The secret transfer of documents became very difficult during the Cold War. Agents relied on the microdot camera to photograph and reduce whole pages of information onto a single tiny piece of film. This piece of film could be embedded into the text of a letter as small as the period at the end of this sentence.
Microdots were also hidden in other things such as rings, hollow coins, or other mailed items. The recipient would read the microdot with the aid of a special viewer, often cleverly concealed as well.
1.6 cm x 2.8 cm
(L x Diameter)