Hughes
The only exception is the double tone F#-G?, which is a curious study. F# as a harmony takes the double tones as sharps, and F? is E#. G? is also a harmony sounding the same tones, by taking the double tones as flats, and B? as C?. F# therefore takes the imperfect tone of E#, and G? the imperfect tone of C?. (See here the harmony of G? in musical clef.) [Harmonies of Tones and Colours, Combinations of dissonance, rests, page 24]