Trethevy Quoit 11
Image by carol27

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.

The 'persistent vision' question is largely irrelevant in this context, but not in others. The fact remains however that the Solar Corona can be observed.
I reiterate: viewed at an angle the solar corona can be gazed upon, in the same way as with a coronograph, but in this case it is the glare of the sky that is obscured and not the solar disc itself. I only know this because it is possible and I, and others, have done it. Viewed in this way the true nature of the sun is revealed. Green, blue, yellow, black; heaving and boiling, and in this way we share a direct physical experience with the builders and users of this structure. This has to be seen to be believed. Again, I say, none of these claims are made lightly. The effect upon the eye and the mind is sensational, and I invite anyone who is sceptical to come and try it. Keep the questions coming.