What is "invisible inks"

Usage examples of "invisible inks".

You must then be skilled in the use of shortwave radio, invisible inks, and so on.

Described as early as the first century AD, invisible inks were commonly used for serious communications until WWII.

Modern invisible inks fluoresce under ultraviolet light and are used as anti-counterfeit devices.

Further analysis dated this 'stain'as invisible inks were then knownto approximately 1700.

Making invisible inks, which possibly began by using milk on ordinary white paper—.

He was going to take it to a place where some one with an understanding of invisible inks could examine it.

The banks, through simple pressure of time, had to be able to spot tracers easily, which put chemical invisible inks out of court.

I'll never get any field experience with him, unless it's in skulking and sneaking and invisible inks.