In 1925 Goldberg demonstrated and published a technique for making microdot (Mikrat nach Goldberg) at a resolution equivalent to the text 50 complete Bibles per square inch. This invention has been widely attributed to a mythical "Professor Zapp" based on J. Edgar Hoover 's erroneous article in the April 1946 Reader's Digest, probably a confusion with Kurt Zapp who trained German spies in microdot photography during the Second World War. In 1937, Goldberg presented a paper at the World Congress of Universal Documentation on an early copying camera he had invented.