The secret transfer of documents became very difficult during the Cold War. Agents relied on microdot cameras to photograph and reduce documents onto a single tiny piece of film. The piece of film could be embedded into the text of a letter as small as a period (.) at the end of this sentence.
Microdots were also hidden in other things such as rings, hollow coins, or even a postcard. The recipient would read the microdot with the aid of a special viewer, often cleverly concealed as well.