§ 8. DifferenceTones.—When two tones are sounded together, certain other tones are heard, occasionally with great distinctness, for which there is no assignable physical stimulus. Within the compass of the same octave, there are mainly two of these. One corresponds to the difference between the vibration numbers of the primary tones, and is called the first differencetone. The other corresponds to the difference between the vibration number of the higher tone and twice the vibration number of the lower tone, and is called the second differencetone. The mode in which these tones are produced has not yet been satisfactorily determined. But it seems that they are due to the structure and function of the organ of hearing, and not to physical conditions. Their explanation forms an important test for any general physiological theory of sound-sensations.