I believe the Merchant Adventurers' Hall sundial has a brass or bronze gnomon like a > which is fixed to a ring of the same metal surrounding the roundel in the dial on the outside. The gnomon root or dial origin is some way above the roundel and the actual gnomon. I think I can provide a clearer outside picture of that dial (and will send it to John off-list). Almost none of the old dials have the gnomon in situ. Feel free to use my wording, John.
Re refraction: if someone painted the glass not in reverse it would need to be inside, though I don't know what is customary. Given that dials can be quite small I think even 3 mm or 1/8" could have a noticeable effect. It would seem simpler, if the Japanese protective cover can be as much as 10" away, to put it outside the whole glass/gnomon assembly, assuming the gnomon can be arranged to project less than that. As long as the cover has parallel faces it would make no difference to the angle of the sun's light. It would reduce the amount of sunlight transmitted at low incident angles - but then perhaps any frame holding the cover will interfere anyway. To have the cover intersecting the gnomon would produce a complication - and why not use it to protect the gnomon as well as the glass? Regards Andrew James -
