Christopher Wren had a problem when designing the Sheldonian. He modeled it on an ancient Roman theater, but that theater had an open roof--in the Oxford climate that was not practical. Wren had to come up with a way to put on a roof that would span the wide extent of the building, and support it without putting big columns into the auditorium that would obstruct the sight lines of the audience. He devised a system of huge wooden trusses in the roof, concealed from view by this fancy painted ceiling.
The painting is called "The Triumph of Religion, Arts and Science over Envy, Hate and Malice." Around the border you can see a canvas cover, like the one the original Roman theater had, being drawn back to reveal the allegory. When you climb up to the cupola of the Sheldonian, you pass through an attic where you can see some of the building's structure.
The cupola of the Sheldonian is one of four structures that one can climb up into for views of the rooftops of Oxford. I think the others are steeper climbs than this one. I was proud of myself for making it up here. In this view you can see Blackwell's down below, and the New Bodleian Library.