It was not long before I observed that it was the most susceptible part of her face, and that, when she turned pale, that mark altered first, and became a dull, lead-coloured streak, lengthening out to its full extent, like a mark in invisible ink
brought to the fire.
If her faithful slate had had the latent qualities of sympathetic paper, and its pencil those of invisible ink
, many a little treatise calculated to astonish the pupils would have come bursting through the dry sums in school-time under the warming influence of Miss Peecher's bosom.
These souvenirs, half effaced and almost obliterated by excess of suffering, were revived by the sombre figure which stood before her, as the approach of fire causes letters traced upon white paper with invisible ink
, to start out perfectly fresh.
SPYING used to be about secret codes and invisible inks
. Now it's about Facebook and Twitter.
In June 1915, Walter Kirke wrote that Mansfield Cumming, the first chief of the SIS, was "making enquiries for invisible inks at the London University".
Spooks at MI6 discovered the bodily fluid could be used as an invisible ink, as it glows brightly under UV light but does not react to detection chemicals.
The technology allows the creation of holograms, hard-to-copy trademarks, invisible inks
and unique threads to DNA-mark yarn, woven fabrics, threads, labels and finished garments.
Later, spies used invisible inks made with special chemicals.
Spies have been sending messages in invisible ink for ages.
It appears that owners of certain HP Indigo six-color presses (1000, s2000 and ws2000 models) can now buy invisible inks
as part of the company's security package.
In George Washington's day, spies relied on secret signals and invisible inks
that would probably amuse 21st century children.
They study invisible inks
and other methods of keeping messages secret.