Discern in a Mirror what an Enemy designs at the Distance (Spell)

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.

The grimoires The Book of Forbidden Knowledge Egyptian Secrets of Albertus Magnus lists this spell.

This spell references the historical event Thirty Year's War

Timeline of related events

1600

1618

Thirty Years' War (this spell references Thirty Year's War)

1900

1950

In this decade: Publication of The Book of Forbidden Magic (lists this spell)