Let us help you identify that mysterious object you’ve found.
Currently in CHALLENGE mode: If you've got something obscure knocking about, post a picture, and let's see how we do. Please prefix such posts with "CHALLENGE:" so we know we've got a fighting chance.
It has 4 lenses in total, two at each end. The only text on it is the number 50, stamped into the bottom of the ring that goes around the outside to attach it to whatever it goes with
https://www.etsy.com/listing/1722174900/antique-pocket-field-microscope
Looks like this antique pocket field microscope
I'd guess some kind of vintage microscope 50mm eyepiece. Maybe telescope
Camera or projector lens, probably of petzval type.
Edit: the lack of an iris or slot for waterhouse stops makes me think projector lens.
My first thought, too. Its almost identical to the image from wikipedia's article on petzval lenses, even.
If you look at the photo of a magic lantern here https://blog.scienceandmediamuseum.org.uk/christmas-magic-lantern-slides/
The front lens cap is very similar.
Looks like a microscope or ~~telephone~~ telescope insert with adjustable eye correction.
Edit: Autocorrect
telephone?
I assume they mean telescope
Thank you, autocorrect to the rescue, again. Edited.
I wonder how many times autocorrect errors cause mayhem in communication compared with misspellings or mistyping?