This is a lot more complicated than I thought. I unscrewed the nipple, thinking the cover would slide right out, but no — nothing. Any ideas? I can't get a very good vantage point and am really hoping this can be done blindly from beneath.

This is a lot more complicated than I thought. I unscrewed the nipple, thinking the cover would slide right out, but no — nothing. Any ideas? I can't get a very good vantage point and am really hoping this can be done blindly from beneath.

Have a go at one of those three finger lookin' things holding the glass. Unscrew the little bolt holding it it. Or, more likely, one of them might be spring loaded. Take care not to drop the glass.