Don't mess with your eyes or your persistence of vision will be about ten minutes. Get some welding glasses and be strict about using them. Otherwise you are risking blindness both temporary and permanent.

The problem with this kind of 'truism' is that it's only a guide. Our eyes, the muscles that control them, and indeed the entire visual cortex, all respond to being exercised. As long as you don't try too much too soon, you can train your focussing capability and increase or decrease the sensitivity of your rod and cone light receptors for day and night vision!

You may be imputing greater skills than the makers possessed. Perhaps the intention of the aperture was simply to illuminate an object inside the vault on a certain day, or range of days. This might have been a carved stone or wooden object. For instance. Similar effects were generated at places such as Maes Howe. A sundial/calendrical calculator would have been more easily constructed in tree and the Quoit will probably have had a more ceremonial/magical function. Perhaps it was where a great leader contemplated a special sunrise or something similar.