Well it has me stumped how it can happen.
The "persistent vision" thing isn't the reason because that can only reflect what's there - and what's there is a corona that's a millionth of the brightness of the sun and the effect of doing what has been suggested by your colleague above "when you stand in the shadow of the capstone and look at the Sun through it, the effect is that the intense glare of the sky around it is almost completely removed" is to reduce the brightness of the corona in direct proportion to the reduction in the brightness of the sky so you're no nearer seeing it than before.
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