The "world in the bell jar" is inspired by all those projects that people keep posting in the Tactile section concerning "canned" or "captive" fairies, and the "fairie garden" projects that have been popping up lately on Crafts pages. I'm thinking that, perhaps, it's a woodland fairy (perhaps an childhood playmate of the Master's?) that the Master of the house caught locally and put into the bell jar to protect her from the Vicar, or maybe an intinerant exterminator of some sort, and that she lives quite happily in her little private county in the bell jar - but she misses the Master, and is shy but excited to see the newcomers to the Library.
If that's acceptable, that is...