I can find no better sub-forum for this. Please move if appropriate.
I remember seeing at least four of these in thrift stores over the years. Not recently though. My dad had one from the seventies that was sadly lost at some point.
I’m talking a plastic lens at least 15cm in diameter probably and hopefully bigger.
I want to build a slide viewer graphoscope with it.
Do you have a link to a concrete example?
What I’m looking for is almost a novelty item. Not something you’d normally think to get from a lab supplier.
Think they where meant for the same bracket of the market that also uses big fresnel lenses.
Only this actually has acceptable optical quality.
5" (12.5cm) magnifying glasses are pretty common, with a smattering of 5.5-5.75" (14.6cm) sizes.
Google "Giant magnifying glass."
The plastic lensed ones are rather excerable - I tried to find a good one for my father some years back with not much luck. I mean OK for reading the paper, but try to get an image ... My father was happy with it.