It is true that Bach, first of all an organist himself, did not always bequeath such an ungrateful task to the organ; besides the organ of accompaniment, he gives us examples of what he calls _organo obligato_. Numerous cantatas furnish such instances; we find one in the Passion according to St. Matthew, with the added interest of an indication of the registration. It is where the chorus, in unison, sings the chorales, _O Lamm Gottes, unschuldig_, and _O Mensch, bewein' dein' Sünde gross_.[173] The register which Bach prescribes here is the _Sesquialtera_, undoubtedly in combination with some foundation stops. The character of this register, thus particularly selected, seems to call for the _tasto solo_; without doubt Bach demanded it because of its decisive quality, for the purpose of bringing out the chorale sung by the _ripieno_ against the other two choruses and the two orchestras, which he treated independently. The brightness of the _Sesquialtera_ would seem to recommend it also for the _sinfonie_, or prelude, of the cantata composed for the election of the Council[174] (August, 1731). This idea is supported by the fact that a _Sesquialtera_ was undoubtedly added to the _positif_ of the organ in St. Thomas' Church in 1730 or 1731, by the organ-builder, J. Scheibe. [Footnote 173: B.-G. iv. [The first and last numbers, respectively, of Part I. In the first the chorale is sung by a special chorus of sopranos, usually boys, while the two mixed choruses and the two orchestras are treated contrapuntally. In the other instance the two choruses are united in one, as are also the two orchestras, and the chorale is sung by all the sopranos, the counterpoint being assigned to the remaining three vocal parts, supported by the orchestra.--TR.]] [Footnote 174: B.-G. v.] A register of quite opposite effect was used to support the whole orchestra in the Reformation Cantata[175] (1717). The Luther chorale-melody is here entrusted to the sixteen-foot _Bombarde_ on the pedal, accompanied in the orchestra by the violoncello and the violone, a similar instrument. [Footnote 175: B.-G. xviii, 10.] In these particular instances we see that Bach departed from the general custom of omitting the reeds and mutation stops;[176] but here the organ derived from its own resources sonorities most individual in character, the accompaniment being furnished by a second instrument (the orchestra). Moreover, as W. Rust, the authorized editor of the Bach cantatas, says, "When the organ is _obbligato_ it does not present itself in a polyphonic capacity, for then it would cover up all the other instruments; but it should be treated as a solo part, like a flute or an oboe."[177] [Footnote 176: The _organo obbligato_ was sometimes written with more delicate intentions; for example, in the alto aria with accompaniment of an _oboe da caccia_, from the cantata _Wer weiss, wie nahe mir mein Ende?_ (B.-G. v.).]