In casual conversation with a composer/theorist the other day--a friend whose musicianship and knowledge I trust--the Beethoven "Spring" Sonata came up (Op. 24 in F for piano and violin). He brought up the extraordinary modulation in the slow movement. At the same instant he said "To F# minor" and I said "To D major."
According to him it is actually a modulation to G-flat minor.
I don't think it is a modulation in any traditional sense, but rather the sound that pivots between piano heaven (G-flat major, the measure preceding the F# minor measure) and violin heaven (D major, the measure following). This is the difference between acoustically generated thought and the need to have things make sense on paper.
I know from a lifetime of involvement in Beethoven's chamber music for mixed ensemble that he was fascinated by the acoustical specificity of instruments, the clash between them furnishing much of the so-called thematic material of the compositions.
