I picked up an 80mm glass sphere the other day. I have plans to eventually turn it into a Campbell-Stokes sunshine recorder, but it’s quite fun to play with regardless.
It’s occurred to me that it’s actually quite a dangerous object to leave laying around. Since the essence of the recorder is to burn a large track on paper with the concentrated sunlight, leaving the lens on a table risks at best turning it into a impromptu recorder, and at worse perhaps burning your house down…
Here’s a little box to keep the lens away from sunlight, and also avoid scratching: