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.