From Egyptian Secrets of Albertus Magnus, Joseph H. Peterson edition:
A Secret and curious Piece of Marvel, to Discern in a Mirror what an Enemy designs at the Distance of Three Miles or more.
Obtain a good plain looking glass, as large as you please, and have it framed on three sides only; upon the left side it should be left open. Such a glass must be held toward the direction where the enemy is existing and you will be able to discern all his markings, manœuverings, his doings and workings. Was effectually used during the thirty years' war.
This spell references the historical event Thirty Year's War
Timeline of related events
1618Thirty Years' War (this spell references Thirty Year's War)
1725Publication of Egyptian Secrets of Albertus Magnus (lists this spell)
1869English-language translation of Egyptian Secrets (lists this spell)