Ok we'll be honest we are really not sure what this is but the consensus of opinion between antique collecting friends is that it may be a fly tray. There is obviously a piece missing, we assume something should be screwed into brass fitting that would raise the blue glass up ( as we have tried to illustrate in photo). Otherwise it would be a useless fly trap as the flies couldn't get in.
If of course you know different please enlighten us on what this is, as we are only really guessing as to what this really is.
In reasonably good condition for age and for whatever purpose it has.
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