It turned out I didn’t need one.

Is there any reference confirming that the Germans ever used the device?

BTW, Feynman misspelled the place name. It is the Frankford Arsenal, not Frankfort. I live in Philadelphia and it’s a common place name (Frankford Arsenal, Frankford Avenue, etc).

When I was a boy I used to look at stream water under a microscope. I often saw paramecia. I always wanted to see an amoeba but never did. One time as the water droplet was drying up, I saw what I thought was an amoeba at the receding edge of the droplet. It was my misfortune, I thought, to see an amoeba for just a second, until the drying overtook it.

I believed that for years, until I read Feynman’s description of a paramecium losing its shape and becoming amoeba-like. Feynman wrote that he had never read about such a thing; I had never read it until I read Feynman’s book, and I haven’t read it since. It’s not even in the wikipedia paramecium entry.