of the two primaries. The result of my own observation is that with mounted forks excited in the gentlest possible way there is but slight dissonance even in an impure Octave.
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