‘The world corrupts me, I think. Or perhaps it's just the weather. It pulls me down and makes me think like you, that one should shrink inside, down and down to a little point of light, preserving one's solitary soul like a flame under a glass.
— Wolf Hall by Hilary Mantel
All I can reasonably visualize is something like the picture below. Am I right? If I am right, why use "under" when a flame is "in" a glass?