Thanks, newpoietikos, for providing more context and background. I had only a little time yesterday morning to look at this and I could not figure out just from the OP what exactly was being described here. Since then I have looked at the original and the examples from Schumann's work.
It's very hard to tell without seeing the musical example.
That's for sure! What I was picturing was much different than the actual examples. The author chooses some of the most exotic examples in the piano repertoire to make his point...
The author also speaks of Schuhmann's youthful exuberance in writing scores.
Yes. Schumann was a little odd in some ways. In his great piano cycle "Carnaval" (also a youthful work), there are 3 "pieces" consisting only of 3 or 4 notes, all of them breves! These are the "Sphinxes".
In diesem Fall soll der Spieler aber erst das b loslassen
...und das C unten in der linken Hand. Es gibt ja 2 Pausenzeichen. ;-)
To get to the translation...
bei liegenbleibender Taste maybe; "while the key remains depressed"
Schumann thus wrote down the accents in the first example
Hmmm. The "so" in the German refers to the manner of setting the accents in the example.
That is, a tied note with an accent mark. This is of course impossible on the piano, modern or ancient. With "thus" it is unclear what is meant. Maybe: In the first example Schumann wrote/placed (hinschreiben denotes a location) the accents in such a manner/thusly, even though the player cannot possibly execute them while the key remains depressed. But the ceasing/dropping out of individual notes automatically results in a (s)light strengthening/reinforcement.[!]
I have a musically educated assistant,
Good. You will need this. :-) I think what you will also need is someone trained in acoustics.
At first reading this does seem impossible, but given the examples it sounds at least plausible. The translation for Verstärkung is crucial here. "Accent" makes one automatically think of loudness and that's not necessarily or entirely what is meant by Stumpf. As far as I can tell he is referring to the psychological perception of the notes becoming louder, due to the fact that the other notes distract us from the top note, but not that there is an actual measurable change in loudness. We tend to hear chords as masses of sound and not as individual notes. The removal of (lower) notes one by one causes us to refocus our attention on the top note and this is what we perceive as an "accent" or strengthening of the note. Hmmm. After playing the two Schumann examples I only find the first one convincing and the "accent" is very slight indeed; others might be less convinced. Aural perception is quite a tricky thing. The other thing he goes into is possible acoustical reasons for the apparent increase in volume due to overtones from the lower notes drowning out the top notes. When the "masking" overtones go away we hear the top notes as "louder". Hmmmm.