John Weeks Moore, Complete Encyclopaedia of Music… (1880)
“ In the music of Corelli, Geminiani, Handel, &c., the general rules of finding the tonic, either in the major mode by the characteristic notes of the signature, or in the minor mode by the leading note accidentally inserted, are not always sufficient. When, instead of the complete series of sharps or flats of the signature, the last sharp or flat is suppressed, and inserted accidentally when requisite, like the leading note of the minor mode, such deviation from the usual method of notation is termed the ancient signature. ”
