"Newton’s earlier studies of light, in which he stuck a needle into his eye and recorded how colors changed as he pressed his retina into different shapes."

That's so unbelievable... (not that I don't believe it, but how could he??).

Because it was a blunt needle, more akin to a knitting needle, and he didn't puncture his eye with it, but inserted it into the socket, between his eye and the bone of his skull, so that he could get it as far back as he could, and deform the back part of his eye.

Ah - still scary, but not nearly as scary as puncturing the eye.

There's a fictional account of this experiment in one of Neal Stephenson's books - Quicksilver I think. It's good.