11. Between the swell[168] and the great organ a coupler shall be constructed. Finally, the whole instrument shall be revoiced, and the tremulant made to vibrate regularly.
II
The document just cited, which is preserved in the archives of Mühlhausen, is full of interest; we will now make a further study of two of its sections, which treat of the same subject.
I refer to the combination of organ with orchestra in the performance of the cantatas.
First, Bach speaks of the Fagotto, whose tone so easily assimilated with that of instruments; here he agrees with his contemporaries, who recommended the use of a sixteen-foot stop of more definite timbre than the bourdons, although not stronger,—it was also called Dulcian,—"dolce suono,"—in performing the basso continuo. The employment of the Stillgedackt, the softest stop in the organ, interests us in its use as a means of filling out this same figured bass. Such a register evidently lacked power, but was sought for that quality of indefiniteness, even of vacuity, which it possessed (still, in German, means quiet); this produced more the effect of diaphony, of a harmonic filling-in, like the sostenuto of certain of our wind-instruments, than of polyphony in real parts, which one could not distinguish.
These lines of J. Th. Mosewius[169] will give us, further, an idea of the rôle which the organ played in the orchestras of Bach and Händel: "It is a widely prevailing impression, and one confirmed by the new instrumentation which Mozart and Mosel made for the Händel oratorios, that by their use of the organ these two masters (Bach and Händel) supplied those features of our instrumentation which were then lacking. Such an opinion is correct, if nothing more is meant than that in concert rooms where no organ is available, it must be replaced by other instruments.[170] It must not be inferred that this new instrumentation maintains reciprocal relations with the original accompaniment. In the former it is the string-quartet which serves as the foundation of the harmony, and it is only the wind-instruments which affect the color. With Händel (and Bach) the organ, which fills out the figured bass, serves as a background for all the other instruments; the color is added by all the other voices of the orchestra, whether strings or wind."
Nothing could be more just than this statement of Mosewius; the organ serves to combine all the parts of the orchestra, unifying them without betraying its agency by any too assertive quality; a gray background, if you will, upon which some livelier colors are displayed, as in paintings of the school of Panselinos.
This testimony of Bach himself, specifying in his plans stops of a very soft and well-rounded quality for the accompanying organ, is corroborated by his contemporaries.
Scheibe, Adlung, and others permitted in the accompaniment of arias and recitatives but a single bourdon of eight feet, called, from its use for such purposes, Musikgedackt. A recitative, especially, was to be sustained lightly, for fear of covering the voice of the singer; a few prolonged notes to guide him, occasional soft chords, and, curiously enough, if one believe in the strict treatment of the organ, arpeggios, as upon a clavecin.
The staccato was generally employed in playing the bass; but this license stopped here, and for ordinary organ pieces Bach exacted from his pupils the strictest legato.