Franklin's Bifocal Spectacles

Franklin invented bifocals; he also experimented with optical filters in his old age. The pair recovered at Independence Hall is a Mason's variant: not a vision aid but a decoder, with green, red, blue, and clear lenses that slide across the eye. Holding the lenses over the back of the Declaration in different combinations causes faint patterns to leap out of the parchment that were invisible to the naked eye. We don't know how this works chemically — fluorescence under specific spectra, residue from layered inks, who can say — but we know it works because the message resolves: "Heere at the wall." Old spelling, deliberately so. Capital H, capital W. A church on a wall.