Interesting. I’m with James. I think it is a glass object that has been painted, and the painting has been finished, but I doubt it is a telescope. There are various geometrical, navigational and surveying instruments scattered round the room. I think the object in question could be a spirit level, with metal end sections and a short central glass section containing the liquid, through which we see the monkey’s arm. Such a level would be an important tool for a painter drawing horizons and buildings. Unlike a builders’ level, you might not make it with a flat base, as there would be no reason to do so.
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