I was reading a short story called "Die Drei Schwestern" by Richard Dehmel, published in 1893, and came across this sentence:
Und so — ja — schritten sie davon — in den brennenden Abendhimmel hinein — schattenhaft schwarz wie ein Wandelbild — bis der Wald sie verschlang.
The context is that the narrator had been accommodating two people, Heinz and Marie, on his family's estate and now the two have gotten engaged and are leaving to live on Heinz's estate. I'm confused about the meaning of this word "Wandelbild." The DWB has the definition "durch einen projectionsapparat hervorgerufene wechselnde bilder, besonders farbenspiele" which is difficult to make sense of. It also has:
auch ein am bühnenhintergrund vorbeigezogenes, die verwandlung der scene andeutendes bild kann man so nennen; bei Campe verdeutschungswb. 108ᵇ wird wandelbild für anamorphose, ein bild (caricatur), das von vorn und von beiden seiten angesehen, drei ganz verschiedene gegenstände darstellt, vorgeschlagen
I looked up the cited book "Verdeutschungswörterbuch" by Joachim Heinrich Campe and accidentally found a different book of the same title by Daniel Sanders, published in 1884, where "Wandelbild" is given as an alternative German term for the more foreign word "Kaleidoskop," which seems to clarify the DWB's first definition. Still, I don't know how any of these meanings make sense in the context of the sentence. I did find a video of a kaleidoscope which shows some dark shapes floating around in the field of view; maybe the author is comparing those shapes to the silhouettes of Heinz and Marie? This seems far-fetched, but I don't know how many kaleidoscopes produced at this time contained those dark floating shapes.
There's also the German wikipedia page for the Isenheim Altarpiece, which is a "winged altarpiece"—a kind of painting that can be folded in different ways to reveal different paintings. The page describes the piece as a "Wandelaltar" with different "Wandelbilder." Still, I can't figure out how this would make sense in context. Maybe I'm missing something.