If you’re brave enough, there is no more convenient scientific subject than your own body: You’re intimately familiar with it, you get near-instant feedback, and you always have it with you (though, to be clear, we are not suggesting you do this at home). Inventors who have used themselves as guinea pigs include polio vaccine inventor Jonas Salk, LSD creator Albert Hofmann, and one of history’s most famous scientists, Sir Isaac Newton. (And no, this is not about the apple that probably didn’t fall on his head.)

Newton had a keen interest in the study of light and vision, especially the way humans perceive color. He wondered if colors were created from inside the eye or by forces outside of it, and if it was the former, if directly manipulating one’s eye could create the sensation of color.

To find out, Newton tried an experiment with his own sight. He took a bodkin (a kind of large sewing needle), inserted it into his eye socket below his actual eye, and pressed it into the back of the eyeball. Then he started to record what he saw, writing, “There appeared severall [sic] white darke & coloured circles.”

It was a neat effect, but not a scientific breakthrough. Newton gained more insight and acclaim through far less invasive experiments with prisms; by demonstrating that a beam of white light could be refracted and then come back together, he found that colors are innate properties of light, not tricks of the eye.