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Spies have used invisible ink for many years to send secret messages. In this section, we'll show you how it's been used and tell you how you can make your own invisible ink.
Courtney de Rysbach was a popular entertainer who performed in shows all over Britain during the First World War (1914-1918). But he had a dark secret - he was a German spy!
Rysbach travelled around the country, giving performances in towns and cities all over Britain. At the same time he was finding out secret information to send back to Germany. In July 1915, he was caught by MI5 and sent to jail for the rest of the war.
It was very difficult for spies to send messages to Germany during the war. Telephone cables to Germany were cut, so they couldn't just phone home. Radios in those days were very big and difficult to hide. Letters sent to foreign countries were opened and read to make sure that they weren't from spies. So they had to find other ways of passing on secrets.
Rysbach had a clever answer. He sent messages written in invisible ink on sheets of music. The messages were concealed between the lines of music. If you looked at the sheets, all you would see would be the musical notes. But if you treated the sheets in a special way you would see the invisible writing.