1873. A. J. Ellis, in Atkinson, trans. Helmholtz’ Pop. Lect. Sci. Subj., iii. 102, note. These [combinational tones] are of two kinds, differential and summational, according as their pitch is the difference or sum of the pitches of the two generating tones.
1881. J. Naylor, Lett., in Nature, XXIV. 2 June, 100/2. I tried in vain however to obtain resonance for a differential and summational tone.
