The progress of classification is distinctly seen in the use of seven letters instead of fifteen, affording a tacit recognition of the most essential underlying facts of harmony--_the equivalence of octaves_. The staff, however, affords the eye no assistance at this point, since the octaves of notes occupy relatively entirely different positions upon it, the octave of a space being invariably a line, and the octave of a line a space. Moreover, the octave of a bass line is always very differently located when it falls upon the treble staff, and, _vice versa_, the octave of a treble note falling in the bass is very differently placed. If a notation had to be made anew it would no doubt facilitate matters to make use of a staff so planned as to bring out the equivalence of octaves more perfectly. A recent American designer, Mrs. Wheeler, has proposed a double staff of six lines, divided into two groups of three, for the treble and bass, thus presenting for the piano score four groups of three lines each, separated by smaller or larger intervals. Upon such a staff every tone would fall in the same place upon the three lines in every octave, the octave of the first line of the lower three would be the first line of the second three, and so on. This, however, is to anticipate. The smaller rhythmic divisions of the measure were very little used in the old music which, if not sung in slow time, was at least written in long notes, and the smaller varieties of notes are the invention of a period perhaps rather later than that at which we have now arrived. They belong to the elaborate rhythmic construction of the music of Haendel, Bach, Scarlatti and Haydn. CHAPTER XVI. MUSICAL INSTRUMENTS. THE VIOLIN, ORGAN, ETC. I.