course in espionage tradecraft. They were taught to write letters in secret inks. The SS had developed a disappearing ink that actually looked like ink, bluish in color and carried in a regular ink bottle; a message written with this ink would turn invisible after a few minutes and could only be unmasked with a certain reagent. They learned how to operate a German-invented “microdot” camera that shrunk documents to the size of the dot above an i, allowing
