Isaac Newton once stuck a bodkin, a long sewing needle, into his eye socket and pressed around, determined to learn about optics, the nature of light and the physiological processes that shape our reality.
How do we know about the event? From his own detailed notes and illustrations.
This is, in part, what he wrote in his notebook at the time:
“I tooke a bodkine gh & put it betwixt my eye & [the] bone as neare to [the] backside of my eye as I could: & pressing my eye [with the] end of it (soe as to make [the] curvature a, bcdef in my eye) there appeared severall white darke & coloured circles r, s, t, &c. Which circles were plainest when I continued to rub my eye [with the] point of [the] bodkine, but if I held my eye & [the] bodkin still, though I continued to presse my eye [with] it yet [the] circles would grow faint & often disappeare untill I removed [them] by moving my eye or [the] bodkin.”