Kim Churchman responded to my treatment of limn with the following comment. I’ll bet it occurred to others, as well:
“About limn: doesn’t limnology mean the study of lakes? How does that fit with your definition of limn?
The answer is straightforward: in no way at all. Yes, the word limne means “lake” in Greek and, I think, Latin, too. But English limn comes from Latin lumen “light”, after good working over down through the ages.
Remember that English has been hauling words out of Latin for centuries. The words borrowed in earlier centuries underwent all the changes English has gone through since Old English. Limnology is a recent addition, only since lakes became a focus of scientific study.