It's essentially a Ramsden configuration but it focuses to a point inside the housing. Spinthariscopes were popular around the turn of the century they were an eyepiece used to see the fissioning of zinc sulfide atoms (they emit visible light photons) under bombardment by alpha particles (helium nucleus) emitted by radioactive radium. The zinc screen and small amount of radium were position on the bottom inside of the magnifying loupe. You would dark-adapt in a room and view the dim fissioning through the eyepiece. The cluster of fissioning atoms appears something like a 10th magnitude galaxy in an 8 inch scope. The diameter of this eyepiece is 40mm.
Edited by RichA, 19 September 2021 - 02:29 AM.