The idea of invisible ink had been around for a long time.
Write using lemon juice as ink, for instance, and the
writing will be invisible. Heat the paper, and the writing
will appear, looking as if it were written in brown ink.
British agents used two types of ink. One could be made
visible by holding the paper over the flame of a candle;
the other could be read by applying a common chemical.
Major John André, the chief British intelligence officer in
New York, told his agents to put an F in the corner of
letters containing invisible ink needing fire to read and A
for those needing acid.
-George Washington, Spymaster,
Thomas B. Allen
What is the central idea of the passage?
* Invisible ink isn't really invisible at all.
Lemon juice is the earliest form of invisible ink.
British agents used invisible ink to send secret
messages.