David Ramsay Hay’s mapping of colour onto musical notes, a diagram from his The Laws of Harmonious Colouring (1838).

This is entirely beside the point of the actual article, but I'm kinda fascinated by that choice of photo. At least to me, the negative is barely legible; I can see all the features, but they barely come together as a coherent face in my mind. So I inverted it and suddenly it went from "I understand conceptually that there's a person there" to "hah this guy looks like he'd be either hilarious or completely insufferable"

Specifically not embedding the image so that everyone that wishes to can see the inverted one from the article first https://imgur.com/a/E2Vws3q