Under glass

I am wondering what the phrase "under glass" means. One may say that such and such is frozen/fixed under glass. I feel that I only have a partial understanding of this phrase.

Maybe it refers to objects protected and displayed under a "bell jar." See illustration here -- www.tiac.net/users/shansen/belljar/ Or maybe it refers to an item pressed and preserved between two panels of glass. Anyone have other ideas?

My number one impression when I read a nonliteral use of "under glass" is fixity. Something kept under glass can be viewed but not changed. Suppose it's a manuscript, for instance, and you spot an error in it . . .
It's like the often heard "These rules aren't carved in stone," but different

How about looking at this in the culinary sense - I ordered pheasant under glass for my dinner. It was (maybe still is) the practice to make an elegant presentation of fine dishes on a silver tray covered with a glass dome. (See reference above to the bell jar idea.)

Perhaps the restaurant did this as a subtle form of advertizing, since newly arrived patrons could see how scrumptious a dish looked as it passed by their table, and might be tempted to order the same.