David H. Keller, "The Jelly-Fish" (1929) from American Fantastic Tales: Terror and the Uncanny from Poe to the Pulps:
The LoA page for this story hits a few important points about Keller: he was a doctor, served in WWI, dealt with shell-shock, wrote all his life but only later started to send stories out, and has some reservations about technological optimism. For more on Keller, see the SF Encyclopedia and Fantasy Encyclopedia entries, which has a lot more info on him and (as usual for those publications) some analysis. There's his Charles Atlas physical education non-fiction! There's his occult detective series! There's his weird sexual hang-ups!
All of which makes Keller sound like a fascinating subject to study; but maybe not the most interesting pulp author to reread nowadays. Curiously, both encyclopedia entries mention a few of his works, but neither mention "The Jelly-Fish." Which is no surprise after we read this rather silly story: on a research yacht studying micro-organisms in the ocean, the hated and smug professor gives a speech about how everything is possible with the will and he will demonstrate this by shrinking down to microscopic jelly-fish size--at which point he gets eaten by a jelly-fish. There's really no surprise there and no shock. The story mostly leaves one with the desire to chuckle. The mad scientist is so over-the-top mad, the action is so ridiculous ("by my will I can change into a microscopic version of myself and not worry about, you know, drowning in the drop of water, but I'm helpless when confronted by a jellyfish"?).
That said, we could put this in the long line of microscopic fiction, from Fitz-James O'Brien's brilliant "The Diamond Lens" to Richard Matheson's unrealistic but thematically interesting The Incredible Shrinking Man. And I wonder if you could do something like this today? Or should we leap over the small scale here and just write Jelly-fishnado?