Hermione is showing Ron how to levitate a feather (from Japanese translation of Harry Potter):
(Hermione recited the spell) and then, the feather left the desk and floated one or two metres above their heads.
I don't understand what ではないか is doing here. I'm used to seeing it as a rhetorical question, but I can see no place for that in this narration. In the original book it is just stated as a fact that the feather floated. There is no additional questioning nuance.
Apologies if this has already been asked, but there are so many question on ではないか and its variants that I failed to find anything obviously relevant.