How to Make Glow-in-the-Dark Ink, 1661

Doing some low-light writing? Grab a fig.
Samuel van Hoogstraten, Self Portrait (1647-9)

"Letters that are not to be read but in the night, must be written with the Gall of a Tortois, or Fig milk, if you put it to dry at the fire, or else [with] Water of Glow-Wormes."

Johann Jacob Wecker, Eighteen Books of the Secrets of Art & Nature (1661)

If I may offer a recommendation: opt for the fig milk. The tortoise dissection might be a little messy on your writing desk.